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The Earth AnalogsEditor: Paul Muljadi

ContentsArticlesDefinition 1

Earth analog 1

The Planets 5

16 Cygni Bb 523 Librae b 955 Cancri f 11Gliese 581 c 14Gliese 581 d 22Gliese 581 g 26Gliese 876 b 35Gliese 876 c 39HD 23127 b 42HD 28185 b 44HD 37124 b 47HD 69830 d 49HD 85512 b 52Kepler-22b 55Mars 57Upsilon Andromedae d 88

Appendix 91

Habitable zone 91

ReferencesArticle Sources and Contributors 99Image Sources, Licenses and Contributors 102

Article LicensesLicense 104

1

Definition

Earth analog

The planet Earth

An Earth analog (also referred to as a Twin Earth, Earth Twin,Second Earth, Alien Earth, Earth 2 or Earth-like planet) is atheoretical other planet (or world) with conditions similar to Earth.

Humans have long speculated on the existence of such a planetand there is considerable interest in the subject expressed inscience, philosophy, science fiction and popular culture.

Before the scientific search for and study of extrasolar planets, thelikeliness of an Earth analog's existence was argued throughphilosophy and science fiction, including the Rare Earthhypothesis, the Mediocrity principle and the Twin Earth thoughtexperiment and alternative reality. More recent searches havegreatly influenced the science of astrobiology, models of planetaryhabitability and also the Search for Extraterrestrial Intelligence.The search is also of interest to advocates of space colonization asa "second home" or space and survival as a potential "new home".

Technological advances in extrasolar planet discovery are rapidly refining the probabilities. Although thedistribution and attributes of the studied planets are still very much unknown, current calculations suggest that Earthanalogs may be relatively common in the universe. 2011 estimates by the NASA's Jet Propulsion Laboratory (JPL)based on observations from the Kepler mission determined that about "1.4 to 2.7 percent" of all sun-like stars areexpected to have earthlike planets "within the habitable zones of their stars". This means there are "two billion" ofthem in our own Milky Way galaxy alone and assuming that all galaxies have a similar number as the Milky Way, inthe 50 billion galaxies in the observable universe there may be as many as a sextillion.[1]

NASA and SETI have proposed categorising the increasing number of Earth-like planets found using a measurecalled the Earth Similarity Index (ESI) based on mass, radius and temperature.[2] [3] According to this measure, theclosest confirmed analogs are Gliese 581 d (0.74), Mars and Gliese 581c (0.70).[4]

AttributesThe probability of finding an Earth analog depends mostly on the attributes which are expected to be similar, andthese vary greatly. Generally it is considered that it would be a terrestrial planet and there have been severalscientific studies aimed at finding such planets. Often implied but not limited to are such criteria as planet size, starsize (i.e. simlar to the Sun), orbital distance and stability, axial tilt and rotation, similar geography, oceans, air andweather conditions, strong magnetosphere and even the presence Earth-like complex life (possibly throughconvergent evolution or parallel evolution). If there is complex life, there could be some forests covering much ofthe land. If there is intelligent life, some parts of land could be covered in cities. Some things that are assumed ofsuch a planet may be unlikely due to Earth's own history. For instance the Earth's atmosphere was not alwaysoxygen-rich and this is a biosignature from the emergence of photosynthetic life. The formation, presence, influenceon these characteristics of the moon (such as tidal forces) may also pose a problem in finding an Earth analog.

Earth analog 2

Candidates in the Solar SystemEarly in the history of astronomy, Venus (and to a lesser extent Mars and Neptune) were thought to be Earth-likeplanets and some even conceptualised them to be home to extraterrestrial civilisation. Although these were laterfound to be misconceptions.Still, scientists continue to find similarities with Mars and postulate that both ancient Venus and Mars could havebeen quite Earth-like.

Mars

Size comparison of Earth and Mars.

Mars, the second closest planet, appears to have had and still havesome similarities to Earth. Like Earth, Mars has an atmosphere with agreenhouse effect, geographical similarities including polar ice caps,similar rotation, volcanic activity and evidence of water. As such, Marsremains a candidate for extraterrestrial life. It also makes humancolonization of Mars a subject of much research.

However Mars is much smaller and lacks a magnetosphere and its yearis almost twice as long. Its freezing climate, lower gravity and thin buttoxic carbon dioxide atmosphere all make it hostile to Earth life.

Ancient Mars

An artist's impression of ancient Mars and itsoceans based on geological data

Ancient Mars may have been quite Earth-like with a similar climateand liquid water.

Mars Ocean Hypothesis

The Mars Ocean Hypothesis states that nearly a third of the surface ofMars was covered by an ocean of liquid water early in the planet’sgeologic history.[5] This primordial ocean, dubbed Oceanus Borealis,[6]

would have filled the Vastitas Borealis basin in the northernhemisphere, a region which lies 4–5 km (2.5–3 miles) below the meanplanetary elevation, at a time period of approximately 3.8 billion yearsago. Evidence for this ocean includes geographic features resemblingancient shorelines, and the chemical properties of the Martian soil andatmosphere. Early Mars would have required a denser atmosphere andwarmer climate to allow liquid water to remain at the surface.[7]

VenusFurther information: Atmosphere of Venus, Volcanism on Venus, and Terraforming of VenusVenus is sometimes called Earth's "sister planet" (see below) due to the similar size, gravity, and bulk composition.Like Earth it has an atmosphere with a greenhouse effect and clouds, rain and is volcanically active. A youngerVenus is believed to have possessed Earth-like oceans,[8] but these evaporated as the temperature rose. However, theextreme heat on present day Venus, combined with the crushing atmosphere composed of toxic carbon dioxide andsulfuric acid rain makes the surface hostile to Earth life. The possibility that a habitable niche exists in the lower andmiddle cloud layers of Venus can not yet be excluded.[9]

Earth analog 3

Saturn's moon Titan

Saturn's moon Titan

Further information: Life on Titan, Lakes of Titan, and Colonization ofTitan

Saturn's moon Titan has some Earth-like properties. Titan's geographyhas similarities to Earth and is known to have a dense atmosphere,[10]

with clouds and rain and the only object other than Earth for whichclear evidence of stable bodies of surface liquid has been found.[11]

The possibility of life on Titan is a subject of ongoing research.In billions of years, Titan may become Earth-like as the solar system'shabitable zone moves farther out.However Titan is much smaller than Earth, it has a lower gravity andits composition including its toxic methane atmosphere andanti-greenhouse effect is hostile to Earth life.

Extrasolar Earth analog

Artist's rendering of Kepler-22b, a possible Earth analogwhich orbits a sun-like star some 600 light years away.

The mediocrity principle suggests that there is a chance thatserendipitous events may have allowed an Earth-like planet toform elsewhere that would allow the emergence of complex,multi-cellular life. However, the Rare Earth hypothesis assertsthat if the strictest criteria are applied, such a planet, if itexists may be so far away that humans may never locate it.

Because the Solar System proved to be devoid of an Earthanalog, the search has widened to extrasolar planets.Astrobiologists assert that Earth analogs would most likely befound in a stellar habitable zone in which liquid water couldexist, providing the conditions for supporting life. It is alsothought by some that a sufficiently massive natural satellitemay form a habitable moon similar to Earth.

A 2011 estimate by the NASA's Jet Propulsion Laboratory (JPL), based on observations from the Kepler mission,determined that about "1.4 to 2.7 percent" of all sun-like stars are expected to have earth-like planets "within thehabitable zones of their stars". This means there are "two billion" such worlds in the Milky Way galaxy. Assumingthat all galaxies have a similar number as the Milky Way, in the 50 billion known galaxies there may be as many asone sextillion.[1]

Perhaps one of the most promising Earth analogs to date, Kepler-22b was confirmed December 5, 2011.[12] orbitingthe habitable zone of a sun-like main sequence star. At 2.4 times the size of Earth it has an estimated surfacetemperature around 22 degrees Celsius, however the nature of the planet is still unknown.

Earth analog 4

Terraforming

Artist's conception of a terraformed Venus, a potential Earthanalog

Terraforming (literally, "Earth-forming") of a planet, moon, orother body is the hypothetical process of deliberatelymodifying its atmosphere, temperature, surface topography orecology to be similar to those of Earth to make it habitable byterran organisms.

Terraforming technology may also, in the distant future,artificially produce an Earth analog planet. Although not real,virtual reality may also create such a world.

References[1] Choi, Charles Q. (21 March 2011). "New Estimate for Alien Earths: 2

Billion in Our Galaxy Alone" (http:/ / www. space. com/11188-alien-earths-planets-sun-stars. html). Space.com. . Retrieved2011-04-24.

[2] http:/ / www. wired. co. uk/ news/ archive/ 2011-11/ 21/exoplanet-indices

[3] Stuart Gary New approach in search for alien life (http:/ / www. abc. net.au/ news/ 2011-11-22/ new-recipe-in-search-for-alien-life/ 3686408/ ) ABC Online. November 22, 2011

[4] Dirk Schulze-Makuch, Abel Méndez, Alberto G. Fairén, Philip von Paris, Carol Turse, Grayson Boyer, Alfonso F. Davila, Marina Resendesde Sousa António, David Catling, and Louis N. Irwin. Astrobiology. doi:10.1089/ast.2010.0592. October 21, 2011

[5] Clifford, S. M. and T. J. Parker, 2001: The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and theCurrent State of the Northern Plains (http:/ / www. sciencedirect. com/ science?_ob=ArticleURL& _udi=B6WGF-457CXN8-4&_user=126524& _rdoc=1& _fmt=& _orig=search& _sort=d& view=c& _acct=C000010360& _version=1& _urlVersion=0&_userid=126524& md5=f78bbc3ae211391e23070bd03d8e1dc6), Icarus 154, 40-79.

[6] Baker, V. R., R. G. Strom, V. C. Gulick, J. S. Kargel, G. Komatsu and V. S. Kale, 1991: Ancient oceans, ice sheets and the hydrological cycleon Mars, Nature, 352, 589-594.

[7] Read, Peter L. and S. R. Lewis, “The Martian Climate Revisited: Atmosphere and Environment of a Desert Planet”, Praxis, Chichester, UK,2004.

[8] Hashimoto, G. L.; Roos-Serote, M.; Sugita, S.; Gilmore, M. S.; Kamp, L. W.; Carlson, R. W.; Baines, K. H. (2008). "Felsic highland crust onVenus suggested by Galileo Near-Infrared Mapping Spectrometer data". Journal of Geophysical Research, Planets 113: E00B24.Bibcode 2008JGRE..11300B24H. doi:10.1029/2008JE003134.

[9] Cockell, C. S. (December 1999), "Life on Venus", Planetary and Space Science 47 (12): 1487–1501, Bibcode 1999P&SS...47.1487C,doi:10.1016/S0032-0633(99)00036-7

[10] "News Features: The Story of Saturn" (http:/ / saturn. jpl. nasa. gov/ news/ features/ saturn-story/ moons. cfm). Cassini-Huygens Mission toSaturn & Titan. NASA & JPL. . Retrieved 2007-01-08.

[11] Stofan, E. R.; et al. (2007). "The lakes of Titan". Nature 445 (1): 61–64. Bibcode 2007Natur.445...61S. doi:10.1038/nature05438.PMID 17203056.

[12] BBC NEWS, "Kepler 22-b: Earth-like planet confirmed" 12/5/2011 http:/ / www. bbc. co. uk/ news/ science-environment-16040655

5

The Planets

16 Cygni Bb

16 Cygni Bb

Extrasolar planet List of extrasolar planets

Radial velocity changes over time of 16 Cygni B caused by the orbit of 16 Cygni B b.

Parent star

Star 16 Cygni B

Constellation Cygnus

Right ascension (α) 19h 41m 51.9720s

Declination (δ) +50° 31′ 03.083″

Distance 70.5 ly(21.6 pc)

Spectral type G2.5Vb

Mass (m) 0.97 M☉

Radius (r) 1.2 R☉

Temperature (T) 5752 ± 3.5[1]  K

Metallicity [Fe/H] 0.09

Age 9.9 Gyr

Orbital elements

Semimajor axis (a) 1.681 ± 0.097 AU

Eccentricity (e) 0.681 ± 0.017

Orbital period (P) 798.5 ± 1.0 d

Argument ofperiastron

(ω) 85.8 ± 2.4°

Time of periastron (T0) 2,446,549.1 ± 6.6 JD

16 Cygni Bb 6

Semi-amplitude (K) 50.5 ± 1.6 m/s

Physical characteristics

Mass (m) >1.68 ± 0.15 MJ

Discovery information

Discovery date 22 October 1996

Discoverer(s) Cochran et al.

Detection method Radial velocity

Discovery site  United States

Discovery status Published

Database references

Extrasolar PlanetsEncyclopaedia

data [2]

SIMBAD data [3]

16 Cygni Bb or 16 Cyg Bb is an extrasolar planet approximately 70 light-years away in the constellation of Cygnus.The planet was discovered orbiting the sun-like star 16 Cygni B, one of two solar-mass components of the triple starsystem 16 Cygni. It makes one revolution every 799 days and was the first eccentric Jupiter to be discovered.

DiscoveryIn October 1996 the discovery of a planetary-mass companion to the star 16 Cygni B was announced, with a mass atleast 1.68 times that of Jupiter. At the time, it had the highest orbital eccentricity of any known extrasolar planet. Thediscovery was made by measuring the star's radial velocity. As the inclination of the orbit is unknown, only a lowerlimit on the mass is known.[4]

Orbit and mass

The orbit of 16 Cygni Bb (black) compared to theplanets in the Solar system.

Unlike the planets in our solar system, the planet's orbit is highlyelliptical, and its distance varies from 0.54 AU at periastron to 2.8 AUat apastron.[5] This high eccentricity may have been caused by tidalinteractions in the binary star system, and the planet's orbit may varychaotically between low and high-eccentricity states over a period oftens of millions of years.[6]

16 Cygni Bb 7

16 Cygni Bb as seen from a desert moon. Star 16Cygni A is seen setting in the moon's atmosphere.

Rendered by Celestia.

The 16 Cygni Bb system

Companion(in order from star)

Mass Semimajoraxis(AU)

Orbitalperiod(days)

Eccentricity

b 1.68 MJ 1.68 799.5 0.689

The lower limit for the object's mass is well below the dividing line between planets and brown dwarfs at 13 Jupitermasses. Preliminary astrometric measurements in 2001 suggested the orbit of 16 Cygni Bb may be highly inclinedwith respect to our line of sight (at around 173°).[7] This would mean the object's mass may be around 14 times thatof Jupiter, making it a low-mass brown dwarf. However these measurements were later proved useful only for upperlimits;.[8]

Physical characteristicsSince the planet has only been detected indirectly by measurements of its parent star, properties such as its radius,composition and temperature are unknown.

Habitable zoneThe planet's highly eccentric orbit means the planet would experience extreme seasonal effects. Despite this,simulations suggest that an Earth-like moon would be able to support liquid water at its surface over the course of ayear.[9] Due to the eccentric orbit of this massive gas giant, it is unlikely that a habitable planet could survive in thissystem.[10]

References[1] Kovtyukh et al.; Soubiran, C.; Belik, S. I.; Gorlova, N. I. (2003). "High precision effective temperatures for 181 F-K dwarfs from line-depth

ratios" (http:/ / www. aanda. org/ articles/ aa/ full/ 2003/ 46/ aa3944/ aa3944. html). Astronomy and Astrophysics 411 (3): 559–564.arXiv:astro-ph/0308429. Bibcode 2003A&A...411..559K. doi:10.1051/0004-6361:20031378. .

[2] http:/ / exoplanet. eu/ planet. php?p1=16+ Cyg+ B& p2=b[3] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=*+ 16+ Cyg+ Bb[4] BUTLER P. & MARCY G., The Lick Observatory Planet Search in: Astronomical and Biochemical Origins and the Search for Life in the

Universe, IAU Colloquium No. 161, Capri 1–5 July 1996, eds. C.B. Cosmovici, S. Bowyer, and D. Werthimer[5] Butler et al.; Wright, J. T.; Marcy, G. W.; Fischer, D. A.; Vogt, S. S.; Tinney, C. G.; Jones, H. R. A.; Carter, B. D. et al. (2006). "Catalog of

Nearby Exoplanets" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 646/ 1/ 505/ 64046. html). The Astrophysical Journal 646 (1): 505–522.arXiv:astro-ph/0607493. Bibcode 2006ApJ...646..505B. doi:10.1086/504701. Archived (http:/ / www. webcitation. org/ 5gTrfcenD) from theoriginal on 2009-05-02. . Retrieved 2009-03-11. ( web version (http:/ / exoplanets. org/ planets. shtml))

16 Cygni Bb 8

[6] Holman, M. et al. (1997). "Chaotic variations in the eccentricity of the planet orbiting 16 Cygni B" (http:/ / www. nature. com/ nature/journal/ v386/ n6622/ abs/ 386254a0. html). Nature 386 (6622): 254–256. Bibcode 1997Natur.386..254H. doi:10.1038/386254a0. .

[7] Han et al.; Black, David C.; Gatewood, George (2001). "Preliminary Astrometric Masses for Proposed Extrasolar Planetary Companions"(http:/ / www. iop. org/ EJ/ article/ 1538-4357/ 548/ 1/ L57/ 005774. html). The Astrophysical Journal Letters 548 (1): L57–L60.Bibcode 2001ApJ...548L..57H. doi:10.1086/318927. Archived (http:/ / www. webcitation. org/ query?id=1241033398789808) from theoriginal on 2009-04-29. . Retrieved 2009-03-11.

[8] Pourbaix, D. and Arenou, F. (2001). "Screening the Hipparcos-based astrometric orbits of sub-stellar objects". Astronomy and Astrophysics372 (3): 935–944. arXiv:astro-ph/0104412. Bibcode 2001A&A...372..935P. doi:10.1051/0004-6361:20010597.

[9] Williams, D., Pollard, D. (2002). "Earth-like worlds on eccentric orbits: excursions beyond the habitable zone" (http:/ / journals. cambridge.org/ action/ displayAbstract?fromPage=online& aid=105145). International Journal of Astrobiology 1 (01): 61–69.Bibcode 2002IJAsB...1...61W. doi:10.1017/S1473550402001064. .

[10] Wittenmyer et al.; Endl, Michael; Cochran, William D.; Levison, Harold F. (2007). "Dynamical and Observational Constraints onAdditional Planets in Highly Eccentric Planetary Systems" (http:/ / www. iop. org/ EJ/ article/ 1538-3881/ 134/ 3/ 1276/ 205882. html). TheAstronomical Journal 134 (3): 1276–1284. Bibcode 2007AJ....134.1276W. doi:10.1086/520880. .

External links• Jean Schneider (2011). "Notes for Planet 16 Cyg B b" (http:/ / exoplanet. eu/ planet. php?p1=16+ Cyg+ B&

p2=b). Extrasolar Planets Encyclopaedia. Retrieved 30 September 2011.• "16 Cygni 2?" (http:/ / www. solstation. com/ stars2/ 16cygni2. htm). SolStation. Retrieved 2008-06-24.• "16 Cygni-B" (http:/ / www. astro. uiuc. edu/ ~kaler/ sow/ 16cyg. html). University of Illinois at

Urbana-Champaign. The Planet Project. Retrieved 2008-06-24.• "16 Cygni B b" (http:/ / www. extrasolar. net/ planettour. asp?StarCatId=& PlanetId=14). Extrasolar Visions.

Retrieved 2008-06-24.• "16 Cyg B" (http:/ / media4. obspm. fr/ exoplanets/ base/ etoile. php?nom=16+ Cyg+ B). Exoplanets. Archived

(http:/ / www. webcitation. org/ query?id=1241644167062691) from the original on 2009-05-06. Retrieved2009-05-03.

23 Librae b 9

23 Librae b

23 Librae b

Extrasolar planet List of extrasolar planets

Artist's conception of 23 Librae b, as a gas giant.

Parent star

Star 23 Librae

Constellation Libra

Right ascension (α) 15h 13m 28s

Declination (δ) –25° 18′ 33″

Apparent magnitude (mV) 6.45

Distance 83.7 ly(25.6 pc)

Mass (m) 1.05 M☉

Radius (r) 1.2 R☉

Temperature (T) 5559 K

Metallicity [Fe/H] 0.23

Age 7.32 Gyr

Orbital elements

Semimajor axis (a) 0.81 ± 0.02 AU(120 Gm)

Periastron (q) 0.62 AU(92 Gm)

Apastron (Q) 0.99 AU(149 Gm)

Eccentricity (e) 0.233 ± 0.002

Orbital period (P) 258.19 ± 0.07 d(0.7069 y)

Argument ofperiastron

(ω) 358.3 ± 3.7°

Time of periastron (T0) 2,450,331.7 ± 2.2 JD

Semi-amplitude (K) 49.52 ± 0.57 m/s

Physical characteristics

Minimum mass (m sin i) 1.59 ± 0.02 MJ

Discovery information

23 Librae b 10

Discovery date November 14, 1999

Discoverer(s) Vogt et al.

Detection method Radial velocity

Discovery site Keck Observatory

Discovery status Published

Other designations

HD 134987 b

Database references

Extrasolar PlanetsEncyclopaedia

data [1]

SIMBAD data [2]

23 Librae b (23 Lib b) is an extrasolar Jovian planet discovered in 1999 orbiting the star 23 Librae. It orbits in itsstar's habitable zone.[3] [4]

As of 1999, the planet was known to have at least 1.5 times Jupiter's mass. The planet orbits 23 Librae at an averagedistance of 0.82 AUs, which is between that of Venus and the Earth in the Solar System.[5]

References[1] http:/ / exoplanet. eu/ planet. php?p1=HD+ 134987& p2=b[2] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=HD+ 134987+ b[3] Vogt et al.; Marcy, Geoffrey W.; Butler, R. Paul; Apps, Kevin (2000). "Six New Planets from the Keck Precision Velocity Survey" (http:/ /

www. iop. org/ EJ/ article/ 0004-637X/ 536/ 2/ 902/ 50799. html). The Astrophysical Journal 536 (2): 902–914. arXiv:astro-ph/9911506.Bibcode 2000ApJ...536..902V. doi:10.1086/308981. .

[4] Butler et al.; Wright, J. T.; Marcy, G. W.; Fischer, D. A.; Vogt, S. S.; Tinney, C. G.; Jones, H. R. A.; Carter, B. D. et al. (2006). "Catalog ofNearby Exoplanets" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 646/ 1/ 505/ 64046. html). The Astrophysical Journal 646 (1): 505–522.arXiv:astro-ph/0607493. Bibcode 2006ApJ...646..505B. doi:10.1086/504701. .

[5] Jones, Hugh; Paul Butler; Tinney; Simon O'Toole; Rob Wittenmyer; Henry; Stefano Meschiari; Steve Vogt et al. (2009). "A long-periodplanet orbiting a nearby Sun-like star". arXiv:0912.2716v1 [astro-ph.EP].

55 Cancri f 11

55 Cancri f

55 Cancri f

Extrasolar planet List of extrasolar planets

An artist's impression of 55 Cancri f.The star-like-looking dots around its sun are the three innermost planets.

Parent star

Star 55 Cancri A

Constellation Cancer

Right ascension (α) 08h 52m 35.8s

Declination (δ) +28° 19′ 51″

Apparent magnitude (mV) 5.95

Distance 40.3 ± 0.4 ly(12.3 ± 0.1 pc)

Spectral type G8V

Mass (m) 0.95 ± 0.10 M☉

Radius (r) 1.152 ± 0.035 R☉

Temperature (T) 5373 ± 9.7 K

Metallicity [Fe/H] 0.29

Age 7.4–8.7 Gyr

Orbital elements

Semimajor axis (a) 0.781 ± 0.007[1] AU(116.9 Gm)

62.5 mas

Periastron (q) 0.730 AU(109.1 Gm)

Apastron (Q) 0.833 AU(124.6 Gm)

Eccentricity (e) 0.2 ± 0.2[1]

Orbital period (P) 260.00 ± 1.1[1] d(0.7118 y)

55 Cancri f 12

Argument ofperiastron

(ω) 181.1 ± 60[1] °

Time of periastron (T0) 2,450,080.9108 ± 1.1[1] JD

Semi-amplitude (K) 4.879 ± 0.6[1] m/s

Physical characteristics

Minimum mass (m sin i) 0.144 ± 0.04[1] MJ(45.7 ± 12.7[1] M⊕)

Discovery information

Discovery date 11 April 2005 (announced)6 November 2007 (published)

Discoverer(s) announced by J. Wisdompublished by D. Fischer

Detection method Doppler spectroscopy

Discovery site  United States

Discovery status Published

Other designations

55 Cancri Af, Rho1 Cancri f, HD 75732 f

Database references

Extrasolar PlanetsEncyclopaedia

data [2]

SIMBAD data [3]

55 Cancri f (abbreviated 55 Cnc f and also referred to as Rho1 Cancri f) is an extrasolar planet approximately 41light-years away from Earth in the constellation of Cancer (the Crab). 55 Cancri f is the fourth known planet (inorder of distance) from the star 55 Cancri and the first planet to have been given the designation of "f".[4]

Discovery

Radial velocity changes over time of 55 Cancri caused by the orbit of 55Cancri f.

The initial presentation of this planet occurred at ameeting of the American Astronomical Society inApril 2005,[5] however it was another two and ahalf years before the planet was to be published ina peer-reviewed journal.[1] It is the first knownplanet outside our solar system to spend its entireorbit within what astronomers call the "habitablezone".[6] Furthermore, its discovery made 55Cancri the first star other than the Sun known tohave five planets.

55 Cancri f 13

Orbit and mass

55 Cnc f's orbit compared to the orbit of Venus(0.72AU).

55 Cancri f is located about 0.781 AU away from the star andtakes 260 days to complete a full orbit.[7] A limitation of the radialvelocity method used to detect 55 Cancri f is that only a minimummass can be obtained, in this case around 0.144 times that ofJupiter, or half the mass of Saturn.[7] A Keplerian fit to the radialvelocity data of 55 Cancri A indicates that the orbit is consistentwith being circular, however changing the value in a rangebetween 0 and 0.4 does not significantly alter the chi-squaredstatistic of the fit, thus a representative eccentricity of 0.2±0.2 wasassumed.[1] In a Newtonian model which takes interactionsbetween the planets into account, the eccentricity comes out as0.0002, almost circular.[1]

Astrometric observations made with the Hubble Space Telescope suggest that the outer planet 55 Cancri d is inclinedat 53° with respect to the plane of the sky.[8] If these measurements are confirmed and the system is assumed to becoplanar, the true mass of 55 Cancri f would therefore be about 25% greater than this lower limit, at around 0.18Jupiter masses.[1]

CharacteristicsSince the planet was detected indirectly through observations of its star, properties such as its radius, compositionand temperature are unknown. With a mass half that of Saturn,[7] 55 Cancri f is likely to be a gas giant with no solidsurface. It orbits in the so-called "habitable zone," which means that liquid water could exist on the surface of apossible moon.[6]

It is not known if the composition and appearance is more like Saturn or Neptune.[4] Based on its temperature, itshould be a Sudarsky Class II planet, covered in water clouds.

References[1] Debra A. Fischer, Geoffrey W. Marcy, R. Paul Butler, Steven S. Vogt, Greg Laughlin, Gregory W. Henry, David Abouav, Kathryn M. G.

Peek, Jason T. Wright, John A. Johnson, Chris McCarthy, Howard Isaacson (23 December 2007). "Five Planets Orbiting 55 Cancri".Astrophysics 675: 790–801. arXiv:0712.3917. Bibcode 2008ApJ...675..790F. doi:10.1086/525512.

[2] http:/ / exoplanet. eu/ planet. php?p1=55+ Cnc& p2=f[3] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=55+ Cnc+ f[4] Shige Abe (12 November 2007). "Researchers Identify First Five-Planet Extrasolar System" (http:/ / astrobiology. nasa. gov/ articles/

researchers-identify-first-five-planet-extrasolar-system/ ). NASA Astrobiology. . Retrieved 17 September 2008.[5] J. Wisdom (11 April 2005). "A Neptune-sized Planet in the rho1 Cancri System" (http:/ / web. archive. org/ web/ 20061216012023/ http:/ /

www. aas. org/ publications/ baas/ v37n2/ dda05/ 29. htm). DDA 36th Meeting, 10–14 April 2005—Session 5 Posters (The AmericanAstronomical Society). Archived from the original (http:/ / www. aas. org/ publications/ baas/ v37n2/ dda05/ 29. htm) on 16 December 2006. .Retrieved 17 September 2008.

[6] Ian Sample, science correspondent (7 November 2007). "Could this be Earth's near twin? Introducing planet 55 Cancri f" (http:/ / www.guardian. co. uk/ science/ 2007/ nov/ 07/ spaceexploration). London: The Guardian. . Retrieved 17 October 2008.

[7] Jean Schneider (2011). "Notes for Planet 55 Cnc f" (http:/ / exoplanet. eu/ planet. php?p1=55+ Cnc& p2=f). Extrasolar PlanetsEncyclopaedia. . Retrieved 8 October 2011.

[8] Han et al. (2001). "Preliminary Astrometric Masses for Proposed Extrasolar Planetary Companions". The Astrophysical Journal Letters 548(1): L57–L60. Bibcode 2001ApJ...548L..57H. doi:10.1086/318927.

55 Cancri f 14

External links• Ward Glen (8 November 2007). "Astronomers Find Fifth Planet Around 55 Cancri" (http:/ / starrymirror. com/

5thplanetorbitingstar. htm). The Starry Mirror. Retrieved 17 September 2008.

Gliese 581 c

Gliese 581 c

Extrasolar planet List of extrasolar planets

Artist's impression of the planetary system around the red dwarf Gliese 581. The five Earth-mass planet (seen in foreground - Gliese581 c) makes a full orbit around the star in 13 days, the other two in 5 (the blue, Neptunian-like planet - Gliese 581 b) and 84 days (the

most remote one, Gliese 581 d).

Parent star

Star Gliese 581

Constellation Libra

Right ascension (α) 15h 19m 26s

Declination (δ) −07° 43′ 20″

Apparent magnitude (mV) 10.55

Distance 20.3 ± 0.3 ly(6.2 ± 0.1 pc)

Spectral type M3V

Mass (m) 0.31 M☉

Radius (r) 0.29 R☉

Temperature (T) 3480 ± 48 K

Metallicity [Fe/H] -0.33 ± 0.12

Age 7 – 11 Gyr

Orbital elementsEpoch JD 2451409.762[1]

Semimajor axis (a) 0.072993 ± 0.000022[1] AU

Eccentricity (e) 0[1]

Gliese 581 c 15

Orbital period (P) 12.9191 ± 0.0058[1] d(0.03537 y)

(310.06 h)

Mean anomaly (M) 33 ± 19[1] °

Semi-amplitude (K) 3.30 ± 0.19[1] m/s

Physical characteristics

Minimum mass (m sin i) 5.6 ± 0.3[1] M⊕

Discovery information

Discovery date April 4, 2007April 24, 2007 (announced)

Discoverer(s) Stéphane Udry et al.

Detection method Radial velocity

Discovery site La Silla Observatory

Discovery status Published

Database references

Extrasolar PlanetsEncyclopaedia

data [2]

SIMBAD data [3]

Gliese 581 c (pronounced /ˈɡliːzə/) or Gl 581 c is a planet orbiting the red dwarf star Gliese 581.[4] It is the secondplanet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth,[1]

it is classified as a super-Earth (a planet of 1 to 10 Earth masses).[5] Assuming that the planet's mass is close to thelower limit determined by radial velocity measurements (the true mass is unknown), it was the smallest knownextrasolar planet around a main sequence star, but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581e, was announced with an approximate mass of 1.9 Earth masses, which is now the smallest known extrasolar planetaround a main sequence star.[6]

Gliese 581 c initially generated interest because it was originally reported to be the first potentially Earth-like planetin the habitable zone of its star, with a temperature right for liquid water on its surface, and by extension, potentiallycapable of supporting extremophile forms of Earth-like life.[4] [7] However, further research on the potential effectsof the planetary atmosphere casts doubt upon the habitability of Gliese 581 c and indicates that the fourth planet inthe system, Gliese 581 g, is a better candidate for habitability.[1] In astronomical terms, the Gliese 581 system isrelatively close to Earth, at 20.3 light years (192 trillion km or 119 trillion miles) in the direction of the constellationof Libra.[8] [9] This distance, along with the declination and right ascension coordinates, give its exact location in ourgalaxy. It is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; it is the 87th closestknown star system to the Sun.[10]

Gliese 581 c 16

DiscoveryThe team released a paper of their findings dated April 27, 2007, published in the July, 2007 journal Astronomy andAstrophysics.[11] In the paper they also announced the discovery of another planet in the system, Gliese 581 d, with aminimum mass of 7.7 Earth masses and a semi-major axis of 0.25 astronomical units. (A reanalysis of the radialvelocity data has since reduced the minimum possible mass of Gliese 581 d to 5.6 Earth masses.)[12]

Physical characteristics

MassThe existence of Gliese 581 c and its mass have been measured by the radial velocity method of detecting extrasolarplanets. The mass of a planet is calculated by the small periodic movements around a common centre of massbetween the host star Gliese 581 and its planets. When all six planets are fit with a Keplerian solution, the minimummass of the planet is determined to be 5.6 Earth masses.[1] The radial velocity method cannot by itself determine thetrue mass, but it cannot be very much larger than this or the system would be dynamically unstable.[11] Dynamicalsimulations of the Gliese 581 system which assume the orbits of the planets are coplanar indicate that the planetscannot exceed approximately 1.6 – 2 times their minimum masses or the planetary system becomes unstable (this isprimarily due to the interaction between planets e and b). For Gliese 581 c, the upper bound is 10.4 Earth masses.[13]

RadiusSince Gliese 581 c has not been detected directly, there are no measurements of its radius. Furthermore, the radialvelocity method used to detect it only puts a lower limit on the planet's mass, which means theoretical models ofplanetary radius and structure can only be of limited use. However, assuming a random orientation of the planet'sorbit, the true mass is likely to be close to the measured minimum mass.Assuming that the true mass is the minimum mass, the radius may be calculated using various models. For example,if Gliese 581 c is a rocky planet with a large iron core, it should have a radius approximately 50% larger than that ofEarth, according to Udry's team.[11] [14] Gravity on such a planet's surface would be approximately 2.24 times asstrong as on Earth. However, if Gliese 581 c is an icy and/or watery planet, its radius would be less than 2 times thatof Earth, even with a very large outer hydrosphere, according to density models compiled by Diana Valencia and herteam for Gliese 876 d.[5] Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times asstrong as on Earth. They claim the real value of the radius may be anything between the two extremes calculated bydensity models outlined above.[15]

Some modelled radii of Gliese 581 c, compared with Earth and Neptune.

Other scientists' views differ. SaraSeager at MIT has speculated thatGliese 581 c and other five-Earth-massplanets could be:[16]

• "rock giants" mostly of silicate.• "cannonball" planets of solid iron.• "gas dwarfs" mostly of helium and

hydrogen.• carbon-rich "diamond worlds"• purely hot "ice VII worlds".• purely "carbon monoxide worlds".If the planet transits the star as seen from our direction, the radius should be measurable, albeit with someuncertainty. Unfortunately, measurements made with the Canadian-built MOST space telescope indicate that transitsdo not occur.[17]

Gliese 581 c 17

For comparison, the smallest confirmed diameter of a planet around a main-sequence star is that of COROT-7b,which is about 70% larger than Earth.

Orbit

The orbits of the Gliese 581 planetary system, as per the 2009analysis excluding planets g and f. In the picture, Gliese 581 c is the

third planet from the star.

Gliese 581 c has an orbital period ("year") of 13 Earthdays[8] and its orbital radius is only about 7% that ofthe Earth, about 11 million km,[18] while the Earth is150 million kilometres from the Sun.[19] Since the hoststar is smaller and colder than the Sun—and thus lessluminous—this distance places the planet on the"warm" edge of the habitable zone around the staraccording to Udry's team.[11] [14] Note that inastrophysics, the "habitable zone" is defined as therange of distances from the star at which a planet couldsupport liquid water on its surface: it should not betaken to mean that the planet's environment would besuitable for humans, a situation which requires a morerestrictive range of parameters. A typical radius for anM0 star of Gliese 581's age and metallicity is 0.00128AU,[20] against the sun's 0.00465 AU. This proximitymeans that the primary star should appear 3.75 timeswider and 14 times larger in area for an observer on theplanet's surface looking at the sky than the Sun appearsto be from Earth's surface.

Tidal lock

Because of its small separation from Gliese 581, the planet has been generally considered to always have onehemisphere facing the star (only day), and the other always facing away (only night), or in other words being tidallylocked.[21] [22] Although a recent orbital fit to the Gliese 581 system uses a circular orbit for Gliese 581 c,[1] olderfits use an eccentricity between 0.10 and 0.22. If the orbit of the planet were eccentric, it would undergo violent tidalflexing.[23] Because tidal forces are stronger when the planet is close to the star, eccentric planets are expected tohave a rotation period which is shorter than its orbital period, also called pseudo-synchronization.[24] An example ofthis effect is seen in Mercury, which is tidally locked in a 3:2 resonance, completing three rotations every two orbits.In any case, even in the case of 1:1 tidal lock, the planet would undergo libration and the terminator would bealternatively lit and darkened during libration.[25]

Models of the evolution of the planet's orbit over time suggest that heating resulting from this tidal locking may playa major role in the planet's geology. Models proposed by scientists predict that tidal heating could yield a surfaceheat flux about three times greater than the Jupiter's moon Io's, which could result in major geological activity suchas volcanoes and plate tectonics.[26]

Gliese 581 c 18

Habitability and climateThe study of Gliese 581 c by the von Bloh et al. team has been quoted as concluding "The super-Earth Gl 581c isclearly outside the habitable zone, since it is too close to the star."[27] The study by Selsis et al. claims even "a planetin the habitable zone is not necessarily habitable" itself, and this planet "is outside what can be considered theconservative habitable zone" of the parent star, and further that if there was any water there then it was lost when thered dwarf was a strong X-ray and EUV emitter, it could have surface temperatures ranging from 700 K to 1000 K(430 to 730 °C), like Venus today.[28] Temperature speculations by other scientists were based on the temperature of(and heat from) the parent star Gliese 581 and have been calculated without factoring in the margin of error(96 °C/K) for the star's temperature of 3432 K to 3528 K, which leads to a large irradiance range for the planet, evenbefore eccentricity is considered.[29]

Effective temperaturesUsing the measured stellar luminosity of Gliese 581 of 0.013 times that of our Sun, it is possible to calculate Gliese581 c's effective temperature a.k.a. black body temperature. (note: this probably differs from its surface temperature).According to Udry's team, the effective temperature for Gliese 581 c, assuming an albedo (reflectivity) such asVenus' (0.64), would be −3 °C (27 °F), and assuming an Earth-like albedo (0.296), then it would be 40 °C(104 °F),[8] [11] a range of temperatures which overlaps with the range that water would be liquid at a pressure of 1atmosphere. However, the effective temperature and actual surface temperature can be very different due to thegreenhouse properties of the planetary atmosphere: for example, Venus has an effective temperature of 34.25 °C(307.40 K; 93.65 °F), but a surface temperature of 463.85 °C (737.00 K; 866.93 °F) (mainly due to a 96.5% carbondioxide atmosphere), a difference of about 430 °C (770 °F).[30] Studies of the habitability (i.e. liquid water forextremophile forms of life)[31] conclude that Gliese 581 c is likely to suffer from a runaway greenhouse effectsimilar to that found on Venus, as such, is highly unlikely to be habitable. Nevertheless, this runaway greenhouseeffect could be prevented by the presence of sufficient reflective cloud cover on the planet's day side.[32]

Alternatively, if the surface were covered in ice, it would have a high albedo (reflectivity), and thus could reflectenough of the incident sunlight back into space to render the planet too cold for habitability, although this situation isexpected to be very unstable except for very high albedos greater than about 0.95 (i.e. ice): release of carbon dioxideby volcanic activity or of water vapor due to heating at the substellar point would trigger a runaway greenhouseeffect.[33]

Liquid waterGliese 581 c is likely to lie outside the habitable zone.[27] [34] No direct evidence has been found for water to bepresent, and it is probably not present in the liquid state. Techniques like the one used to measure the extrasolarplanet HD 209458 b may in the future be used to determine the presence of water in the form of vapor in the planet'satmosphere, but only in the rare case of a planet with an orbit aligned so as to transit its star, which Gliese 581 c isnot known to do.

Tidally-locked modelsTheoretical models predict that volatile compounds such as water and carbon dioxide, if present, might evaporate in the scorching heat of the sunward side, migrate to the cooler night side, and condense to form ice caps. Over time, the entire atmosphere might freeze into ice caps on the night side of the planet. However, it remains unknown if water and/or carbon dioxide are even present on the surface of Gliese 581c. Alternatively, an atmosphere large enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.[35] For example, although Venus has a small axial inclination, very little sunlight reaches the surface at the poles. A slow rotation rate approximately 117 times slower than Earth's produces prolonged days and nights. Despite the uneven distribution of sunlight cast on Venus at any given time, polar areas and the night side of Venus are kept almost as

Gliese 581 c 19

hot as on the day side by globally circulating winds.[36]

References[1] Vogt, S. S.; et al. (2010). "The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese

581". arXiv:1009.5733 [astro-ph.EP].[2] http:/ / exoplanet. eu/ planet. php?p1=Gl+ 581& p2=c[3] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Gl+ 581+ c[4] Than, Ker (2007-04-24). "Major Discovery: New Planet Could Harbor Water and Life" (http:/ / www. space. com/ scienceastronomy/

070424_hab_exoplanet. html). space.com. . Retrieved 2007-04-29.[5] Valencia et al.; Sasselov, Dimitar D.; O'Connell, Richard J. (2006). "Radius and Structure Models of the First Super-Earth Planet". The

Astrophysical Journal 656 (1): 545–551. arXiv:astro-ph/0610122. Bibcode 2007ApJ...656..545V. doi:10.1086/509800.[6] "Catalog of Nearby Exoplanets—Planets Table" (http:/ / exoplanets. org/ planets. shtml). Catalog of Nearby Exoplanets—Planets Table.

Exoplanets.org. 2008-01-26. . Retrieved 2008-10-05.[7] Than, Ker (2007-02-24). "Planet Hunters Edge Closer to Their Holy Grail" (http:/ / www. space. com/ scienceastronomy/

070424_exoplanet_side. html). space.com. . Retrieved 2007-04-29.[8] "New 'super-Earth' found in space" (http:/ / news. bbc. co. uk/ 1/ hi/ sci/ tech/ 6589157. stm). BBC News. April 25, 2007. . Retrieved

2007–04–25.[9] van Leeuwen, F. (2007). "HIP 74995" (http:/ / webviz. u-strasbg. fr/ viz-bin/ VizieR-5?-out. add=. & -source=I/ 311/ hip2& recno=74749).

Hipparcos, the New Reduction. . Retrieved 2008-08-18.[10] "The 100 Nearest Stars" (http:/ / www. chara. gsu. edu/ RECONS/ TOP100. 2007. 0426. htm). RECONS. . Retrieved 2007–05–10.[11] Udry et al.; Bonfils, X.; Delfosse, X.; Forveille, T.; Mayor, M.; Perrier, C.; Bouchy, F.; Lovis, C. et al. (2007). "The HARPS search for

southern extra-solar planets, XI. Super-Earths (5 and 8 M⊕) in a 3-planet system". Astronomy and Astrophysics 469 (3): L43–L47.Bibcode 2007A&A...469L..43U. doi:10.1051/0004-6361:20077612.

[12] p. 29, Vogt 2010.[13] Mayor, M.; Bonfils, X.; Forveille, T.; Delfosse, X.; Udry, S.; Bertaux, J. -L.; Beust, H.; Bouchy, F. et al. (2009). "The HARPS search for

southern extra-solar planets". Astronomy and Astrophysics 507 (1): 487–494. Bibcode 2009A&A...507..487M.doi:10.1051/0004-6361/200912172.

[14] "Astronomers Find First Earth-like Planet in Habitable Zone" (http:/ / www. eso. org/ outreach/ press-rel/ pr-2007/ pr-22-07. html). ESO. .Retrieved 2007–05–10.

[15] Valencia and Sasselov; Sasselov, Dimitar D.; O’connell, Richard J. (2007). "Detailed Models of Super-Earths: How Well Can We Infer BulkProperties?". The Astrophysical Journal 665 (2): 1413–1420. Bibcode 2007ApJ...665.1413V. doi:10.1086/519554.

[16] Seager (2008). "Alien Earths from A to Z" (http:/ / pqasb. pqarchiver. com/ skyandtelescope/ access/ 1478225761. html?dids=1478225761&FMT=ABS& FMTS=ABS& date=Jan+ 2008& author=Sara+ Seager& desc=Alien+ Earths+ from+ A+ to+ Z). Sky & Telescope. ISSN0037-6604 (January): 22–25. .

[17] "Boring Star May Mean Livelier Planet" (http:/ / www. spaceref. com/ news/ viewpr. html?pid=22805). Spaceref.com. . Retrieved2008-09-15.

[18] Overbye, Dennis (2007–04–25). "20 light years away, the most Earthlike planet yet" (http:/ / www. iht. com/ articles/ 2007/ 04/ 25/ news/planet. php). International Herald Tribune. . Retrieved 2007–05–10.

[19] "The Earth Worldbook" (http:/ / www. nasa. gov/ worldbook/ earth_worldbook. html). NASA. . Retrieved 2007–05–10.[20] Girardi L., Bressan A., Bertelli G., Chiosi C. (2000). "Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to

7 M☉, and from Z=0.0004 to 0.03". Astron. Astrophys. Suppl. Ser. 141 (3): 371. arXiv:astro-ph/9910164. Bibcode 2000A&AS..141..371G.doi:10.1051/aas:2000126.

[21] Vergano, Dan (2007–04–25). "Out of our world: Earthlike planet" (http:/ / www. usatoday. com/ printedition/ news/ 20070425/1a_bottomstrip25_dom. art. htm). USA Today. . Retrieved 2007–05–10.

[22] Selsis 2.4.1 "becomes tidally locked in less than 1 Gyr. "[23] Beust, H. et al. (2008). "Dynamical evolution of the Gliese 581 planetary system". Astronomy and Astrophysics 479 (1): 277–282.

Bibcode 2008A&A...479..277B. doi:10.1051/0004-6361:20078794.[24] Hut, P. (1981). "Tidal Evolution in Close Binary Systems". Astronomy and Astrophysics 99 (1): 126–140. Bibcode 1981A&A....99..126H.[25] Perlman, David (2007-04-24). "New planet found: It might hold life" (http:/ / www. sfgate. com/ cgi-bin/ article. cgi?f=/ c/ a/ 2007/ 04/ 24/

BAG33PE14U26. DTL). San Francisco Chronicle. . Retrieved 2007–04–24.[26] Jackson, Brian; Richard Greenberg, Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ 681 (2): 1631. arXiv:0803.0026.

Bibcode 2008ApJ...681.1631J. doi:10.1086/587641.[27] von Bloh et al. (2007). "The Habitability of Super-Earths in Gliese 581". Astronomy and Astrophysics 476 (3): 1365–1371.

Bibcode 2007A&A...476.1365V. doi:10.1051/0004-6361:20077939.[28] Selsis et al.; Kasting, J. F.; Levrard, B.; Paillet, J.; Ribas, I.; Delfosse, X. (2007). "Habitable planets around the star Gl 581?". Astronomy

and Astrophysics 476 (3): 1373–1387. Bibcode 2007A&A...476.1373S. doi:10.1051/0004-6361:20078091.[29] Bean, J. L.; Benedict, G. F.; Endl, M. (2006). "Metallicities of M Dwarf Planet Hosts from Spectral Synthesis". The Astrophysical Journal

653 (1): L65–L68. arXiv:astro-ph/0611060. Bibcode 2006ApJ...653L..65B. doi:10.1086/510527.

Gliese 581 c 20

[30] "Venus Fact Sheet" (http:/ / nssdc. gsfc. nasa. gov/ planetary/ factsheet/ venusfact. html). NASA. . Retrieved 2008-09-20.[31] Selsis 5. "Gl 581c is very unlikely to be habitable"[32] Selsis 3.1 "would be habitable only if clouds with the highest reflectivity covered most of the daytime hemisphere. "[33] Selsis 3.1.2[34] Selsis Abstract, 3. Figure 4.[35] Alpert, Mark (2005-11-07). "Red Star Rising" (http:/ / www. sciam. com/ article. cfm?chanID=sa004&

articleID=000CC344-B043-1353-AF3383414B7FFE9F). Scientific American. . Retrieved 2007–04–25.[36] Ralph D Lorenz, Jonathan I Lunine, Paul G Withers, Christopher P. McKay (2001). "Titan, Mars and Earth: Entropy Production by

Latitudinal Heat Transport" (http:/ / sirius. bu. edu/ withers/ pppp/ pdf/ mepgrl2001. pdf) (PDF). Ames Research Center, University of ArizonaLunar and Planetary Laboratory. . Retrieved 2007-08-21.

Further reading

News media reports• Dennis Overbye (June 12, 2007). "A Planet Is Too Hot for Life, but Another May Be Just Right; So much for the

Goldilocks planet." (http:/ / www. nytimes. com/ 2007/ 06/ 12/ science/ space/ 12plan. html?_r=2& ref=science&oref=slogin). New York Times. Retrieved 2009-07-11.

• "Astronomers Find First Earth-like Planet in Habitable Zone" (http:/ / www. eso. org/ outreach/ press-rel/pr-2007/ pr-22-07. html). European Southern Observatory. 2007-04-25. Retrieved 2008-06-20.

• "New 'super-Earth' found in space" (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 6589157. stm). BBC News.2007-04-25. Retrieved 2008-06-20.

• Than, Ker (2007-04-24). "Major Discovery: New Planet Could Harbor Water and Life" (http:/ / www. space.com/ scienceastronomy/ 070424_hab_exoplanet. html). SPACE.com. Retrieved 2008-06-20.

• Hazel Muir (April 25, 2007). "'Goldilocks' planet may be just right for life" (http:/ / space. newscientist. com/article/ dn11710). NewScientistSpace.

• "Astronomers find first habitable Earth-like planet" (http:/ / www. scientificblogging. com/ news/astronomers_find_first_habitable_earth_like_planet). Scientificblogging.com. April 24, 2007.

• "Found 20 light years away:the new Earth" (http:/ / www. dailymail. co. uk/ pages/ live/ articles/ technology/technology. html?in_article_id=450467& in_page_id=1965). Daily Mail. April 26, 2007.

• Ian Sample (April 24, 2007). "'Second Earth' may mean we're not alone" (http:/ / www. hindu. com/ thehindu/holnus/ 008200704250305. htm). The Hindu.

• J. R. Minkle (April 24, 2007). "All Wet? Astronomers Claim Discovery of Earth-like Planet" (http:/ / www.sciam. com/ article. cfm?articleID=25A261F0-E7F2-99DF-313249A4883E6A86& chanID=sa007). ScientificAmerican.

• "Distant planet judged possibly habitable" (http:/ / www. world-science. net/ othernews/ 070425-habitable-planet.htm). World Science. April 23, 2007.

• ANI (April 23, 2007). "First habitable Earth like planet outside Solar System discovered" (http:/ / www.dailyindia. com/ show/ 135806. php/ First-habitable-Earth-like-planet-outside-Solar-System-discovered).DailyIndia.com.

Gliese 581 c 21

Non-news media• "Artist conceptions of extrasolar planet Gliese 581 c" (http:/ / cosmographica. com/ gallery/ extrasolar/

Gliese581c/ index. html). Cosmographica. Retrieved 2008-06-20.• "The Neighbor: Gliese 581c" (http:/ / www. geochemsoc. org/ publications/ geochemicalnews/ gn131apr07/

theneighborgliese581c. htm). The Geochemical Society. Retrieved 2007-12-06.• "Red, Willing, and Able: 2001 New Scientist article on types of planets likely to be around red dwarf stars" (http:/

/ www. kencroswell. com/ reddwarflife. html). KenCroswell.com. Retrieved 2008-06-20.• "Sunrise from the Surface of Gliese 581c" (http:/ / apod. nasa. gov/ apod/ ap070502. html). NASA. Astronomy

Picture of the Day. 2007-05-02. Retrieved 2008-06-20.

External links• Gliese 581 - The "Red Dwarf" and implications for its "earthlike" planet Gliese 581c (http:/ / members. misty.

com/ don/ g581. html)

Gliese 581 d 22

Gliese 581 d

Gliese 581 d

Extrasolar planet List of extrasolar planets

An artist's impression of Gliese 581 d and speculative moons.

Parent star

Star Gliese 581

Constellation Libra

Right ascension (α) 15h 19m 26s[1]

Declination (δ) −07° 43′ 20″[1]

Apparent magnitude (mV) 10.5[1]

Distance 20.3 ± 0.3 ly(6.2 ± 0.1 pc)

Spectral type M3V

Mass (m) 0.31 M☉

Radius (r) 0.29 R☉

Temperature (T) 3480 ± 48 K

Metallicity [Fe/H] -0.33 ± 0.12

Age 7 – 11 Gyr

Orbital elementsEpoch JD 2451409.762[2]

Semimajor axis (a) 0.21847 ± 0.00028[2] AU

Eccentricity (e) 0[2]

Orbital period (P) 66.87 ± 0.13[2] d(0.183 y)

(1600 h)

Mean anomaly (M) 56 ± 27[2] °

Semi-amplitude (K) 1.91 ± 0.22[2] m/s

Physical characteristics

Minimum mass (m sin i) 5.6 ± 0.6[2] M⊕

Discovery information

Discovery date 24 April 2007

Gliese 581 d 23

Discoverer(s) Udry et al.

Detection method Radial velocity

Discovery site La Silla Observatory, Chile

Discovery status Published[3]

Other designations

HO Librae d, HO Lib d, BD−07°4003 d, GJ 581 d, HIP 74995 d, LFT 1195 d, LHS 394 d, LPM 564 d, LTT 6112 d, NLTT 39886 d,TYC 5594-1093-1 d, Wolf 562 d.

Database references

Extrasolar PlanetsEncyclopaedia

data [4]

SIMBAD data [5]

Gliese 581 d (pronounced /ˈɡliːzə/) or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in orderfrom the star.Because of its mass, at least 5.6 times that of Earth,[2] the planet is classified as a super-Earth. Originally believed tobe outside the habitable zone, in late April 2009 new observations made by the original discovery team concludedthat the planet is on the outskirts of the habitable zone where liquid water may exist. In May 2011 researchers inFrance released a study of a three-dimensional climate simulation concluding that it is plausible that the planet has astable atmosphere and liquid water on the surface, concurring that it is the first discovered terrestrial-mass exoplanetin the habitable zone[6] , followed by HD 85512 b several months later.

DiscoveryA team of astronomers led by Stéphane Udry of the Geneva Observatory used the HARPS instrument on theEuropean Southern Observatory 3.6 meter telescope in La Silla, Chile to discover the planet in 2007. Udry's teamemployed the radial velocity technique, in which the mass of a planet is determined based on the small perturbationsit induces in its parent star’s orbit via gravity.[3]

Physical characteristicsThe motion of the parent star indicates a minimum mass for Gliese 581 d of 5.6 Earth masses (earlier analyses gavehigher values).[2] Dynamical simulations of the Gliese 581 system assuming that the orbits of the three planets arecoplanar show that the system becomes unstable if the masses of the planets exceed 1.6 – 2 times the minimumvalues. Using earlier minimum mass values for Gliese 581 d, this implies an upper mass limit for Gliese 581 d of13.8 Earth masses.[7]

Climate and habitabilityIt was originally thought that Gliese 581 d orbits outside the habitable zone of its star. However, in 2009 the originaldiscovery team revised its original estimate of the planet's orbital parameters, finding that it orbits closer to its starthan originally believed. They concluded that the planet is within the habitable zone where liquid water couldexist,[7] [8] corroborated in 2011 by a climate simulation study.[6] According to Stéphane Udry, "It could be coveredby a 'large and deep ocean'; it is the first serious ocean planet candidate."[9]

On average, the light that Gliese 581 d receives from its star has about 30% of the intensity of sunlight on Earth. By comparison, sunlight on Mars has about 40% of the intensity of that on Earth. That might seem to suggest that Gliese 581 d is too cold to support liquid water and hence is inhospitable to life. However, an atmospheric greenhouse

Gliese 581 d 24

effect can significantly raise planetary temperatures. For example, Earth's own mean temperature would be about−18°C[10] without any greenhouse gases, ranging from around 100°C on the day side to −150°C at night, much likethat found on the moon. If the atmosphere of Gliese 581 d produces a sufficiently large greenhouse effect, and theplanet's geophysics stabilizes the CO2 levels (as Earth's does via plate tectonics), then the surface temperature mightpermit a liquid water cycle, conceivably allowing the planet to support life.[11] [12] [13] [14] Calculations by Barnes etal. suggest, however, that tidal heating is too low to keep plate tectonics active on the planet, unless radiogenicheating is somewhat higher than expected.[15] In recent years, discoveries have led scientists to believe that Gliese581d may be tidally locked, which means that it has permanent day and night side.[16]

Gliese 581 d is probably too massive to be made only of rocky material. It may have originally formed on a moredistant orbit as an icy planet that then migrated closer to its star.[17] [18]

Messages from Earth

Another artist impression of Gliese 581 d as aSuper-Earth.

In October 2008, members of the networking website Bebo beamed AMessage From Earth, a high-power transmission at Gliese 581, usingthe RT-70 radio telescope belonging to the National Space Agency ofUkraine. This transmission is due to arrive in the Gliese 581 system'svicinity by the year 2029; the earliest possible arrival for a response,should there be one, would be in 2049.[19]

As part of the 2009 National Science Week celebrations in Australia,Cosmos Magazine launched a website called Hello From Earth tocollect messages for transmission to Gliese 581d. The maximum length

of the messages was 160 characters, and they were restricted to the English language. In total, 25,880 messages werecollected from 195 countries around the world. The messages were transmitted from the DSS-43 70 m radiotelescope at the Canberra Deep Space Communication Complex at Tidbinbilla, Australia on the 28th of August,2009.[20]

References[1] "Gliese 581 (V* HO Lib -- Variable of BY Dra type)" (http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=GJ+ 581). SIMBAD Astronomical

Database. . Retrieved 2010-04-10.[2] Vogt, S. S.; et al. (2010). "The Lick-Carnegie Exoplanet Survey: A 3.1 M⊕ Planet in the Habitable Zone of the Nearby M3V Star Gliese

581". arXiv:1009.5733 [astro-ph.EP].[3] "The HARPS search for southern extra-solar planets" (http:/ / exoplanet. eu/ papers/ udry_terre_HARPS-1. pdf), S. Udry. X. Bonfils. X.

Delfosse. T. Forveille. M. Mayor. C. Perrier. F. Bouchy. C. Lovis. F. Pepe. D. Queloz. J.-L. Bertaux. The Extrasolar Planets Encyclopaedia.April 4, 2007. Accessed June 10, 2011

[4] http:/ / exoplanet. eu/ planet. php?p1=Gl+ 581& p2=d[5] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Gl+ 581+ d[6] Wordsworth, R.; et al. (2011). "Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone".

arXiv:1105.1031 [astro-ph.EP].[7] M. Mayor, X. Bonfils, T. Forveille, X. Delfosse, S. Udry, J.-L. Bertaux, H. Beust, F. Bouchy, C. Lovis, F. Pepe, C. Perrier, D. Queloz, N. C.

Santos (2009). "The HARPS search for southern extra-solar planets,XVIII. An Earth-mass planet in the GJ 581 planetary system".arXiv:0906.2780 [astro-ph].

[8] "Lightest exoplanet yet discovered" (http:/ / www. eso. org/ public/ outreach/ press-rel/ pr-2009/ pr-15-09. html). eso.org. 2009-04-21. .Retrieved 2009-04-21.

[9] "Lightest exoplanet yet discovered" (http:/ / www. eso. org/ public/ news/ eso0915/ ), European Southern Observatory. April 21, 2009.Accessed June 10, 2011

[10] "Global Warming Frequently Asked Questions" (http:/ / lwf. ncdc. noaa. gov/ oa/ climate/ globalwarming. html#q1). Lwf.ncdc.noaa.gov.2008-05-08. . Retrieved 2009-01-18.

[11] von Bloh, W. et al. (2008). "Habitability of Super-Earths: Gliese 581c and 581d". Proceedings of the International Astronomical Union 3.arXiv:0712.3219. doi:10.1017/S1743921308017031.

Gliese 581 d 25

[12] "Centauri Dreams » Blog Archive » Gliese 581d: A Habitable World After All?" (http:/ / www. centauri-dreams. org/ ?p=1625).Centauri-dreams.org. 2007-12-13. . Retrieved 2009-01-18.

[13] Posted at 12:06 AM in Space Exploration (2007-06-15). "New 'Super Earth 2' Discovered in Constellation Libra" (http:/ / www. dailygalaxy.com/ my_weblog/ 2007/ 06/ another_super_e. html). Dailygalaxy.com. . Retrieved 2009-01-18.

[14] Wordsworth, Robin; François Forget; Franck Selsis; Ehouarn Millour; Benjamin Charnay; Jean-Baptiste Madeleine (1970). "Gliese 581d isthe first discovered terrestrial-mass exoplanet in the habitable zone". arXiv:1105.1031 [astro-ph.EP].

[15] Barnes, Rory; Jackson, Brian; Greenberg, Richard; Raymond, Sean N. (2009-06-09). "Tidal Limits to Planetary Habitability". TheAstrophysical Journal 700: L30–L33. arXiv:0906.1785. Bibcode 2009ApJ...700L..30B. doi:10.1088/0004-637X/700/1/L30.

[16] "Hot topics " Deep Space " Alien Life " Gliese 581d: A Habitable Exoplanet? : Gliese 581d: A Habitable Exoplanet?" (http:/ / www.astrobio. net/ pressrelease/ 3980/ gliese-581d-a-habitable-exoplanet). Astrobiology Magazine. 2011-05-20. . Retrieved 2011-05-21.

[17] SPACE.com - Hopes Dashed for Life on Distant Planet (http:/ / www. space. com/ scienceastronomy/ 070618_mm_gliese_581d. html)[18] von Bloh, W.; Bounama, C.; Cuntz, M.; Franck, S. (2007). "The Habitability of Super-Earths in Gliese 581". Astronomy & Astrophysics 476

(3): 1365–1371. Bibcode 2007A&A...476.1365V. doi:10.1051/0004-6361:20077939.[19] "Zimbio Pilot - Gliese 581c" (http:/ / www. zimbio. com/ pilot?ID=bwaIrlOhfhm& ZURL=/ Gliese+ 581c/ news& URL=http:/ / news.

google. com/ news/ url?sa=T& ct=us/ 3-0& fd=R& url=http:/ / blogs. discovermagazine. com/ discoblog/ 2008/ 10/ 13/britney-pics-beamed-into-space-thus-far-aliens-remain-silent/ & cid=0& ei=Z58MSeCgKpLmyASDtuz3AQ&usg=AFQjCNFY_rY3wSipj2eD2pMLrI_lbcEWAg). Zimbio.com. 2008-10-13. . Retrieved 2009-04-23.

[20] Jenkins, Simon (2009-08-28). "Earth sends 25,000 hellos to outer space" (http:/ / news. brisbanetimes. com. au/ breaking-news-national/earth-sends-25000-hellos-to-outer-space-20090828-f284. html). Brisbane Times. . Retrieved 2009-09-16.

External links• National Science Week 2009 - Send A Message To Gliese 581d (http:/ / www. hellofromearth. net)• Wordsworth, R. D. et al. (2010). "Is Gliese 581d habitable? Some constraints from radiative-convective climate

modeling". Astronomy & Astrophysics 522: A22. arXiv:1005.5098. Bibcode 2010A&A...522A..22W.doi:10.1051/0004-6361/201015053.

Gliese 581 g 26

Gliese 581 g

Gliese 581 g

Extrasolar planet List of extrasolar planets

Artist's impression of Gliese 581 g orbiting its parent star.

Parent star

Star Gliese 581

Constellation Libra

Right ascension (α) 15h 19m 26s

Declination (δ) −07° 43′ 20″

Apparent magnitude (mV) 10.55

Distance 20.3 ± 0.3 ly(6.2 ± 0.1 pc)

Spectral type M3V

Mass (m) 0.31 M☉

Radius (r) 0.29 R☉

Temperature (T) 3480 ± 48 K

Metallicity [Fe/H] −0.33 ± 0.12

Age 7 – 11 Gyr

Orbital elementsEpoch JD 2451409.762[1]

Semimajor axis (a) 0.14601 ± 0.00014[1] AU

Eccentricity (e) 0[1]

Orbital period (P) 36.562 ± 0.052[1] d(0.100 y)

(877 h)

Mean anomaly (M) 271 ± 48[1] °

Semi-amplitude (K) 1.29 ± 0.19[1] m/s

Gliese 581 g 27

Physical characteristics

Minimum mass (m sin i) 3.1 ± 0.4[1] M⊕

Discovery information

Discovery date September 29, 2010

Discoverer(s) Steven S. Vogt et al.

Detection method Radial Velocity

Discovery site Keck Observatory, Hawaii

Discovery status May be "unlikely" to exist according to one recent study.[2]

Database references

Extrasolar PlanetsEncyclopaedia

data [3]

SIMBAD data [4]

Gliese 581 g (pronounced /ˈɡliːzə/), also Gl 581 g or GJ 581 g, is an extrasolar planet of the red dwarf star Gliese581, 20.5 light-years from Earth in the constellation of Libra.[5] It is the sixth planet discovered in the Gliese 581planetary system and the fourth in order of increasing distance from the star. The discovery was announced by theLick-Carnegie Exoplanet Survey in late September 2010, after a decade of observation. However, according to onerecent study the planet may be "unlikely" to exist.[2]

Gliese 581 g has attracted attention because, if it exists, it is near the middle of the habitable zone of its parent star.That means it could sustain liquid water on its surface and could potentially host life similar to that on Earth. (Theplanet is expected to be around 20 Fahrenheit degrees cooler than Earth, however). If it is a rocky planet, favorableatmospheric conditions could permit the presence of liquid water, a necessity for all known life, on its surface. Witha mass 3.1 to 4.3 times Earth's, Gliese 581 g is considered a super-Earth and was the planet closest in size to Earthknown in a habitable zone until the discovery of Kepler-22b (600 light years away). This makes it one of the mostEarth-like Goldilocks planet found outside the Solar System and the exoplanet with one of the greatest recognizedpotential for harboring life from a distance in close proximity to Earth (20 light years away).[6]

The detection of Gliese 581 g after such a short period of searching and at such close proximity has led someastronomers to hypothesize that the proportion of stars with habitable planets may be greater than ten percent.[1]

Gliese 581 g 28

Discovery

The six-planet model of the Gliese 581 planetarysystem with circular orbits.

The planet was detected by astronomers in the Lick-CarnegieExoplanet Survey, led by principal investigator Steven S. Vogt,professor of astronomy and astrophysics at the University ofCalifornia, Santa Cruz and co-investigator R. Paul Butler of theCarnegie Institution of Washington. The discovery was made usingradial velocity measurements, combining 122 observations obtainedover 11 years from the High Resolution Echelle Spectrometer (HIRES)instrument of the Keck 1 telescope with 119 measurements obtainedover 4.3 years from the High Accuracy Radial Velocity PlanetSearcher (HARPS) instrument of the European Southern Observatory's3.6 m telescope at La Silla Observatory.[1] [7]

After subtracting the signals of the previously known Gliese 581planets, b, c, d and e, the signals of two additional planets wereapparent: a 445-day signal from a newly recognized outermost planetdesignated f, and the 37-day signal from Gliese 581 g.[1] [8] Theprobability that the detection of the latter was spurious was estimatedat only 2.7 in a million.[1] The authors stated that while the 37-daysignal is "clearly visible in the HIRES data set alone", "the HARPSdata set alone is not able to reliably sense this planet" and concluded,"It is really necessary to combine both data sets to sense all theseplanets reliably."[1] The Lick–Carnegie team explained the results oftheir research in a paper published in the Astrophysical Journal.Although not sanctioned by the IAU's naming conventions, Vogt'steam informally refers to the planet as "Zarmina's World" after hiswife,[9] and some cases simply as Zarmina.

Nondetection in new HARPS data analysis

Two weeks after the announcement of the discovery of Gliese 581 g, astronomer Francesco Pepe of the GenevaObservatory reported that in a new analysis of 179 measurements taken by the HARPS spectrograph over 6.5 years,neither planet g nor planet f was detectable.[10] [11] Vogt responded to the latest concerns by saying, "I am not overlysurprised by this as these are very weak signals, and adding 60 points onto 119 does not necessarily translate to biggains in sensitivity." He cautioned that not finding the planet in this study does not make a strong case for it notexisting, because both data sets may be needed to detect it.[12] More recently, Vogt added, "I feel confident that wehave accurately and honestly reported our uncertainties and done a thorough and responsible job extracting whatinformation this data set has to offer. I feel confident that anyone independently analyzing this data set will come tothe same conclusions."[13]

Differences in the two groups' results may involve the planetary orbital characteristics assumed in calculations. According to MIT astronomer Sara Seager, Vogt postulated the planets around Gliese 581 had perfectly circular orbits whereas the Swiss group thought the orbits were more eccentric.[14] This difference in approach may be the reason for the disagreement, according to Alan Boss.[14] Butler remarked that with additional observations, "I would expect that on the time scale of a year or two this should be settled."[10] Other astronomers also supported a deliberate evaluation: Seager stated, "We will have consensus at some point; I don't think we need to vote right now." and Ray Jayawardhana noted, "Given the extremely interesting implications of such a discovery, it's important to have independent confirmation."[14] Gliese 581 g is listed as "unconfirmed" in the Extrasolar Planets

Gliese 581 g 29

Encyclopaedia.[15]

Erroneous data analysisIn December 2010, a methodological error has been revealed in the data analysis that led to the "discovery" of Gliese581 f and g.[16] The team around Steven Vogt inferred the number of exoplanets by using a reduced chi-square,although the orbital models are nonlinear in the model parameters. Therefore, reduced chi-square is not a trustworthydiagnostic. In fact, an investigation of the fit residuals showed that the data used by Vogt's team actually prefers amodel with four planets, not six, in agreement with the results of Francesco Pepe's team.

Further analyses of HIRES/HARPS dataAnother re-analysis found no clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set.[17]

The claim was made that the HARPS data only provided some evidence for 5 planet signals, while incorporation ofboth data sets actually degraded the evidence for more than four planets (i.e., none for 581 f or 581 g)."I have studied [the paper] in detail and do not agree with his conclusions,"[18] Steven Vogt said in reply, concernedthat Gregory has considered the HIRES data as more uncertain.[19] The question of Gliese 581g's existence won't besettled definitively until researchers gather more high-precision radial velocity data, Vogt said.By performing a number of statistical tests, Guillen Anglada-Escude of the Carnegie Institute of Washingtonconcluded that the existence of Gl 581 g was well supported by the available data. This despite the presence of astatistical degeneracy that derives from an alias of the first eccentric harmonic of another planet in the system.[20] Ina forthcoming paper, Anglada-Escude and Rebekah Dawson claim “With the data we have, the most likelyexplanation is that this planet is still there.”[21]

Mikko Tuomi of the University of Turku performed a Bayesian re-analysis of the HARPS and HIRES data with theresult that they "do not imply the conclusion that there are two additional companions orbiting GJ 581".[22]

Physical characteristicsGliese 581 g has an orbital period of 37 days, orbiting at a distance of 0.146 AU from its parent star.[1] It is believedto have a mass of 3.1 to 4.3 times Earth's and a radius of 1.3 to 2.0 times Earth's (1.3 to 1.5 times larger ifpredominantly rocky, 1.7 to 2.0 times larger if predominantly water ice). Its mass indicates it is probably a rockyplanet with a solid surface. The planet's surface gravity is expected to be in the range of 1.1 to 1.7 times Earth's,enough to hold on to an atmosphere likely to be denser than Earth's.[1]

Tidal locking and habitabilityBecause of Gliese 581 g's proximity to its parent star, it is predicted to be tidally locked to Gliese 581. Just as Earth'sMoon always presents the same face to the Earth, the length of Gliese 581 g's sidereal day would then preciselymatch the length of its year, meaning it would be permanently light on one half and permanently dark on the otherhalf of its surface.[1] [23] Tidal locking also means the planet would have no axial tilt and therefore no seasonality ina conventional sense.With one side of the planet always facing the star, temperatures could range from blazing hot in the bright side tofreezing cold in the dark side if atmospheric heat transport is limited. The atmosphere's inventory of volatilecompounds such as water and carbon dioxide could then permanently freeze out on the dark side. However, anatmosphere of the expected density would be likely to moderate these extremes.

Gliese 581 g 30

Atmospheric effects

Planetary orbits in the Gliese 581 systemcompared to those of our own Solar System ("g"

designates Gliese 581g)

An atmosphere that is dense will circulate heat, potentially allowing awide area on the surface to be habitable.[24] For example, Venus has asolar rotation rate approximately 117 times slower than Earth's,producing prolonged days and nights. Despite the uneven distributionof sunlight over time intervals shorter than several months,unilluminated areas of Venus are kept almost as hot as the day side byglobally circulating winds.[25] Simulations have shown that anatmosphere containing appropriate levels of greenhouse CO2 and H2Oneed only be a tenth the pressure of Earth's atmosphere (100 mb) toeffectively distribute heat to the night side.[26] Current technologycannot determine the atmospheric or surface composition of the planetdue to the overpowering light of its parent star.[27]

Whether or not a tide-locked planet with the orbital characteristics of Gliese 581g is actually habitable depends onthe composition of the atmosphere and the nature of the planetary surface. A comprehensive modeling study [28]

including atmospheric dynamics, realistic radiative transfer and the physics of formation of sea ice (if the planet hasan ocean) indicates that the planet can become as hot as Venus if it is dry and allows carbon dioxide to accumulate inits atmosphere. The same study identified two habitable states for a water-rich planet. If the planet has a very thinatmosphere, a thick ice crust forms over most of the surface, but the substellar point remains hot enough to yield aregion of thin ice or even episodically open water. If the planet has an atmosphere with Earthlike pressures,containing approximately 20% (molar) carbon dioxide, then the greenhouse effect is sufficiently strong to maintain apool of open water under the substellar point with temperatures comparable to the Earth's tropics. This state has beendubbed "Eyeball Earth" by the author.Modeling of the effect of tidal locking on Gliese 581 g's possible atmosphere, using a general circulation modelemploying an atmosphere with Earthlike surface pressure but a highly idealized representation of radiative processes,indicates that for a solid-surface planet the locations of maximum warmth would be distributed in a sidewayschevron-shaped pattern centered near the substellar point.[29] [30]

TemperaturesIt is estimated that the average global equilibrium temperature (the temperature in the absence of atmosphericeffects) of Gliese 581 g ranges from 209 to 228 K (−64 to −45 °C, or −84 to −49 °F) for Bond albedos (reflectivities)from 0.5 to 0.3 (with the latter being more characteristic of the inner Solar System). Adding an Earth-like greenhouseeffect yields an average surface temperature in the range of 236 to 261 K (−37 to −12 °C, or −35 to 10 °F).[1] [31]

Gliese 581g is in an orbit where a silicate weathering thermostat can operate, and this can lead to accumulation ofsufficient carbon dioxide in the atmosphere to permit liquid water to exist at the surface, provided the planet'scomposition and tectonic behavior can support sustained outgassing.[28]

Gliese 581 g 31

Temperaturecomparisons

Venus Earth Gliese 581 g Mars

Globalequilibriumtemperature

307 K34 °C93 °F

255 K−18 °C−0.4 °F

209 K to 228 K−64 °C to −45°C−83 °F to −49 °F

206 K−67 °C−88.6 °F

+ Venus'GHG effect

737 K464 °C867 °F

+ Earth'sGHG effect

288 K15 °C59 °F

236 K to 261 K−37 °C to −12°C−35 °F to 10 °F

+ Mars'GHG effect

210 K−63 °C−81 °F

Tidallylocked

Almost No Probably No

GlobalBond Albedo

0.9 0.29 0.5 to 0.3 0.25

Refs.[1] [31] [32] [33] [34]

By comparison, Earth's present global equilibrium temperature is 255 K (−18 °C), which is raised to 288 K (15 °C)by greenhouse effects. However, when life evolved early in Earth's history, the Sun's energy output is thought tohave been only about 75% of its current value,[35] which would have correspondingly lowered Earth's equilibriumtemperature under the same albedo conditions. Yet Earth maintained equable temperatures in that era, perhaps with amore intense greenhouse effect,[36] or a lower albedo,[37] than at present.Current Martian surface temperatures vary from lows of about −87 °C (−125 °F) during polar winter to highs of upto −5 °C (23 °F) in summer.[32] The wide range is due to the rarefied atmosphere, which cannot store much solarheat, and the low thermal inertia of the soil.[38] Early in its history, a denser atmosphere may have permitted theformation of an ocean on Mars.[39]

Two previously discovered planets in the same system, Gliese 581 c and d (inward and outward from planet g,respectively), were also regarded as potentially habitable following their discovery.[40] Both were later evaluated asbeing outside the conservatively defined habitable zone, leading Vogt et al. to remark that "The GJ 581 system has asomewhat checkered history of habitable planet claims".[1] However, a subsequent downward revision of the periodof planet d from 83 to 67 days has bolstered its habitability prospects, although a large greenhouse effect would beneeded.[1]

Potential for lifeFurther information: Planetary habitabilityIn an interview with Lisa-Joy Zgorski of the National Science Foundation, Steven Vogt was asked what he thought about the chances of life existing on Gliese 581 g. Vogt was optimistic: "I'm not a biologist, nor do I want to play one on TV. Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that, my own personal feeling is that the chances of life on this planet are 100%. I have almost no doubt about it."[41] In the same article Dr. Seager is quoted as saying "Everyone is so primed to say here's the next place we’re going to find life, but this isn’t a good planet for follow-up."[41] According to Vogt, the long lifetime of red dwarfs improves the chances of life being present. "It's pretty hard to stop life once you give it the right conditions", he said.[42] "Life on

Gliese 581 g 32

other planets doesn't mean E.T. Even a simple single-cell bacteria or the equivalent of shower mold would shakeperceptions about the uniqueness of life on Earth."[42]

Implications

Keck telescope at twilight

Scientists have monitored only a relatively small number of stars in thesearch for exoplanets. The discovery of a potentially habitable planetlike Gliese 581 g so early in the search might mean that habitableplanets are more widely distributed than had been previously believed.According to Vogt, the discovery "implies an interesting lower limit onthe fraction of stars that have at least one potentially habitable planet asthere are only ~116 known solar-type or later stars out to the 6.3 parsecdistance of Gliese 581."[43] This finding foreshadows what Vogt calls anew, second Age of Discovery in exoplanetology:[44]

Confirmation by other teams through additionalhigh-precision RVs would be most welcome. But if GJ 581g is confirmed by further RV scrutiny, themere fact that a habitable planet has been detected this soon, around such a nearby star, suggests that η⊕could well be on the order of a few tens of percent, and thus that either we have just been incrediblylucky in this early detection, or we are truly on the threshold of a second Age of Discovery.[1]

If the fraction of stars with potentially habitable planets (η⊕, "eta-Earth") is on the order of a few tens of percent asVogt proposes, and the Sun's stellar neighborhood is a typical sample of the galaxy, then the discovery of Gliese 581g in the habitable zone of its star points to the potential of billions of Earth-like planets in our Milky Way galaxyalone.[45]

Notes and references[1] Vogt, Steven S.; Butler, R. Paul; Rivera, Eugenio J.; Haghighipour, Nader; Henry, Gregory W.; Williamson, Michael H. (2010-09-29). "The

Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581".arXiv:1009.5733 [astro-ph.EP].

[2] Forveille, T.; Bonfils, X.; Delfosse, X.; Alonso, R.; Udry, S.; Bouchy, F.; Gillon, M.; Lovis, C.; Neves, V.; Mayor, M.; Pepe, F.; Queloz, D.;Santos, N. C.; Segransan, D.; Almenara, J. M.; eeg, H.; Rabus. M. (2011-09-12). "The HARPS search for southern extra-solar planets XXXII.Only 4 planets in the Gl~581 system". arXiv:1109.2505v1 [astro-ph.EP]. "...Our dataset therefore has strong diagnostic power for planets withthe parameters of Gl 581f and Gl 581g, and we conclude that the Gl 581 system is unlikely to contain planets with those characteristics..."(http:/ / arxiv. org/ PS_cache/ arxiv/ pdf/ 1109/ 1109. 2505v1. pdf)

[3] http:/ / exoplanet. eu/ planet. php?p1=Gl+ 581& p2=g[4] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Gl+ 581+ g[5] Times of India website (http:/ / timesofindia. indiatimes. com/ home/ science/ Found-Planet-thats-just-like-Earth/ articleshow/ 10899740.

cms)[6] The planet is much too far away to reach with today's technology: at the speed of the space shuttle, "it would take 766,000 years to get to

Gliese 581 g—that's more than three times longer than homo sapiens have been around." - Palmer, Brian (2010-09-30). "Gliese 581 g orBust!" (http:/ / www. slate. com/ id/ 2269244). Slate, Washingtonpost Newsweek Interactive. . Retrieved 2010-10-08.

[7] Alleyne, Richard (2010-09-30). "Gliese 581g the most Earth like planet yet discovered" (http:/ / www. telegraph. co. uk/ science/ space/8033124/ Gliese-581g-the-most-Earth-like-planet-yet-discovered. html). The Daily Telegraph. . Retrieved 2010-09-30.

[8] Alexander, Amir (2010-10-06). "Billions and Billions? Discovery of Habitable Planet Suggests Many More are Out There" (http:/ / www.planetary. org/ news/ 2010/ 1006_Billions_and_Billions_Discovery_of. html). The Planetary Society web site (http:/ / www. planetary. org/home/ ). The Planetary Society. . Retrieved 2010-10-08.

[9] Meichsner, Von Irene (2010-09-30). "Erdähnlicher Planet entdeckt" (http:/ / www. ksta. de/ html/ artikel/ 1284751479322. shtml). KölnerStadt-Anzeiger. . Retrieved 2010-10-05.

[10] Kerr, Richard A. (2010-10-12). "Recently Discovered Habitable World May Not Exist" (http:/ / news. sciencemag. org/ sciencenow/ 2010/10/ recently-discovered-habitable-world. html). Science Now. AAAS. . Retrieved 2010-10-12.

[11] Mullen, Leslie (2010-10-12). "Doubt Cast on Existence of Habitable Alien World" (http:/ / www. astrobio. net/ exclusive/ 3647/doubt-cast-on-existence-of-habitable-alien-world). Astrobiology Magazine. . Retrieved 2010-10-12.

Gliese 581 g 33

[12] Grossman, Lisa (2010-10-12). "Exoplanet Wars: "First Habitable World" May Not Exist" (http:/ / www. wired. com/ wiredscience/ 2010/10/ gliese581g-may-not-exist). Wired. . Retrieved 2010-10-12.

[13] Wall, Mike (2010-10-13). "Astronomer Stands By Discovery of Alien Planet Gliese 581g Amid Doubts" (http:/ / www. space. com/scienceastronomy/ gliese-581g-discoverer-responds-101013. html). Space.com. . Retrieved 2010-10-13.

[14] Cowen, Ron (2010-10-13). "Swiss team fails to confirm recent discovery of an extrasolar planet that might have right conditions for life"(http:/ / www. sciencenews. org/ view/ generic/ id/ 64308/ title/ Existence_of_habitable_exoplanet_questioned). Science News. . Retrieved2010-10-13.

[15] "Notes for star Gl 581" (http:/ / exoplanet. eu/ star. php?st=Gl+ 581). The Extrasolar Planets Encyclopaedia. . Retrieved 2010-10-11.[16] Rene Andrae; Tim Schulze-Hartung; Peter Melchior (2010). "Dos and don'ts of reduced chi-squared". arXiv:1012.3754 [astro-ph.IM].[17] Gregory (2011). "Bayesian Re-analysis of the Gliese 581 Exoplanet System". arXiv:1101.0800 [astro-ph.SR].[18] http:/ / www. cbc. ca/ technology/ story/ 2011/ 01/ 14/ habitable-planet-gliese-vogt-gregory. html "Habitable planet find doubted by B.C.

scientist."[19] http:/ / www. space. com/ 10897-alien-planet-gliese-581g-great-debate. html[20] Guillem Anglada-Escudé (2010). "Aliases of the first eccentric harmonic : Is GJ 581g a genuine planet candidate?".

arXiv:1011.0186 [astro-ph.EP].[21] http:/ / www. wired. com/ wiredscience/ 2011/ 01/ gliese-581g-questioned/[22] Mikko Tuomi (2011). "Bayesian re-analysis of the radial velocities of Gliese 581. Evidence in favour of only four planetary companions".

arXiv:1102.3314 [astro-ph.EP].[23] Berardelli, Phil (2010-09-29). "Astronomers Find Most Earth-like Planet to Date" (http:/ / news. sciencemag. org/ sciencenow/ 2010/ 09/

astronomers-find-most-earth-like. html). ScienceNOW. . Retrieved 2010-09-30.[24] Alpert, Mark (2005-11-07). "Red Star Rising" (http:/ / www. sciam. com/ article. cfm?chanID=sa004&

articleID=000CC344-B043-1353-AF3383414B7FFE9F). Scientific American. . Retrieved 2007–04–25.[25] Ralph D Lorenz, Jonathan I Lunine, Paul G Withers, Christopher P. McKay (2001). "Titan, Mars and Earth: Entropy Production by

Latitudinal Heat Transport" (http:/ / sirius. bu. edu/ withers/ pppp/ pdf/ mepgrl2001. pdf) (PDF). Ames Research Center, University of ArizonaLunar and Planetary Laboratory. . Retrieved 2007-08-21.

[26] Joshi, M. M.; Haberle, R. M.; Reynolds, R. T. (October 1997). "Simulations of the Atmospheres of Synchronously Rotating TerrestrialPlanets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability". Icarus 129 (2): 450–465.Bibcode 1997Icar..129..450J. doi:10.1006/icar.1997.5793.

[27] Shiga, David (2010-09-29). "Found: first rocky exoplanet that could host life" (http:/ / www. newscientist. com/ article/dn19519-found-first-rocky-exoplanet-that-could-host-life. html). New Scientist. . Retrieved 2010-09-30.

[28] Pierrehumbert, R. T. (2010-12-27). "A Palette of climates for Gliese 581g" (http:/ / geosci. uchicago. edu/ ~rtp1/ papers/Gliese581gPalletesApJL2011. pdf). Astrophysical Journal Letters (IOP) 726 (1): L8. Bibcode 2011ApJ...726L...8P.doi:10.1088/2041-8205/726/1/L8. . Retrieved 2010-12-27.

[29] Heng, Kevin; Vogt, Steven S. (2010-10-25). "Gliese 581g as a scaled-up version of Earth: atmospheric circulation simulations".arXiv:1010.4719 [astro-ph.EP].

[30] Grossman, Lisa (2010-11-01). "Climate Model Suggests Where the Aliens Are" (http:/ / www. wired. com/ wiredscience/ 2010/ 11/gliese-581g-climate/ ). Wired News (http:/ / www. wired. com/ ). Condé Nast Publications. . Retrieved 2010-11-03.

[31] Stephens, Tim (2010-09-29). "Newly discovered planet may be first truly habitable exoplanet" (http:/ / news. ucsc. edu/ 2010/ 09/ planet.html). University News & Events. University of California, Santa Cruz. .

[32] "NASA, Mars: Facts & Figures" (http:/ / solarsystem. jpl. nasa. gov/ planets/ profile. cfm?Object=Mars& Display=Facts). . Retrieved2010-01-28.

[33] Mallama, A.; Wang, D.; Howard, R.A. (2006). "Venus phase function and forward scattering from H2SO4". Icarus 182 (1): 10–22.Bibcode 2006Icar..182...10M. doi:10.1016/j.icarus.2005.12.014.

[34] Mallama, A. (2007). "The magnitude and albedo of Mars". Icarus 192 (2): 404–416. Bibcode 2007Icar..192..404M.doi:10.1016/j.icarus.2007.07.011.

[35] Sagan, C.; Mullen, G. (1972). "Earth and Mars: Evolution of Atmospheres and Surface Temperatures". Science 177 (4043): 52–56.Bibcode 1972Sci...177...52S. doi:10.1126/science.177.4043.52. PMID 17756316.

[36] Pavlov, Alexander A.; Kasting, James F.; Brown, Lisa L.; Rages, Kathy A.; Freedman, Richard (May 2000). "Greenhouse warming by CH4in the atmosphere of early Earth". Journal of Geophysical Research 105 (E5): 11981−11990. Bibcode 2000JGR...10511981P.doi:10.1029/1999JE001134.

[37] Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; Bjerrum, Christian J. (2010-04-01). "No climate paradox under the faint early Sun".Nature 464 (7289): 744–747. Bibcode 2010Natur.464..744R. doi:10.1038/nature08955. PMID 20360739.

[38] "Mars' desert surface..." (http:/ / www-mgcm. arc. nasa. gov/ mgcm/ HTML/ WEATHER/ surface. html). MGCM Press release. NASA. .Retrieved 2007-02-25.

[39] Boyce, J. M.; Mouginis, P.; Garbeil, H. (2005). "Ancient oceans in the northern lowlands of Mars: Evidence from impact craterdepth/diameter relationships". Journal of Geophysical Research (American Geophysical Union) 110 (E03008): (15 pp.).Bibcode 2005JGRE..11003008B. doi:10.1029/2004JE002328.

[40] Udry, S.; Bonfils, X.; Delfosse, X.; Forveille, T.; Mayor, M.; Perrier, C.; Bouchy, F.; Lovis, C.; Pepe, F.; Queloz, D.; Bertaux, J.-L. (2007). "The HARPS search for southern extra-solar planets. XI. Super-Earths (5 and 8 M⊕) in a 3-planet system". Astronomy and Astrophysics 469

Gliese 581 g 34

(3): L43 – L47. Bibcode 2007A&A...469L..43U. doi:10.1051/0004-6361:20077612.[41] NSF. Press Release 10-172 - Video (http:/ / www. nsf. gov/ news/ news_videos. jsp?cntn_id=117765& media_id=68454& org=NSF). Event

occurs at 41:25-42:31. See Overbye, Dennis (2010-09-29). "New Planet May Be Able to Nurture Organisms" (http:/ / www. nytimes. com/2010/ 09/ 30/ science/ space/ 30planet. html?_r=1). The New York Times'. . Retrieved 2010-09-30.

[42] Borenstein, Seth (2010-09-30). "Could 'Goldilocks' planet be just right for life?" (http:/ / www. lankanewspapers. com/ news/ 2010/ 9/60768_space. html). Associated Press. . Retrieved 2010-10-20.

[43] Vogt 2010, pp.32-33. For more information, see Turnbull, Margaret C.; Tarter, Jill C. (March 2003). "Target Selection for SETI: 1. ACatalog of Nearby Habitable Stellar Systems". The Astrophysical Journal (Institute of Physics Publishing). arXiv:astro-ph/0210675.Bibcode 2003ApJS..145..181T. doi:10.1086/345779.

[44] NSF. Press Release 10-172 - Video. Event occurs at 17:00-17:46.[45] Vogt 2010, p.2. See Berardelli, Phil (2010-09-29). "Astronomers Find Most Earth-like Planet to Date" (http:/ / news. sciencemag. org/

sciencenow/ 2010/ 09/ astronomers-find-most-earth-like. html). AAAS. . Retrieved 2010-09-30.

External links• National Science Foundation (2010-09-29). "Steven Vogt and Paul Butler lead a team that discovered the first

potentially habitable exoplanet" (http:/ / www. nsf. gov/ news/ news_videos. jsp?cntn_id=117765&media_id=68454& org=NSF). "Video: Steven Vogt of UC Santa Cruz and UC Observatories and Paul Butler ofthe Carnegie Institution of Washington join NSF's Lisa-Joy Zgorski to announce the discovery of the firstexoplanet that has the potential to support life."

• "NASA and NSF-Funded Research Finds First Potentially Habitable Exoplanet" (http:/ / www. nasa. gov/ home/hqnews/ 2010/ sep/ HQ_10-237_Exoplanet_Findings. html). Release 10-237. NASA. 2010-09-29.

Gliese 876 b 35

Gliese 876 b

Gliese 876 b

Extrasolar planet List of extrasolar planets

An artist's concept of Gliese 876 b -- Cloudless but with water clouds at the poles --.

Parent star

Star Gliese 876

Constellation Aquarius

Right ascension (α) 22h 53m 16.73s

Declination (δ) −14° 15′ 49.3″

Apparentmagnitude

(mV) 10.15

Distance 15.3 ± 0.1 ly(4.70 ± 0.04 pc)

Spectral type M4V

Mass (m) 0.334 ± 0.030 M☉

Radius (r) 0.36 R☉

Temperature (T) 3350 ± 300 K

Metallicity [Fe/H] 0.05 ± 0.20

Age 0.1 – 5.0 Gyr

Orbital elementsEpoch HJD 2,450,602.093

Semimajor axis (a) 0.208317 ± 0.000020 [1] [2] AU

Eccentricity (e) 0.0324 ± 0.0013 [1]

Orbital period (P) 61.1166 ± 0.0086 [1] d

Inclination (i) 59 [1] °

Argument ofperiastron

(ω) 50.3 ± 3.2 [1] °

Mean anomaly (M) 325.7 ± 3.2 [1] °

Gliese 876 b 36

Semi-amplitude (K) 214.00 ± 0.42 [1] m/s

Physical characteristics

Mass (m) 2.2756 ± 0.0045 [1] MJ

Discovery information

Discovery date June 23, 1998

Discoverer(s) California and Carnegie Planet Search Team and independently by the Geneva Extrasolar PlanetSearch Team

Detection method Radial velocity

Discovery site Lick, Keck, Haute-Provence and La Silla Observatories [3] [4]

Discovery status Published

Database references

Extrasolar PlanetsEncyclopaedia

data [5]

SIMBAD data [6]

Gliese 876 b is an extrasolar planet orbiting the red dwarf star Gliese 876. It completes one orbit in approximately 61days. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf star.

DiscoveryGliese 876 b was discovered independently by two different teams, one led by Geoffrey Marcy (with data from KeckObservatory and Lick Observatory)[3] and the other by Xavier Delfosse (at Geneva Observatory).[4] Like themajority of known extrasolar planets, it was discovered by detecting variations in its star's radial velocity as a resultof the planet's gravity. This was done by making sensitive measurements of the Doppler shift of the spectral lines ofGliese 876. It was the first discovered of four known planets in the Gliese 876 system.[3] [4] [7] [8] [1]

Orbit and mass

The orbits of the planets of Gliese 876. Gliese876 b is the third planet from the star.

Gliese 876 b is in a 1:2:4 Laplace resonance with the inner planetGliese 876 c and the outer planet Gliese 876 e: in the time it takesplanet e to complete one orbit, planet b completes two and planet ccompletes four. This is the second known example of a Laplaceresonance, the first being Jupiter's moons Io, Europa and Ganymede. [1]

As a result, the orbital elements of the planets change fairly rapidly asthey dynamically interact with one another.[9] The planet's orbit has alow eccentricity, similar to the planets in our solar system. Thesemimajor axis of the orbit is only 0.208 AU, less than that of Mercuryin our solar system.[1] However Gliese 876 is such a faint star that thisputs it in the outer part of the habitable zone.[10]

A limitation of the radial velocity method used to detect Gliese 876 b isthat only a lower limit on the planet's mass can be obtained. This lowerlimit is around 1.93 times the mass of Jupiter.[8] The true mass dependson the inclination of the orbit, which in general is unknown. However

Gliese 876 b 37

because Gliese 876 is only 15 light years from earth Benedict et al. (2002) were able to use one of the Fine GuidanceSensors on the Hubble Space Telescope to detect the astrometric wobble created by Gliese 876 b.[11] This constitutedthe first unambiguous astrometric detection of an extrasolar planet.[1] Their analysis suggested that the orbitalinclination is 84°±6° (close to edge-on).[11] In the case of Gliese 876 b, modelling the planet-planet interactions fromthe Laplace resonance shows that the actual inclination of the orbit is 59° resulting in a true mass of 2.2756 times themass of Jupiter.[1]

Physical characteristicsGiven the planet's high mass, it is likely that Gliese 876 b is a gas giant with no solid surface. Since the planet hasonly been detected indirectly through its gravitational effects on the star, properties such as its radius, composition,and temperature are unknown. Assuming a composition similar to Jupiter and an environment close to chemicalequilibrium, it is predicted that the atmosphere of Gliese 876 b is cloudless, though cooler regions of the planet maybe able to form water clouds.[12]

Gliese 876 b lies within the habitable zone of Gliese 876 as defined by the ability of an Earth-mass planet to retainliquid water at its surface. While the prospects for life on a gas giant are unknown, large moons may be able tosupport a habitable environment. Models of tidal interactions between a hypothetical moon, the planet and the starsuggest that large moons should be able to survive in orbit around Gliese 876 b for the lifetime of the system.[13] Onthe other hand, it is unclear whether such moons could form in the first place.[14]

Notes[1] Rivera, Eugenio J. et al. (July 2010). "The Lick-Carnegie Exoplanet Survey: A Uranus-mass Fourth Planet for GJ 876 in an Extrasolar

Laplace Configuration" (http:/ / iopscience. iop. org/ 0004-637X/ 719/ 1/ 890/ fulltext). The Astrophysical Journal 719 (1): 890–899.arXiv:1006.4244. Bibcode 2010ApJ...719..890R. doi:10.1088/0004-637X/719/1/890. .

[2] Uncertainties in the planetary masses and semimajor axes do not take into account the uncertainty in the mass of the star.[3] Marcy, Geoffrey W. et al. (1998). "A Planetary Companion to a Nearby M4 Dwarf, Gliese 876" (http:/ / iopscience. iop. org/ 1538-4357/ 505/

2/ L147/ fulltext). The Astrophysical Journal Letters 505 (2): L147–L149. arXiv:astro-ph/9807307. Bibcode 1998ApJ...505L.147M.doi:10.1086/311623. .

[4] Delfosse, X. et al. (1998). "The closest extrasolar planet. A giant planet around the M4 dwarf GL 876" (http:/ / aa. springer. de/ papers/8338002/ 2300l43/ small. htm). Astronomy and Astrophysics 338: L67–L70. arXiv:astro-ph/9808026. Bibcode 1998A&A...338L..67D. .

[5] http:/ / exoplanet. eu/ planet. php?p1=Gliese+ 876& p2=b[6] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Gliese+ 876+ b[7] Marcy, Geoffrey W. et al. (2001). "A Pair of Resonant Planets Orbiting GJ 876" (http:/ / iopscience. iop. org/ 0004-637X/ 556/ 1/ 296/

fulltext). The Astrophysical Journal 556 (1): 296–301. Bibcode 2001ApJ...556..296M. doi:10.1086/321552. .[8] Rivera, Eugenio J. et al. (2005). "A ~7.5 M⊕ Planet Orbiting the Nearby Star, GJ 876" (http:/ / iopscience. iop. org/ 0004-637X/ 634/ 1/ 625/

fulltext). The Astrophysical Journal 634 (1): 625–640. arXiv:astro-ph/0510508. Bibcode 2005ApJ...634..625R. doi:10.1086/491669. .[9] Butler, R. P. et al. (2006). "Catalog of Nearby Exoplanets" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 646/ 1/ 505/ 64046. html). The

Astrophysical Journal 646 (1): 505–522. arXiv:astro-ph/0607493. Bibcode 2006ApJ...646..505B. doi:10.1086/504701. .[10] Jones, Barrie W. et al. (2005). "Prospects for Habitable "Earths" in Known Exoplanetary Systems" (http:/ / iopscience. iop. org/ 0004-637X/

622/ 2/ 1091/ fulltext). The Astrophysical Journal 622 (2): 1091–1101. arXiv:astro-ph/0503178. Bibcode 2005ApJ...622.1091J.doi:10.1086/428108. .

[11] Benedict, G. F. et al. (2002). "A Mass for the Extrasolar Planet Gliese 876b Determined from Hubble Space Telescope Fine GuidanceSensor 3 Astrometry and High-Precision Radial Velocities" (http:/ / iopscience. iop. org/ 1538-4357/ 581/ 2/ L115/ fulltext). The AstrophysicalJournal 581 (2): L115–L118. arXiv:astro-ph/0212101. Bibcode 2002ApJ...581L.115B. doi:10.1086/346073. .

[12] Sudarsky, David et al. (2003). "Theoretical Spectra and Atmospheres of Extrasolar Giant Planets" (http:/ / iopscience. iop. org/ 0004-637X/588/ 2/ 1121/ fulltext). The Astrophysical Journal 588 (2): 1121–1148. arXiv:astro-ph/0210216. Bibcode 2003ApJ...588.1121S.doi:10.1086/374331. .

[13] Barnes, Jason W.; O'Brien, D. P. (2002). "Stability of Satellites around Close-in Extrasolar Giant Planets" (http:/ / iopscience. iop. org/0004-637X/ 575/ 2/ 1087/ fulltext). The Astrophysical Journal 575 (2): 1087–1093. arXiv:astro-ph/0205035. Bibcode 2002ApJ...575.1087B.doi:10.1086/341477. . (paper incorrectly refers to Gliese 876 b as GJ876c)

[14] Canup, Robin M.; Ward, William R. (2006). "A common mass scaling for satellite systems of gaseous planets". Nature 441 (7095):834–839. Bibcode 2006Natur.441..834C. doi:10.1038/nature04860. PMID 16778883.

Gliese 876 b 38

References

External links• "A planet for Gliese 876" (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap980626. html). NASA. Astronomy Picture of

the Day. 1998-06-26. Retrieved 2008-06-21.• "Gliese 876 : THE CLOSEST EXTRASOLAR PLANET" (http:/ / www. obs-hp. fr/ www/ nouvelles/ gl876.

html). Observatoire de Haute Provence. Retrieved 2008-06-21.• www.extrasolar.net (http:/ / www. extrasolar. net/ planettour. asp?StarCatID=normal& PlanetID=2)

Gliese 876 c 39

Gliese 876 c

Gliese 876 c

Extrasolar planet List of extrasolar planets

Artists concept of Gliese 876 c

Parent star

Star Gliese 876

Constellation Aquarius

Right ascension (α) 22h 53m 16.73s

Declination (δ) −14° 15′ 49.3″

Apparent magnitude (mV) 10.17

Distance 15.3 ly(4.72 pc)

Spectral type M4V

Mass (m) 0.334 ± 0.030 M☉

Radius (r) 0.36 R☉

Temperature (T) 3350 ± 300 K

Metallicity [Fe/H] 0.05 ± 0.20

Age 0.1 – 5.0 Gyr

Orbital elementsEpoch HJD 2,450,602.093

Semimajor axis (a) 0.129590 ± 0.000024 [1] [2] AU

Eccentricity (e) 0.25591 ± 0.00093 [1]

Orbital period (P) 30.0881 ± 0.0082 [1] d

Inclination (i) 59 [1] °

Argument ofperiastron

(ω) 48.76 ± 0.70 [1] °

Mean longitude (λ) 294.59 ± 0.94 [1] °

Semi-amplitude (K) 88.34 ± 0.47 [1] m/s

Physical characteristics

Mass (m) 0.7142 ± 0.0039 [1] MJ

Discovery information

Gliese 876 c 40

Discovery date April 4, 2001

Discoverer(s) Marcy et al.

Detection method Radial velocity

Discovery site California andCarnegie Planet Search

Discovery status Published

Database references

Extrasolar PlanetsEncyclopaedia

data [3]

SIMBAD data [4]

Gliese 876 c is an extrasolar planet orbiting the red dwarf star Gliese 876, taking about 30 days to complete an orbit.The planet was discovered in April 2001 and is the second planet in order of increasing distance from its star.

DiscoveryAt the time of discovery, Gliese 876 was already known to host an extrasolar planet designated Gliese 876 b. In2001, further analysis of the star's radial velocity revealed the existence of a second planet in the system, which wasdesignated Gliese 876 c.[5] The orbital period of Gliese 876 c was found to be exactly half that of the outer planet,which meant that the radial velocity signature of the second planet was initially interpreted as a higher eccentricity ofthe orbit of Gliese 876 b.

Orbit and mass

The orbits of the planets of Gliese 876. Gliese876 c is the second planet from the star.

Gliese 876 c is in a 1:2:4 Laplace resonance with the outer planetsGliese 876 b and Gliese 876 e: for every orbit of planet e, planet bcompletes two orbits and planet c completes four.[1] This leads tostrong gravitational interactions between the planets,[6] causing theorbital elements to change rapidly as the orbits precess. [1] [7] This isthe second known example of a Laplace resonance, the first beingJupiter's moons Io, Europa and Ganymede.

The orbital semimajor axis is only 0.13 AU, around a third of theaverage distance between Mercury and the Sun, and is more eccentricthan the orbit of any of the major planets of our solar system.[8] Despitethis, it is located in the inner regions of the system's habitable zone,since Gliese 876 is such an intrinsically faint star. [9]

A limitation of the radial velocity method used to detect Gliese 876 c isthat only a lower limit on the planet's mass can be obtained. This isbecause the measured mass value depends on the inclination of the

orbit, which is not determined by the radial velocity measurements. However in a resonant system such as Gliese876, gravitational interactions between the planets can be used to determine the true masses. Using this method, theinclination of the orbit can be determined, revealing the planet's true mass to be 0.72 times that of Jupiter.[1]

Gliese 876 c 41

CharacteristicsBased on its high mass, Gliese 876 c is likely to be a gas giant with no solid surface. Since it was detected indirectlythrough its gravitational effects on the star, properties such as its radius, composition, and temperature are unknown.Assuming a composition similar to Jupiter and an environment close to chemical equilibrium, the planet is predictedto have a cloudless upper atmosphere. [10]

Gliese 876 c lies at the inner edge of the system's habitable zone. While the prospects for life on gas giants areunknown, it might be possible for a large moon of the planet to provide a habitable environment. Unfortunately tidalinteractions between a hypothetical moon, the planet, and the star could destroy moons massive enough to behabitable over the lifetime of the system. [11] In addition it is unclear whether such moons could form in the firstplace.[12]

Notes[1] Rivera, Eugenio J. et al. (July 2010). "The Lick-Carnegie Exoplanet Survey: A Uranus-mass Fourth Planet for GJ 876 in an Extrasolar

Laplace Configuration" (http:/ / iopscience. iop. org/ 0004-637X/ 719/ 1/ 890/ fulltext). The Astrophysical Journal 719 (1): 890–899.arXiv:1006.4244. Bibcode 2010ApJ...719..890R. doi:10.1088/0004-637X/719/1/890. .

[2] Uncertainties in the planetary masses and semimajor axes do not take into account the uncertainty in the mass of the star.[3] http:/ / exoplanet. eu/ planet. php?p1=Gliese+ 876& p2=c[4] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Gliese+ 876+ c[5] Marcy, Geoffrey W. et al. (2001). "A Pair of Resonant Planets Orbiting GJ 876" (http:/ / iopscience. iop. org/ 0004-637X/ 556/ 1/ 296/

fulltext). The Astrophysical Journal 556 (1): 296–301. Bibcode 2001ApJ...556..296M. doi:10.1086/321552. .[6] Rivera, E., Lissauer, J. (2001). "Dynamical Models of the Resonant Pair of Planets Orbiting the Star GJ 876". The Astrophysical Journal 558

(1): 392–402. Bibcode 2001ApJ...558..392R. doi:10.1086/322477.[7] Butler et al.; Wright, J. T.; Marcy, G. W.; Fischer, D. A.; Vogt, S. S.; Tinney, C. G.; Jones, H. R. A.; Carter, B. D. et al. (2006). "Catalog of

Nearby Exoplanets" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 646/ 1/ 505/ 64046. html). The Astrophysical Journal 646 (1): 505–522.arXiv:astro-ph/0607493. Bibcode 2006ApJ...646..505B. doi:10.1086/504701. . ( web version (http:/ / exoplanets. org/ planets. shtml))

[8] Correia et al.; Couetdic; Laskar; Bonfils; Mayor; Bertaux; Bouchy; Delfosse et al. (2010). "The HARPS search for southern extra-solarplanets XIX. Characterization and dynamics of the GJ876 planetary system". Astronomy & Astrophysics. arXiv:1001.4774.Bibcode 2010A&A...511A..21C. doi:10.1051/0004-6361/200912700.

[9] Jones, Barrie W. et al. (2005). "Prospects for Habitable "Earths" in Known Exoplanetary Systems" (http:/ / iopscience. iop. org/ 0004-637X/622/ 2/ 1091/ fulltext). The Astrophysical Journal 622 (2): 1091–1101. arXiv:astro-ph/0503178. Bibcode 2005ApJ...622.1091J.doi:10.1086/428108. .

[10] Sudarsky, David et al. (2003). "Theoretical Spectra and Atmospheres of Extrasolar Giant Planets" (http:/ / iopscience. iop. org/ 0004-637X/588/ 2/ 1121/ fulltext). The Astrophysical Journal 588 (2): 1121–1148. arXiv:astro-ph/0210216. Bibcode 2003ApJ...588.1121S.doi:10.1086/374331. .

[11] Barnes, Jason W.; O'Brien, D. P. (2002). "Stability of Satellites around Close-in Extrasolar Giant Planets" (http:/ / iopscience. iop. org/0004-637X/ 575/ 2/ 1087/ fulltext). The Astrophysical Journal 575 (2): 1087–1093. arXiv:astro-ph/0205035. Bibcode 2002ApJ...575.1087B.doi:10.1086/341477. . (paper incorrectly refers to Gliese 876 b as GJ876c)

[12] Canup, Robin M.; Ward, William R. (2006). "A common mass scaling for satellite systems of gaseous planets". Nature 441 (7095):834–839. Bibcode 2006Natur.441..834C. doi:10.1038/nature04860. PMID 16778883.

References

External links• "A planet for Gliese 876" (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap980626. html). NASA. Astronomy Picture of

the Day. 1998-06-26. Retrieved 2008-06-21.• "Gliese 876 : THE CLOSEST EXTRASOLAR PLANET" (http:/ / www. obs-hp. fr/ www/ nouvelles/ gl876.

html). Observatoire de Haute Provence. Retrieved 2008-06-21.• www.extrasolar.net (http:/ / www. extrasolar. net/ planettour. asp?StarCatID=normal& PlanetID=156)

HD 23127 b 42

HD 23127 b

HD 23127 b

Extrasolar planet List of extrasolar planets

Parent star

Star HD 23127

Constellation Reticulum

Right ascension (α) 03h 39m 23.639s

Declination (δ) –60° 04′ 40.230″

Apparent magnitude (mV) 8.58

Distance 290.73 ly(89.14 pc)

Spectral type G2V

Orbital elements

Semimajor axis (a) 2.29 AU(343 Gm)

25.7 mas

Periastron (q) 1.28 AU(192 Gm)

Apastron (Q) 3.30 AU(493 Gm)

Eccentricity (e) 0.44 ± 0.07

Orbital period (P) 1214 ± 9 d(3.32 y)

Argument ofperiastron

(ω) 190 ± 6°

Time of periastron (T0) 2,400,229 ± 19 JD

Semi-amplitude (K) 27.5 ± 1 m/s

Physical characteristics

Minimum mass (m sin i) 1.37 MJ

Discovery information

Discovery date February 9, 2007

Discoverer(s) O'Toole et al.

HD 23127 b 43

Detection method radial velocity

Discovery site  Australia

Discovery status Published

Database references

Extrasolar PlanetsEncyclopaedia

data [1]

SIMBAD data [2]

HD 23127 b is a jovian extrasolar planet orbiting the star HD 23127 at the distance of 2.29 AU, taking 3.32 years toorbit. The orbit is very eccentric, a so-called "eccentric Jupiter". At periastron, the distance is 1.28 AU, putting in itsouter edge of the habitable zone, and at apastron, the distance is 3.30 AU. The mass is at least 1.37 times Jupiter.Only the minimum mass is known because the inclination is not known.

References• O'Toole et al. (2007). "New Planets around Three G Dwarfs" [3]. The Astrophysical Journal 660 (2): 1636–1641.

arXiv:astro-ph/0702213. Bibcode 2007ApJ...660.1636O. doi:10.1086/513563.

References[1] http:/ / exoplanet. eu/ planet. php?p1=HD+ 23127& p2=b[2] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=HD+ 23127+ b[3] http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 660/ 2/ 1636/ 70956. html

HD 28185 b 44

HD 28185 b

HD 28185 b

Extrasolar planet List of extrasolar planets

An artist's impression of HD 28185 b as seen fromthe horizon of a fictional Earth-like moon.

Parent star

Star HD 28185

Constellation Eridanus

Right ascension (α) 04h 26m 26.3205s

Declination (δ) −10° 33′ 02.955″

Apparent magnitude (mV) 7.81

Distance 138 ly(42 pc)

Spectral type G5V

Mass (m) 0.99 ± 0.07 M☉

Radius (r) 1.04 R☉

Temperature (T) 5705 K

Metallicity [Fe/H] 0.24

Age 7.5 Gyr

Orbital elements

Semimajor axis (a) 1.031 ± 0.060 AU(154.2 Gm)

Periastron (q) 0.959 AU(143.5 Gm)

Apastron (Q) 1.102 AU(164.9 Gm)

Eccentricity (e) 0.070 ± 0.040

Orbital period (P) 383.0 ± 2.0 d(1.049 y)

Orbital speed (υ) 29.37 km/s

Argument ofperiastron

(ω) 351 ± 25°

Time of periastron (T0) 2,451,863 ± 26 JD

HD 28185 b 45

Semi-amplitude (K) 161 ± 11 m/s

Physical characteristics

Minimum mass (m sin i) 5.72 ± 0.93 MJ

Discovery information

Discovery date April 4, 2001

Discoverer(s) Santos et al.

Detection method Radial velocity (CORALIE)

Discovery site  Chile La Silla Observatory

Discovery status Published

Database references

Extrasolar PlanetsEncyclopaedia

data [1]

SIMBAD data [2]

HD 28185 b is an extrasolar planet approximately 138 light-years away from Earth in the constellation of Eridanus.The planet was discovered orbiting the Sun-like star HD 28185 in April 2001 as a part of the CORALIE survey forsouthern extrasolar planets, and its existence was independently confirmed by the Magellan Planet Search Survey in2008.[3] HD 28185 b orbits its sun in a circular orbit that is at the inner edge of its star's habitable zone.[4]

DiscoveryLike the majority of known extrasolar planets, HD 28185 b was discovered by detecting small periodic variations inthe radial velocity of its parent star caused by the gravitational attraction of the planet. This was achieved bymeasuring the Doppler shift of the star's spectrum. In 2001 it was announced that HD 28185 exhibited a wobblealong the line-of-sight with a period of 383 days, with an amplitude indicating a minimum mass 5.72 times that ofJupiter.[5]

Orbit and massHD 28185 b takes 1.04 years to orbit its parent star. Unlike most known long-period planets, the orbit of HD 28185 bhas a low eccentricity, comparable to that of Mars in our solar system.[6] The orbit lies entirely within its star'shabitable zone.[4]

The amplitude of the radial velocity oscillations means that the planet has a mass at least 5.7 times that of Jupiter inour solar system. However, the radial velocity method only yields a minimum value on the planet's mass, dependingon the orbital inclination to our line-of-sight. Therefore, the true mass of the planet may be much greater than thislower limit.

HD 28185 b 46

CharacteristicsGiven the planet's high mass, it is most likely to be a gas giant with no solid surface. Since the planet has only beendetected indirectly through observations of the star, properties such as its radius, composition, and temperature areunknown.Since HD 28185 b orbits in its star's habitable zone, some have speculated on the possibility of life on worlds in theHD 28185 system.[7] While it is unknown whether gas giants can support life, simulations of tidal interactionssuggest that HD 28185 b could harbor Earth-mass satellites in orbit around it for many billions of years.[8] Suchmoons, if they exist, may be able to provide a habitable environment, though it is unclear whether such satelliteswould form in the first place.[9] Additionally, a small planet in one of the gas giant's Trojan points could survive in ahabitable orbit for long periods.[10] In addition, the high mass of HD 28185 b of over six Jupiter masses actuallymakes either of these scenarios more likely than if the planet was about Jupiter's mass or less.

References[1] http:/ / exoplanet. eu/ planet. php?p1=HD+ 28185& p2=b[2] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=HD+ 28185+ b[3] Minniti, Dante et al. (2009). "Low-Mass Companions for Five Solar-Type Stars From the Magellan Planet Search Program" (http:/ /

iopscience. iop. org/ 0004-637X/ 693/ 2/ 1424/ fulltext). The Astrophysical Journal 693 (2): 1424–1430. Bibcode 2009ApJ...693.1424M.doi:10.1088/0004-637X/693/2/1424. .

[4] Jones, Barrie W.; Sleep, P. Nick; Underwood, David R. (2006). "Habitability of Known Exoplanetary Systems Based on Measured StellarProperties". The Astrophysical Journal 649 (2): 1010–1019. arXiv:astro-ph/0603200. Bibcode 2006ApJ...649.1010J. doi:10.1086/506557.

[5] Santos, N. et al. (2001). "The CORALIE survey for southern extra-solar planets VI. New long-period giant planets around HD 28185 and HD213240" (http:/ / www. edpsciences. org/ articles/ aa/ abs/ 2001/ 45/ aah3054/ aah3054. html). Astronomy and Astrophysics 379 (3):999–1004. Bibcode 2001A&A...379..999S. doi:10.1051/0004-6361:20011366. .

[6] Butler et al.; Wright, J. T.; Marcy, G. W.; Fischer, D. A.; Vogt, S. S.; Tinney, C. G.; Jones, H. R. A.; Carter, B. D. et al. (2006). "Catalog ofNearby Exoplanets" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 646/ 1/ 505/ 64046. html). The Astrophysical Journal 646 (1): 505–522.arXiv:astro-ph/0607493. Bibcode 2006ApJ...646..505B. doi:10.1086/504701. . ( web version (http:/ / exoplanets. org/ planets. shtml))

[7] Mullen, L. (2001). "Extrasolar Planets with Earth-like Orbits" (http:/ / nai. nasa. gov/ news_stories/ news_detail. cfm?ID=126). . Retrieved 22July 2006.

[8] Barnes, J., O'Brien, D. (2002). "Stability of Satellites around Close-in Extrasolar Giant Planets". Astrophysical Journal 575 (2): 1087–1093.arXiv:astro-ph/0205035. Bibcode 2002ApJ...575.1087B. doi:10.1086/341477.

[9] Canup, R., Ward, W. (2006). "A common mass scaling for satellite systems of gaseous planets" (http:/ / www. nature. com/ nature/ journal/v441/ n7095/ abs/ nature04860. html). Nature 441 (7095): 834–839. Bibcode 2006Natur.441..834C. doi:10.1038/nature04860.PMID 16778883. .

[10] Schwarz, R.; Dvorak, R.; Süli, Á.; Érdi, B. (2007). "Survey of the stability region of hypothetical habitable Trojan planets". Astronomy andAstrophysics 474 (3): 1023–1029. Bibcode 2007A&A...474.1023S. doi:10.1051/0004-6361:20077994.

External links• "HD 28185" (http:/ / media4. obspm. fr/ exoplanets/ base/ etoile. php?nom=HD+ 28185). Exoplanets.• HD 28185 b (http:/ / www. extrasolar. net/ planettour. asp?StarCatId=normal& PlanetId=158) on Extrasolar

Visions (http:/ / www. extrasolar. net/ )

HD 37124 b 47

HD 37124 b

HD 37124 b

Extrasolar planet List of extrasolar planets

Parent star

Star HD 37124

Constellation Taurus

Right ascension (α) 05h 37m 02.4864s

Declination (δ) +20° 43′ 50.836″

Apparent magnitude (mV) 7.68

Distance 107.63 ly(33.2 pc)

Spectral type G4V

Mass (m) 0.91 M☉

Radius (r) 0.82 R☉

Temperature (T) 5610 ± 70 K

Metallicity [Fe/H] -0.32

Age 3.33 Gyr

Orbital elements

Semimajor axis (a) 0.53 AU

Eccentricity (e) 0.055

Orbital period (P) 154.46 ± 0.369 d

Argument ofperiastron

(ω) 140.5°

Time of periastron (T0) 2,450,000.11 JD

Semi-amplitude (K) 27.5 m/s

Physical characteristics

Minimum mass (m sin i) 0.61 MJ

Discovery information

Discovery date 14 November 1999

Discoverer(s) Marcy et al.

Detection method Radial velocity

Discovery site  United States

Discovery status Confirmed

Database references

Extrasolar PlanetsEncyclopaedia

data [1]

SIMBAD data [2]

HD 37124 b 48

HD 37124 b is an extrasolar planet approximately 108 light-years away in the constellation of Taurus (the Bull). Theplanet was discovered in 1999 orbiting the star HD 37124. Based on its mass, it is most likely that this planet is aJovian planet (like Jupiter).

References• Vogt et al.; Marcy, Geoffrey W.; Butler, R. Paul; Apps, Kevin (2000). "Six New Planets from the Keck Precision

Velocity Survey". The Astrophysical Journal 536 (2): 902–914. arXiv:astro-ph/9911506.Bibcode 2000ApJ...536..902V. doi:10.1086/308981.

External links• "A dynamical analysis of the HD 37124 planetary system" [3] (in French). CAT.INIST. Retrieved 2008-06-22.• Jianghui Ji et al. (2003-05-23). "The Librating Companions in HD 37124, HD 12661, HD 82943, 47 Uma and GJ

876: Alignment or Antialignment?". arXiv:astro-ph/0305448 [astro-ph].

References[1] http:/ / exoplanet. eu/ planet. php?p1=HD+ 37124& p2=b[2] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=HD+ 37124+ b[3] http:/ / cat. inist. fr/ ?aModele=afficheN& cpsidt=14462421

HD 69830 d 49

HD 69830 d

HD 69830 d

Extrasolar planet List of extrasolar planets

HD 69830 and HD 69830 d

Parent star

Star HD 69830

Constellation Puppis

Right ascension (α) 08h 18m 23.9473s

Declination (δ) −12° 37′ 55.824″

Apparent magnitude (mV) 5.95

Distance 41 ly(12.6 pc)

Spectral type K0V

Mass (m) 0.86 ± 0.03 M☉

Radius (r) 0.89 R☉

Temperature (T) 5385 ± 20 K

Metallicity [Fe/H] −0.05 ± 0.02

Age 5.1–6.1 Gyr

Orbital elements

Semimajor axis (a) 0.63 AU

Eccentricity (e) 0.07 ± 0.07

Orbital period (P) 197 ± 3 d

Argument ofperiastron

(ω) 224 ± 61°

Time of periastron (T0) 24513358 ± 34 JD

Semi-amplitude (K) 2.20 ± 0.19 m/s

Physical characteristics

Minimum mass (m sin i) 0.058 MJ

Temperature (T) ~284 K

Discovery information

Discovery date May 18, 2006

Discoverer(s) Lovis et al.

HD 69830 d 50

Detection method HARPS

Discovery status confirmed

Database references

Extrasolar PlanetsEncyclopaedia

data [1]

SIMBAD data [2]

HD 69830 d is an extrasolar planet orbiting the orange dwarf star HD 69830 every 197 days. It is the outermostknown planet in its planetary system and possibly lies within its habitable zone.

DiscoveryPlanet HD 69830 d was discovered in 2006.

Orbit and massThe planet's orbit has a low eccentricity, like most of the planets in our solar system. The semimajor axis of the orbitis only 0.63 AU, similar to that of Venus. However, HD 69830 is a less massive and energetic star than the Sun,thereby putting the planet within its habitable zone.

CharacteristicsGiven the planet's Neptune-type mass, it is likely that HD 69830 d is a gas giant with no solid surface. Since theplanet has only been detected indirectly through its gravitational effects on the star, properties such as its radius andcomposition are unknown.

References In Popular CultureIn the Bestiarum included with the special editions of Halo 3, HD 69830 d is described as the home star of theJackals, specifically stating that they come from a hypothetical moon of the third planet at the inner edge of theasteroid belt.It is mentioned as a potentially habitable planet on the History Channel´s program The Universe.

References• Lovis et al. (2006). "An extrasolar planetary system with three Neptune-mass planets" [3]. Nature 441 (7091):

305–309. arXiv:astro-ph/0703024. Bibcode 2006Natur.441..305L. doi:10.1038/nature04828. PMID 16710412.

HD 69830 d 51

External links• SST: Signs of Alien Asteroid Belt [4]

• SolStation: HD 69830 / HR 3259 [5]

• SPACE.com: Planets Found in Potentially Habitable Setup (May 17, 2006) [6]

• SpaceDaily: Trio Of Neptunes And Their Belt (May 18, 2006) [7]

References[1] http:/ / exoplanet. eu/ planet. php?p1=HD+ 69830& p2=d[2] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=HD+ 69830+ d[3] http:/ / www. nature. com/ nature/ journal/ v441/ n7091/ abs/ nature04828. html[4] http:/ / www. spitzer. caltech. edu/ Media/ releases/ ssc2005-10/ release. shtml[5] http:/ / www. solstation. com/ stars2/ hd69830. htm[6] http:/ / www. space. com/ scienceastronomy/ 060517_netpune_planets. html[7] http:/ / www. spacedaily. com/ reports/ Trio_Of_Neptunes_And_Their_Belt. html

HD 85512 b 52

HD 85512 b

HD 85512 b

Extrasolar planet List of extrasolar planets

Plot of the orbit of HD 85512 b compared to the star's habitable zone

Parent star

Star HD 85512

Constellation Vela

Right ascension (α) 09h 51m 07.1s

Declination (δ) −43° 30′ 10″

Apparent magnitude (mV) 7.66

Distance 36 [1]  ly(11.1 ± 0.1[2]  pc)

Spectral type K5 V

Mass (m) 0.69 M☉

Temperature (T) 4715 ± 102 K

Metallicity [Fe/H] −0.33 ± 0.03

Age 5.61 ± 0.61 Gyr

Orbital elements

Semimajor axis (a) 0.26 ± 0.005[2] AU

Eccentricity (e) 0.11 ± 0.1[2]

Orbital period (P) 54.43 ± 0.13[2] d

Orbital speed (υ) 94.913 ± 0.038 km/s

Physical characteristics

Minimum mass (m sin i) 3.6 ± 0.5 M⊕

Temperature (T) 298[3] K

Discovery information

Discovery date August 17, 2011

Discoverer(s) Pepe et al.

Detection method Radial velocity (HARPS)

HD 85512 b 53

Discovery site La Silla Observatory

Discovery status Published

Other designations

CD-42 5678 b, GJ 370 b, Gliese 370 b, HIP 48331 b, LHS 2201 b, NStars 0951-4330 b, YPC 2340 b

Database references

Extrasolar PlanetsEncyclopaedia

data [4]

SIMBAD data [5]

HD 85512 b (known as Gliese 370 b or Gl 370 b) is an extrasolar planet orbiting Gliese 370 (a K-type or "orangedwarf") star approximately 36 light-years distance from Earth in the constellation of Vela (the Sail).[1] [2]

Due to its mass of at least 3.6 times the mass of Earth, HD 85512 b is classified as a super-Earth and is one of thesmallest exoplanets discovered to be in the habitable zone.[2] HD 85512 b, along with Gliese 581 d is considered tobe one of the best candidates for habitability as of August 25, 2011.[2] [6]

Detection and discoveryThe planet was discovered by scientists at University of Geneva, Switzerland,[6] [7] led by Swiss astronomerStéphane Udry[8] of the Guaranteed Time Observations (GTO) program of High Accuracy Radial velocity PlanetSearcher (HARPS), a high-precision echelle spectrograph installed on ESO's 3.6 m telescope at La Silla Observatoryin Chile.[3] The team used the Doppler spectroscopy technique which determines the minimum mass of the planetthrough slight changes in motion of the parent star.

Physical characteristicsOn August 17, 2011, researchers released a study of the planet, concluding that HD 85512 b is the most habitableexoplanet discovered up to that point[2] and one of the most stable exoplanets discovered by the High AccuracyRadial Velocity Planet Searcher.[3] The planet has a minimum Earth mass of 3.6 ± 0.5, surface gravity of about 1.4 gand an estimated temperature of 298 K (25 °C or 77 °F) at the top of its atmosphere. The estimated temperature isnoted to be similar to temperatures in Southern France,[3] [9] but various atmospheric conditions prevalent in theplanet have to be analyzed to estimate the temperature of the surface.[3] It orbits the parent star at a distance of about0.26 AU,[6] with an orbital period of about 54 days.[6]

Habitability and climateFor the temperature to be below 270 K, for a circular orbit, the planetary albedo should be 0.48 ± 0.05 and for aneccentricity of 0.11, the planetary albedo should be 0.52.[2] If the planet has 50% cloud cover, water may exist inliquid form on the planet[8] provided its atmosphere is similar to our own, thus making the planet habitable.[7] [10] [11]

Also, if the albedo of the planet is increased due to cloud cover, water could be present in its liquid form on theplanet, which would mean that the planet is on the edge of habitability.[2] [11]

HD 85512 b 54

Effective temperaturesUsing the measured stellar luminosity of Gliese 370 of 0.126 times that of our Sun, it is possible to calculate HD85512 b's effective temperature a.k.a. black body temperature, which probably differs from its surface temperature.The effective temperature for HD 85512 b, assuming an aforementioned albedo, would be 24.138 °C (75 °F). [12]

References[1] Pepe, F; et al. (2011). The HARPS search for Earth-like planets in the habitable zone: I – Very low-mass planets around HD20794, HD85512

and HD192310. arXiv:1108.3447. Bibcode 2011yCat..35349058P. doi:10.1051/0004-6361/201117055.[2] Kaltenegger, L; Udry, S; Pepe, F (2011). A Habitable Planet around HD 85512?. arXiv:1108.3561. Bibcode 2011arXiv1108.3561K.[3] "HARPS: Hunting for Nearby Earth-like Planets" (http:/ / www. centauri-dreams. org/ ?p=19560). centauri-dreams.org. . Retrieved

2011-08-25.[4] http:/ / exoplanet. eu/ planet. php?p1=HD+ 85512& p2=b[5] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=HD+ 85512+ b[6] "Researchers find potentially habitable planet" (http:/ / www. maxisciences. com/ planète-habitable/

des-chercheurs-decouvrent-une-planete-potentiellement-habitable_art16635. html) (in French). maxisciences.com. . Retrieved 2011-08-31.[7] "Found a planet where life could exist" (http:/ / kauno. diena. lt/ naujienos/ mokslas-ir-it/

rasta-planeta-kurioje-galetu-egzistuoti-gyvybe-373401) (in Lithuanian). maxisciences.com. . Retrieved 2011-08-31.[8] "Super Earth circulating in ekosferze?" (http:/ / technologie. gazeta. pl/ internet/ 1,114257,10139412,Super_Ziemia_krazaca_w_ekosferze_.

html) (in Polish). technologie.gazeta.pl. . Retrieved 2011-08-31.[9] "Italian helps find planet that could sustain life" (http:/ / lagazzettadelmezzogiorno. it/ notizia. php?IDNotizia=452563& IDCategoria=2694).

La Gazzetta del Mezzogiorno. September 2, 2011. . Retrieved September 7, 2011.[10] "Exoplanet Looks Potentially Lively" (http:/ / www. scientificamerican. com/ podcast/ episode.

cfm?id=exoplanet-looks-potentially-lively-11-08-22). scientificamerican.com. . Retrieved 2011-08-25.[11] "Is There A Habitable Planet Circling HD 85512?" (http:/ / www. spaceref. com/ news/ viewsr. html?pid=38197). spaceref.com. . Retrieved

2011-08-31.[12] http:/ / io9. com/ 5839395/ potentially-habitable-super+ earth-is-among-50-newly-discovered-exoplanets

External links• A Habitable Planet around HD 85512 (https:/ / www. cfa. harvard. edu/ ~lkaltenegger/ Home_files/

KalteneggerHD85512. pdf)

Kepler-22b 55

Kepler-22b

Kepler-22b

Extrasolar planet List of extrasolar planets

A diagram of the Kepler-22b System, compared to our Inner Solar System.

Parent star

Star Kepler-22

Orbital elements

Orbital period (P) 289.9 [1] d

Physical characteristics

Radius (r) 2.4 [2] R⊕

Discovery information

Discovery date

Discoverer(s)

Detection method Transit

Discovery site Kepler telescope

Discovery status Published

Other designations

Database references

Extrasolar PlanetsEncyclopaedia

data [3]

SIMBAD data [4]

Kepler-22b 56

Artist's conception of Kepler-22b.

Kepler-22b is the first extrasolar planet confirmed by NASA's KeplerSpace Telescope to orbit in the habitable zone of a Sun-like star.

The discovery was announced December 5, 2011.[5] The planet's radiusis roughly 2.4 times the radius of Earth; it is 600 light years away fromEarth, in orbit around the G-type star Kepler 22.[5] [6]

To date, its mass and surface composition remain unknown.[5] [6] If ithas an Earth-like density (5.515 g/cm3) then it would mass 13.8 Earths[7] while its surface gravity would be 2.4 times Earth's.[8] If it has waterlike density (1 g/cm3) then it would mass 2.5 Earths [9] and have asurface gravity of 0.43 times Earth's [10] .

The distance from Kepler-22b to its star is about 15% less than the distance from Earth to the Sun, hence its orbit isabout 85% of Earth's orbit. One orbital revolution around its star takes 289.9 days.[1] [11]

The light output of Kepler-22b's star is about 25% less than that of the Sun.[5] The combination of a shorter distancefrom the star and a lower light output are consistent with a moderate surface temperature. Scientists estimate that inthe absence of atmosphere, the equilibrium temperature would be approximately -11°C. If the greenhouse effectcaused by the atmosphere is Earthlike, this corresponds to approximately 22 °C (72°F) average surfacetemperature.[5] [6]

References[1] Alien Planet Could Host Life (http:/ / news. discovery. com/ space/ alien-planet-found-in-habitable-zone-111205. html), Discovery.com[2] "NASA Telescope Confirms Alien Planet in Habitable Zone" (http:/ / www. space. com/ 13821-nasa-kepler-alien-planets-habitable-zone.

html), Space.com 12/5/2011[3] http:/ / exoplanet. eu/ planet. php?p1=Kepler-22& p2=b[4] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=[5] BBC NEWS, "Kepler 22-b: Earth-like planet confirmed" 12/5/2011 http:/ / www. bbc. co. uk/ news/ science-environment-16040655[6] NASA Press Release, "NASA's Kepler Confirms Its First Planet In Habitable Zone", 12/5/2011, http:/ / www. nasa. gov/ centers/ ames/ news/

releases/ 2011/ 11-99AR. html[7] m ~ V ~ R3, hence 2.43 = 13.8 is the proportion[8] Gravity is proportional to m/r^2. (See Newtonian gravity) If m=13.4 and r=2.4, then G=13.4/(2.4^2)=2.4 . (13.4 is 2.4^3).[9] (13.8/5.5)[10] 2.5/2.42

[11] NASA - NASA's Kepler Mission Confirms Its First Planet in Habitable Zone of Sun-like Star (http:/ / www. nasa. gov/ mission_pages/kepler/ news/ kepscicon-briefing. html)

Footnotes: Calculations

External Links• "NASA Telescope Confirms Alien Planet in Habitable Zone" (http:/ / www. space. com/

13821-nasa-kepler-alien-planets-habitable-zone. html) (Space.com)• "Kepler 22-b: Earth-like planet confirmed" (http:/ / www. bbc. co. uk/ news/ science-environment-16040655)

(BBC)• "NASA's Kepler Confirms Its First Planet In Habitable Zone" (http:/ / www. jpl. nasa. gov/ news/ news.

cfm?release=2011-373) (NASA)• Kepler discoveries: Kepler-22b "a yearly orbit of 289 days" (http:/ / www. nasa. gov/ mission_pages/ kepler/

multimedia/ images/ kepler-22b-diagram. html) (NASA)• "View of Kepler 22-b Sky Location" (http:/ / www. worldwidetelescope. org/ webclient/ default.

aspx?wtml=http:/ / www. worldwidetelescope. org/ wwtweb/ goto. aspx?object=No+ Object& ra=19.

Kepler-22b 57

2810972852724& dec=47. 8846744786791& zoom=0. 445987757026504& wtml=true) (Worldwide Telescope)

Mars

Mars  

Mars in 1980 as seen by the Viking 1 orbiter

Designations

Pronunciation i/ˈmɑrz/

Adjective Martian

Orbital characteristics[1]

Epoch J2000

Aphelion 249,209,300 km1.665 861 AU

Perihelion 206,669,000 km1.381 497 AU

Semi-major axis 227,939,100 km1.523 679 AU

Eccentricity 0.093 315

Orbital period 686.971 days1.8808 Julian years668.5991 sols

Synodic period 779.96 days2.135 Julian years

Average orbital speed 24.077 km/s

Mean anomaly 19.3564°

Inclination 1.850° to ecliptic5.65° to Sun's equator1.67° to invariable plane[2]

Longitude of ascending node 49.562°

Argument of perihelion 286.537°

Satellites 2

Physical characteristics

Mars 58

Equatorial radius 3,396.2 ± 0.1 km[a][3]

0.533 Earths

Polar radius 3,376.2 ± 0.1 km[a][3]

0.531 Earths

Flattening 0.005 89 ± 0.000 15

Surface area 144,798,500 km2

0.284 Earths

Volume 1.6318×1011 km3[4]

0.151 Earths

Mass 6.4185×1023 kg[4]

0.107 Earths

Mean density 3.9335 ± 0.0004[4] g/cm³

Equatorial surface gravity 3.711 m/s²[4]

0.376 g

Escape velocity 5.027 km/s

Sidereal rotationperiod

1.025 957 day24.622 9 h[4]

Equatorial rotation velocity 868.22 km/h (241.17 m/s)

Axial tilt 25.19°

North pole right ascension 21 h 10 min 44 s317.681 43°

North pole declination 52.886 50°

Albedo 0.170 (geometric)[5]

0.25 (Bond)[6]

Surface temp.   Kelvin   Celsius

min mean max

186 K 210 K[6] 293 K[7]

−87 °C −63 °C 20 °C

Apparent magnitude +1.6 to −3.0[8]

Angular diameter 3.5–25.1"[6]

Atmosphere[6] [9]

Surface pressure 0.636 (0.4–0.87) kPa

Mars 59

Composition (mole fractions)95.32% carbon dioxide2.7% nitrogen1.6% argon0.13% oxygen0.08% carbon monoxide210 ppm water vapor100 ppm nitric oxide15 ppm molecular hydrogen[10]

2.5 ppm neon850 ppb HDO300 ppb krypton130 ppb formaldehyde80 ppb xenon30 ppb ozone18 ppb hydrogen peroxide[11]

10 ppb methane[12]

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars.It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance.[1]

Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters ofthe Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cyclesof Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of OlympusMons, the highest known mountain within the Solar System, and of Valles Marineris, the largest canyon. The smoothBorealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.[2] [3]

Until the first successful flyby of Mars occurred in 1965, by Mariner 4, many speculated about the presence of liquidwater on the planet's surface. This was based on observed periodic variations in light and dark patches, particularlyin the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some asirrigation channels for liquid water. These straight line features were later explained as optical illusions, thoughgeological evidence gathered by unmanned missions suggest that Mars once had large-scale water coverage on itssurface.[4] In 2005, radar data revealed the presence of large quantities of water ice at the poles,[5] and atmid-latitudes.[6] [7] The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007.The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.[8]

Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids,similar to 5261 Eureka, a Martian trojan asteroid. Mars is currently host to three functional orbiting spacecraft: MarsOdyssey, Mars Express, and the Mars Reconnaissance Orbiter. On the surface are the Mars Exploration RoverOpportunity and its recently decommissioned twin, Spirit, along with several other inert landers and rovers, bothsuccessful and unsuccessful. The Phoenix lander completed its mission on the surface in 2008. Observations byNASA's now-defunct Mars Global Surveyor show evidence that parts of the southern polar ice cap have beenreceding.[9] Observations by NASA's Mars Reconnaissance Orbiter have revealed possible flowing water during thewarmest months on Mars.[10]

Mars can easily be seen from Earth with the naked eye. Its apparent magnitude reaches −3.0[8] a brightness surpassedonly by Jupiter, Venus, the Moon, and the Sun. Optical ground based telescopes are typically limited to resolvingfeatures about 300 km (186 miles) across when Earth and Mars are closest, because of Earth's atmosphere.[11]

Mars 60

Physical characteristics

Size comparison of Earth and Mars.

Mars has approximately half the diameter of Earth. It is less dense thanEarth, having about 15% of Earth's volume and 11% of the mass. Itssurface area is only slightly less than the total area of Earth's dryland.[6] While Mars is larger and more massive than Mercury, Mercuryhas a higher density. This results in the two planets having a nearlyidentical gravitational pull at the surface—that of Mars is stronger byless than 1%. The red-orange appearance of the Martian surface iscaused by iron(III) oxide, more commonly known as hematite, orrust.[12]

GeologyMars is a terrestrial planet that consists of minerals containing silicon and oxygen, metals, and other elements thattypically make up rock. The surface of Mars is primarily composed of tholeiitic basalt,[13] although parts are moresilica-rich than typical basalt and may be similar to andesitic rocks on Earth or silica glass. Regions of low albedoshow concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normalconcentrations of sheet silicates and high-silicon glass. Parts of the southern highlands include detectable amounts ofhigh-calcium pyroxenes. Localized concentrations of hematite and olivine have also been found.[14] Much of thesurface is deeply covered by finely grained iron(III) oxide dust.[15] [16]

Like Earth, this planet has undergone differentiation, resulting in a dense, metallic core region overlaid by less densematerials.[17] Current models of the planet's interior imply a core region about 1794 ± 65 km in radius, consistingprimarily of iron and nickel with about 16–17% sulfur.[18] This iron sulfide core is partially fluid, and has twice theconcentration of the lighter elements than exist at Earth's core. The core is surrounded by a silicate mantle thatformed many of the tectonic and volcanic features on the planet, but now appears to be dormant. Besides silicon andoxygen, the most abundant elements in the martian crust are iron, magnesium, aluminum, calcium, and potassium.The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km.[19] Earth's crust,averaging 40 km, is only one third as thick as Mars’ crust, relative to the sizes of the two planets.Although Mars has no evidence of a current structured global magnetic field,[20] observations show that parts of theplanet's crust have been magnetized, and that alternating polarity reversals of its dipole field have occurred in thepast. This paleomagnetism of magnetically susceptible minerals has properties that are very similar to the alternatingbands found on the ocean floors of Earth. One theory, published in 1999 and re-examined in October 2005 (with thehelp of the Mars Global Surveyor), is that these bands demonstrate plate tectonics on Mars four billion years ago,before the planetary dynamo ceased to function and the planet's magnetic field faded away.[21]

During the Solar System's formation, Mars was created as the result of a stochastic process of run-away accretion outof the protoplanetary disk that orbited the Sun. Mars has many distinctive chemical features caused by its position inthe Solar System. Elements with comparatively low boiling points such as chlorine, phosphorus and sulphur aremuch more common on Mars than Earth; these elements were probably removed from areas closer to the Sun by theyoung star's energetic solar wind.[22]

After the formation of the planets, all were subjected to the so-called "Late Heavy Bombardment". About 60% of thesurface of Mars shows a record of impacts from that era,[23] [24] [25] while much of the remaining surface is probablyunderlain by immense impact basins caused by those events. There is evidence of an enormous impact basin in thenorthern hemisphere of Mars, spanning 10,600 km by 8,500 km, or roughly four times larger than the Moon's SouthPole – Aitken basin, the largest impact basin yet discovered.[2] [3] This theory suggests that Mars was struck by aPluto-sized body about four billion years ago. The event, thought to be the cause of the Martian hemisphericdichotomy, created the smooth Borealis basin that covers 40% of the planet.[26] [27]

Mars 61

The geological history of Mars can be split into many periods, but the following are the three primary periods:[28] [29]

• Noachian period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 4.5 billion yearsago to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge, avolcanic upland, is thought to have formed during this period, with extensive flooding by liquid water late in theperiod.

• Hesperian period (named after Hesperia Planum): 3.5 billion years ago to 2.9–3.3 billion years ago. TheHesperian period is marked by the formation of extensive lava plains.

• Amazonian period (named after Amazonis Planitia): 2.9–3.3 Gyr ago billion years ago to present. Amazonianregions have few meteorite impact craters, but are otherwise quite varied. Olympus Mons formed during thisperiod, along with lava flows elsewhere on Mars.

Top down view of Olympus Mons, the highestknown mountain in the solar system

Some geological activity is still taking place on Mars. The AthabascaValles is home to sheet-like lava flows up to about 200 Mya. Waterflows in the grabens called the Cerberus Fossae occurred less than 20Mya, indicating equally recent volcanic intrusions.[30] On February 19,2008, images from the Mars Reconnaissance Orbiter showed evidenceof an avalanche from a 700 m high cliff.[31]

Soil

The Phoenix lander returned data showing Martian soil to be slightlyalkaline and containing elements such as magnesium, sodium,potassium and chloride. These nutrients are found in gardens on Earth,and are necessary for growth of plants.[32] Experiments performed bythe Lander showed that the Martian soil has a basic pH of 8.3, and maycontain traces of the salt perchlorate.[33] [34]

Annotated image of Tharsis Tholusdark streak, as seen by Hirise. It islocated in the middle left of this

picture. Tharsis Tholus is just off tothe right.

Streaks are common across Mars and new ones appear frequently on steep slopesof craters, troughs, and valleys. The streaks are dark at first and get lighter withage. Sometimes the streaks start in a tiny area which then spreads out forhundreds of metres. They have also been seen to follow the edges of bouldersand other obstacles in their path. The commonly accepted theories include thatthey are dark underlying layers of soil revealed after avalanches of bright dust ordust devils.[35] Several explanations have been put forward, some of whichinvolve water or even the growth of organisms.[36] [37]

Mars 62

Hydrology

Microscopic photo taken by Opportunity showinga gray hematite concretion, indicative of the past

presence of liquid water

Liquid water cannot exist on the surface of Mars due to lowatmospheric pressure, except at the lowest elevations for shortperiods.[38] [39] The two polar ice caps appear to be made largely ofwater.[40] [41] The volume of water ice in the south polar ice cap, ifmelted, would be sufficient to cover the entire planetary surface to adepth of 11 meters.[42] A permafrost mantle stretches from the pole tolatitudes of about 60°.[40]

Large quantities of water ice are thought to be trapped underneath thethick cryosphere of Mars. Radar data from Mars Express and the MarsReconnaissance Orbiter show large quantities of water ice both at thepoles (July 2005)[5] [43] and at mid-latitudes (November 2008).[6] ThePhoenix lander directly sampled water ice in shallow Martian soil onJuly 31, 2008.[8]

Landforms visible on Mars strongly suggest that liquid water has atleast at times existed on the planet's surface. Huge linear swathes of scoured ground, known as outflow channels, cutacross the surface in around 25 places. These are thought to record erosion which occurred during the catastrophicrelease of water from subsurface aquifers, though some of these structures have also been hypothesised to resultfrom the action of glaciers or lava.[44] [45] The youngest of these channels are thought to have formed as recently asonly a few million years ago.[46] Elsewhere, particularly on the oldest areas of the martian surface, finer-scale,dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys andtheir distribution very strongly imply that they were carved by runoff resulting from rain or snow fall in early Marshistory. Subsurface water flow and groundwater sapping may play important subsidiary roles in some networks, butprecipitation was probably the root cause of the incision in almost all cases.[47]

There are also thousands of features along crater and canyon walls that appear similar to terrestrial gullies. Thegullies tend to be in the highlands of the southern hemisphere and to face the Equator; all are poleward of 30°latitude. A number of authors have suggested that their formation process demands the involvement of liquid water,probably from melting ice,[48] [49] although others have argued for formation mechanisms involving carbon dioxidefrost or the movement of dry dust.[50] [51] No partially degraded gullies have formed by weathering and nosuperimposed impact craters have been observed, indicating that these are very young features, possibly even activetoday.[49]

Other geological features, such as deltas and alluvial fans preserved in craters, also argue very strongly for warmer,wetter conditions at some interval or intervals in earlier Mars history.[52] Such conditions necessarily require thewidespread presence of crater lakes across a large proportion of the surface, for which there is also independentmineralogical, sedimentological and geomorphological evidence.[53] Some authors have even gone so far as to arguethat at times in the martian past, much of the low northern plains of the planet were covered with a true oceanhundreds of meters deep, though this remains controversial.[54]

Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific mineralssuch as hematite and goethite, both of which sometimes form in the presence of water.[55] Some of the evidencebelieved to indicate ancient water basins and flows has been negated by higher resolution studies by the MarsReconnaissance Orbiter.[56] In 2004, Opportunity detected the mineral jarosite. This forms only in the presence ofacidic water, which demonstrates that water once existed on Mars.[57]

Mars 63

Polar caps

Viking Orbiter's view of the northern ice cap of Mars

South polar cap in 2000

Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface andcausing the deposition of 25–30% of the atmosphere into slabs of CO2 ice (dry ice).[58] When the poles are againexposed to sunlight, the frozen CO2 sublimes, creating enormous winds that sweep off the poles as fast as 400 km/h.These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and largecirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004.[59]

The polar caps at both poles consist primarily of water ice. Frozen carbon dioxide accumulates as a comparativelythin layer about one metre thick on the north cap in the northern winter only, while the south cap has a permanentdry ice cover about eight metres thick.[60] The northern polar cap has a diameter of about 1,000 kilometres during thenorthern Mars summer,[61] and contains about 1.6 million cubic km of ice, which if spread evenly on the cap wouldbe 2 km thick.[62] (This compares to a volume of 2.85 million cubic km (km3) for the Greenland ice sheet.) Thesouthern polar cap has a diameter of 350 km and a thickness of 3 km.[63] The total volume of ice in the south polarcap plus the adjacent layered deposits has also been estimated at 1.6 million cubic km.[64] Both polar caps showspiral troughs, which recent analysis of SHARAD ice penetrating radar has shown are a result of katabatic winds thatspiral due to the Coriolis Effect.[65] [66]

The seasonal frosting of some areas near the southern ice cap results in the formation of transparent 1 metre thickslabs of dry ice above the ground. With the arrival of spring, sunlight warms the subsurface and pressure fromsubliming CO2 builds up under a slab, elevating and ultimately rupturing it. This leads to geyser-like eruptions ofCO2 gas mixed with dark basaltic sand or dust. This process is rapid, observed happening in the space of a few days,weeks or months, a rate of change rather unusual in geology—especially for Mars. The gas rushing underneath a slabto the site of a geyser carves a spider-like pattern of radial channels under the ice.[67] [68] [69] [70]

Mars 64

Geography

Volcanic plateaus (red) and impact basins (blue)dominate this topographic map of Mars

Although better remembered for mapping the Moon, Johann HeinrichMädler and Wilhelm Beer were the first "areographers". They beganby establishing that most of Mars’ surface features were permanent,and more precisely determining the planet's rotation period. In 1840,Mädler combined ten years of observations and drew the first map ofMars. Rather than giving names to the various markings, Beer andMädler simply designated them with letters; Meridian Bay (SinusMeridiani) was thus feature "a."[71]

Today, features on Mars are named from a variety of sources. Albedofeatures are named for classical mythology. Craters larger than 60kilometres (37 mi) are named for deceased scientists and writers andothers who have contributed to the study of Mars. Craters smaller that60 km are named for towns and villages of the world with populationsof less than 100,000. Large valleys are named for the word mars or star in various languages, small valleys arenamed for rivers.[72]

Large albedo features retain many of the older names, but are often updated to reflect new knowledge of the natureof the features. For example, Nix Olympica (the snows of Olympus) has become Olympus Mons (MountOlympus).[73] The surface of Mars as seen from Earth is divided into two kinds of areas, with differing albedo. Thepaler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian 'continents' andgiven names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features werethought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark featureseen from Earth is Syrtis Major Planum.[74] The permanent northern polar ice cap is named Planum Boreum, whilethe southern cap is called Planum Australe.Mars’ equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (atGreenwich), by choice of an arbitrary point; Mädler and Beer selected a line in 1830 for their first maps of Mars.After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0),located in the Sinus Meridiani ("Middle Bay" or "Meridian Bay"), was chosen for the definition of 0.0° longitude tocoincide with the original selection.[75]

Since Mars has no oceans and hence no 'sea level', a zero-elevation surface also had to be selected as a referencelevel; this is also called the areoid[76] of Mars, analogous to the terrestrial geoid. Zero altitude is defined by theheight at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure.[77] This pressure corresponds to the triplepoint of water, and is about 0.6% of the sea level surface pressure on Earth (0.006 atm).[78] In practice, today thissurface is defined directly from satellite gravity measurements.

An approximate true-color image, taken by Mars Exploration Rover Opportunity, shows the view of Victoria Craterfrom Cape Verde. It was captured over a three-week period, from October 16 – November 6, 2006.

Mars 65

Impact topography

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southernhighlands, pitted and cratered by ancient impacts. Research in 2008 has presented evidence regarding a theoryproposed in 1980 postulating that, four billion years ago, the northern hemisphere of Mars was struck by an objectone-tenth to two-thirds the size of the Moon. If validated, this would make the northern hemisphere of Mars the siteof an impact crater 10,600 km long by 8,500 km wide, or roughly the area of Europe, Asia, and Australia combined,surpassing the South Pole – Aitken basin as the largest impact crater in the Solar System.[2] [3]

Mars is scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 km or greater have beenfound.[79] The largest confirmed of these is the Hellas impact basin, a light albedo feature clearly visible fromEarth.[80] Due to the smaller mass of Mars, the probability of an object colliding with the planet is about half that ofthe Earth. Mars is located closer to the asteroid belt, so it has an increased chance of being struck by materials fromthat source. Mars is also more likely to be struck by short-period comets, i.e., those that lie within the orbit ofJupiter.[81] In spite of this, there are far fewer craters on Mars compared with the Moon because the atmosphere ofMars provides protection against small meteors. Some craters have a morphology that suggests the ground becamewet after the meteor impacted.[82]

Tectonic sites

The shield volcano, Olympus Mons (Mount Olympus), at 27 km is the highest known mountain in the SolarSystem.[83] It is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes.Olympus Mons is over three times the height of Mount Everest, which in comparison stands at just over 8.8 km.[84]

The large canyon, Valles Marineris (Latin for Mariner Valleys, also known as Agathadaemon in the old canal maps),has a length of 4,000 km and a depth of up to 7 km. The length of Valles Marineris is equivalent to the length ofEurope and extends across one-fifth the circumference of Mars. By comparison, the Grand Canyon on Earth is only446 km long and nearly 2 km deep. Valles Marineris was formed due to the swelling of the Tharsis area whichcaused the crust in the area of Valles Marineris to collapse. Another large canyon is Ma'adim Vallis (Ma'adim isHebrew for Mars). It is 700 km long and again much bigger than the Grand Canyon with a width of 20 km and adepth of 2 km in some places. It is possible that Ma'adim Vallis was flooded with liquid water in the past.[85]

Caves

THEMIS image of probable Mars cave entrances,informally named (A) Dena, (B) Chloe, (C)

Wendy, (D) Annie, (E) Abby (left) and Nikki,and (F) Jeanne.

Images from the Thermal Emission Imaging System (THEMIS) aboardNASA's Mars Odyssey orbiter have revealed seven possible caveentrances on the flanks of the Arsia Mons volcano.[86] The caves,named after loved ones of their discoverers, are collectively known asthe "seven sisters."[87] Cave entrances measure from 100 m to 252 mwide and they are believed to be at least 73 m to 96 m deep. Becauselight does not reach the floor of most of the caves, it is likely that theyextend much deeper than these lower estimates and widen below thesurface. "Dena" is the only exception; its floor is visible and wasmeasured to be 130 m deep. The interiors of these caverns may beprotected from micrometeoroids, UV radiation, solar flares and highenergy particles that bombard the planet's surface.[88]

Mars 66

Atmosphere

The tenuous atmosphere of Mars, visible on thehorizon in this low-orbit photo

Mars lost its magnetosphere 4 billion years ago,[89] so the solar windinteracts directly with the Martian ionosphere, lowering theatmospheric density by stripping away atoms from the outer layer.Both Mars Global Surveyor and Mars Express have detected theseionised atmospheric particles trailing off into space behind Mars.[89]

[90] Compared to Earth, the atmosphere of Mars is quite rarefied.Atmospheric pressure on the surface ranges from a low of 30 Pa(0.030 kPa) on Olympus Mons to over unknown operator:u','unknown operator: u','unknown operator: u',' (unknownoperator: u'strong'unknown operator: u','kPa) in the HellasPlanitia, with a mean pressure at the surface level of 600 Pa(0.60 kPa).[91] The surface pressure of Mars at its thickest is equal tothe pressure found 35 km[92] above the Earth's surface. This is less than1% of the Earth's surface pressure (101.3 kPa). The scale height of theatmosphere is about 10.8 km,[93] which is higher than Earth's (6 km)because the surface gravity of Mars is only about 38% of Earth's, aneffect offset by both the lower temperature and 50% higher average molecular weight of the atmosphere of Mars.

The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon and contains traces of oxygenand water.[6] The atmosphere is quite dusty, containing particulates about 1.5 µm in diameter which give the Martiansky a tawny color when seen from the surface.[94]

Methane map

Methane has been detected in the Martian atmosphere with a molefraction of about 30 ppb;[12] [95] it occurs in extended plumes, and theprofiles imply that the methane was released from discrete regions. Innorthern midsummer, the principal plume contained 19,000 metric tonsof methane, with an estimated source strength of 0.6 kilogram persecond.[96] [97] The profiles suggest that there may be two local sourceregions, the first centered near 30° N, 260° W and the second near 0°,310° W.[96] It is estimated that Mars must produce 270 ton/year ofmethane.[96] [98]

The implied methane destruction lifetime may be as long as about 4 Earth years and as short as about 0.6 Earthyears.[96] [99] This rapid turnover would indicate an active source of the gas on the planet. Volcanic activity,cometary impacts, and the presence of methanogenic microbial life forms are among possible sources. Methanecould also be produced by a non-biological process called serpentinization[b] involving water, carbon dioxide, andthe mineral olivine, which is known to be common on Mars.[100]

Mars 67

Climate

Mars from Hubble Space Telescope October 28,2005 with dust storm visible.

Of all the planets in the Solar System, the seasons of Mars are the mostEarth-like, due to the similar tilts of the two planets' rotational axes.The lengths of the Martian seasons are about twice those of Earth's, asMars’ greater distance from the Sun leads to the Martian year beingabout two Earth years long. Martian surface temperatures vary fromlows of about −87 °C (−125 °F) during the polar winters to highs of upto −5 °C (23 °F) in summers.[38] The wide range in temperatures is dueto the thin atmosphere which cannot store much solar heat, the lowatmospheric pressure, and the low thermal inertia of Martian soil.[101]

The planet is also 1.52 times as far from the sun as Earth, resulting injust 43% of the amount of sunlight.[102]

If Mars had an Earth-like orbit, its seasons would be similar to Earth'sbecause its axial tilt is similar to Earth's. The comparatively largeeccentricity of the Martian orbit has a significant effect. Mars is near

perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winterin the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are moreextreme and the seasons in the northern are milder than would otherwise be the case. The summer temperatures inthe south can reach up to 30 °C (86 °F) warmer than the equivalent summer temperatures in the north.[103]

Mars also has the largest dust storms in our Solar System. These can vary from a storm over a small area, to giganticstorms that cover the entire planet. They tend to occur when Mars is closest to the Sun, and have been shown toincrease the global temperature.[104]

Orbit and rotation

Mars’ average distance from the Sun is roughly230 million km (1.5 AU) and its orbital period is687 (Earth) days as depicted by the red trail, with

Earth's orbit shown in blue.(Animation)

Mars’ average distance from the Sun is roughly 230 million km (1.5AU) and its orbital period is 687 (Earth) days. The solar day (or sol) onMars is only slightly longer than an Earth day: 24 hours, 39 minutes,and 35.244 seconds. A Martian year is equal to 1.8809 Earth years, or1 year, 320 days, and 18.2 hours.[6]

The axial tilt of Mars is 25.19 degrees, which is similar to the axial tiltof the Earth.[6] As a result, Mars has seasons like the Earth, though onMars they are nearly twice as long given its longer year. Currently theorientation of the north pole of Mars is close to the star Deneb.[9] Marspassed its perihelion in April 2009[105] and its aphelion in March2010.[105] The next perihelion comes in March 2011 and the nextaphelion in February 2012.

Mars has a relatively pronounced orbital eccentricity of about 0.09; ofthe seven other planets in the Solar System, only Mercury showsgreater eccentricity. It is known that in the past Mars has had a muchmore circular orbit than it does currently. At one point 1.35 million Earth years ago, Mars had an eccentricity ofroughly 0.002, much less than that of Earth today.[106] The Mars cycle of eccentricity is 96,000 Earth yearscompared to the Earth's cycle of 100,000 years.[107] Mars also has a much longer cycle of eccentricity with a period

of 2.2 million Earth years, and this overshadows the 96,000-year cycle in the eccentricity graphs. For the last 35,000 years the orbit of Mars has been getting slightly more eccentric because of the gravitational effects of the other

Mars 68

planets. The closest distance between the Earth and Mars will continue to mildly decrease for the next 25,000years.[108]

Images comparing Mars' orbit with Ceres, a dwarf planet in the asteroid belt. The left is shown from the north ecliptic pole. The right is shown fromthe ascending node. The segments of orbits south of the ecliptic are plotted in darker colors. The perihelia (q) and aphelia (Q) are labelled with the

date of nearest passage. The orbit of Mars is red, Ceres is yellow.

Moons

Phobos in color by Mars Reconnaissance Orbiter – HiRISE, on March 23, 2008

Deimos in color on February 21, 2009 by the same (not to scale)

Mars has two relatively small natural moons, Phobos and Deimos, which orbit close to the planet. Asteroid capture is a long-favored theory but their origin remains uncertain.[109] Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology,

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accompanied their father Ares, god of war, into battle. Ares was known as Mars to the Romans.[110] [111]

From the surface of Mars, the motions of Phobos and Deimos appear very different from that of our own moon.Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outsidesynchronous orbit—where the orbital period would match the planet's period of rotation—rises as expected in theeast but very slowly. Despite the 30 hour orbit of Deimos, it takes 2.7 days to set in the west as it slowly falls behindthe rotation of Mars, then just as long again to rise.[112]

Because the orbit of Phobos is below synchronous altitude, the tidal forces from the planet Mars are graduallylowering its orbit. In about 50 million years it will either crash into Mars’ surface or break up into a ring structurearound the planet.[112]

The origin of the two moons is not well understood. Their low albedo and carbonaceous chondrite composition havebeen regarded as similar to asteroids, supporting the capture theory. The unstable orbit of Phobos would seem topoint towards a relatively recent capture. But both have circular orbits, very near the equator, which is very unusualfor captured objects and the required capture dynamics are complex. Accretion early in the history of Mars is alsoplausible but would not account for a composition resembling asteroids rather than Mars itself, if that is confirmed.A third possibility is the involvement of a third body or some kind of impact disruption.[113] More recent lines ofevidence for Phobos having a highly porous interior[114] and suggesting a composition containing mainlyphyllosilicates and other minerals known from Mars,[115] point toward an origin of Phobos from material ejected byan impact on Mars that reaccreted in Martian orbit,[116] similar to the prevailing theory for the origin of Earth'smoon. While the VNIR spectra of the moons of Mars resemble those of outer belt asteroids, the thermal infraredspectra of Phobos are reported to be inconsistent with chondrites of any class.[115]

Search for life

Viking Lander 2 site May 1979

Viking Lander 1 site February 1978.

The current understanding of planetary habitability—the ability of a world to develop and sustain life—favorsplanets that have liquid water on their surface. This most often requires that the orbit of a planet lie within thehabitable zone, which for the Sun currently extends from just beyond Venus to about the semi-major axis ofMars.[117] During perihelion Mars dips inside this region, but the planet's thin (low-pressure) atmosphere preventsliquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates theplanet's potential for habitability. Some recent evidence has suggested that any water on the Martian surface mayhave been too salty and acidic to support regular terrestrial life.[118]

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The lack of a magnetosphere and extremely thin atmosphere of Mars are a challenge: the planet has little heattransfer across its surface, poor insulation against bombardment of the solar wind and insufficient atmosphericpressure to retain water in a liquid form (water instead sublimates to a gaseous state). Mars is also nearly, or perhapstotally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals andminerals between the surface and interior of the planet.[119]

Evidence suggests that the planet was once significantly more habitable than it is today, but whether livingorganisms ever existed there remains unknown. The Viking probes of the mid-1970s carried experiments designed todetect microorganisms in Martian soil at their respective landing sites and had positive results, including a temporaryincrease of CO2 production on exposure to water and nutrients. This sign of life was later disputed by somescientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have foundlife. A re-analysis of the Viking data, in light of modern knowledge of extremophile forms of life, has suggested thatthe Viking tests were not sophisticated enough to detect these forms of life. The tests could even have killed a(hypothetical) life form.[120] Tests conducted by the Phoenix Mars lander have shown that the soil has a very alkalinepH and it contains magnesium, sodium, potassium and chloride.[121] The soil nutrients may be able to support lifebut life would still have to be shielded from the intense ultraviolet light.[122]

At the Johnson Space Center lab, some fascinating shapes have been found in the meteorite ALH84001, which isthought to have originated from Mars. Some scientists propose that these geometric shapes could be fossilizedmicrobes extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-yearvoyage to Earth. An exclusively inorganic origin for the shapes has also been proposed.[123]

Small quantities of methane and formaldehyde recently detected by Mars orbiters are both claimed to be hints forlife, as these chemical compounds would quickly break down in the Martian atmosphere.[124] [125] It is remotelypossible that these compounds may instead be replenished by volcanic or geological means such asserpentinization.[100]

Exploration

Mars 3 lander on a 1972 Soviet stamp.

Dozens of spacecraft, including orbiters, landers, and rovers, have beensent to Mars by the Soviet Union, the United States, Europe, and Japanto study the planet's surface, climate, and geology. As of 2008, theprice of transporting material from the surface of Earth to the surfaceof Mars is approximately US$309,000 per kilogram.[126]

Roughly two-thirds of all spacecraft destined for Mars have failed inone manner or another before completing or even beginning theirmissions, including the difficult late 20th century period of earlypioneers and first-timers. Mission failures are typically ascribed totechnical problems, and missions planners have to balance technologyand mission goals.[127] Failures since 1995 include Mars 96 (1996), Mars Climate Orbiter (1999), Beagle 2 (2003),and Fobos-Grunt (2011).

Past missionsThe first successful fly-by of Mars was on July 14–15, 1965, by NASA's Mariner 4. On November 14, 1971 Mariner 9 became the first space probe to orbit another planet when it entered into orbit around Mars.[128] The first objects to successfully land on the surface were two Soviet probes: Mars 2 on November 27 and Mars 3 on December 2, 1971, but both ceased communicating within seconds of landing. The 1975 NASA launches of the Viking program consisted of two orbiters, each having a lander; both landers successfully touched down in 1976. Viking 1 remained operational for six years, Viking 2 for three. The Viking landers relayed color panoramas of Mars[129] and the

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orbiters mapped the surface so well that the images remain in use.The Soviet probes Phobos 1 and 2 were sent to Mars in 1988 to study Mars and its two moons. Phobos 1 lost contacton the way to Mars. Phobos 2, while successfully photographing Mars and Phobos, failed just before it was set torelease two landers to the surface of Phobos.[130]

Following the 1992 failure of the Mars Observer orbiter, the NASA Mars Global Surveyor achieved Mars orbit in1997. This mission was a complete success, having finished its primary mapping mission in early 2001. Contact waslost with the probe in November 2006 during its third extended program, spending exactly 10 operational years inspace. The NASA Mars Pathfinder, carrying a robotic exploration vehicle Sojourner, landed in the Ares Vallis onMars in the summer of 1997, returning many images.[131]

Spirit's lander on Mars, 2004

View from the Phoenix lander, 2008

The NASA Phoenix Mars lander arrived on the north polar region of Mars on May 25, 2008.[132] Its robotic arm wasused to dig into the Martian soil and the presence of water ice was confirmed on June 20.[133] [134] [134] The missionconcluded on November 10, 2008 after contact was lost.[135]

The Dawn spacecraft flew by Mars in February 2009 for a gravity assist on its way to investigate Vesta and thenCeres.[136]

Current missionsThe NASA Mars Odyssey orbiter entered Mars orbit in 2001.[137] Odyssey's Gamma Ray Spectrometer detectedsignificant amounts of hydrogen in the upper metre or so of regolith on Mars. This hydrogen is thought to becontained in large deposits of water ice.[138]

The Mars Express mission of the European Space Agency (ESA) reached Mars in 2003. It carried the Beagle 2lander, which failed during descent and was declared lost in February, 2004.[139] In early 2004 the Planetary FourierSpectrometer team announced the orbiter had detected methane in the Martian atmosphere. ESA announced in June2006 the discovery of aurorae on Mars.[140]

In January 2004, the NASA twin Mars Exploration Rovers named Spirit (MER-A) and Opportunity (MER-B) landedon the surface of Mars. Both have met or exceeded all their targets. Among the most significant scientific returns hasbeen conclusive evidence that liquid water existed at some time in the past at both landing sites. Martian dust devilsand windstorms have occasionally cleaned both rovers' solar panels, and thus increased their lifespan.[141] SpiritRover (MER-A) was active until 2010, when it stopped sending data.On March 10, 2006, the NASA Mars Reconnaissance Orbiter (MRO) probe arrived in orbit to conduct a two-yearscience survey. The orbiter began mapping the Martian terrain and weather to find suitable landing sites forupcoming lander missions. The MRO snapped the first image of a series of active avalanches near the planet's northpole, scientists said March 3, 2008.[142]

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The Mars Science Laboratory, named Curiosity, launched on November 26, 2011, and is expected to reach Mars inAugust 2012. It is larger and more advanced than the Mars Exploration Rovers, with a movement rate of 90 m/h.Experiments include a laser chemical sampler that can deduce the make-up of rocks at a distance of 13 m.[143]

Future missionsIn 2008, NASA announced MAVEN, a robotic mission in 2013 to provide information about the atmosphere ofMars.[144] In 2018 the ESA plans to launch its first Rover to Mars; the ExoMars rover will be capable of drilling 2 minto the soil in search of organic molecules.[145]

The Finnish-Russian MetNet, is mission concept where multiple small vehicles on Mars to establish a widespreadobservation network to investigate the planet's atmospheric structure, physics and meteorology.[146] MetNet wasconsidered for a piggyback launch on the Russian Fobos-Grunt mission, but not selected.[147]

Manned mission goalsThe ESA hopes to land humans on Mars between 2030 and 2035.[148] This will be preceded by successively largerprobes, starting with the launch of the ExoMars probe[149] and a joint NASA-ESA Mars sample return mission.[150]

Manned exploration by the United States was identified as a long-term goal in the Vision for Space Explorationannounced in 2004 by then US President George W. Bush.[151] The planned Orion spacecraft would be used to senda human expedition to Earth's moon by 2020 as a stepping stone to a Mars expedition. On September 28, 2007,NASA administrator Michael D. Griffin stated that NASA aims to put a man on Mars by 2037.[152]

Mars Direct, a low-cost human mission proposed by Robert Zubrin, founder of the Mars Society, would useheavy-lift Saturn V class rockets, such as the Space X Falcon X, or, the Ares V, to skip orbital construction, LEOrendezvous, and lunar fuel depots. A modified proposal, called "Mars to Stay", involves not returning the firstimmigrant explorers immediately, if ever (see Colonization of Mars).[153]

Astronomy on Mars

Phobos transits the Sun, as seen by Mars RoverOpportunity on March 10, 2004

With the existence of various orbiters, landers, and rovers, it is nowpossible to study astronomy from the Martian skies. While Mars’ moonPhobos appears about one third the angular diameter of the full Moonas it appears from Earth, Deimos appears more or less star-like, andappears only slightly brighter than Venus does from Earth.[154]

There are various phenomena, well-known on Earth, that have beenobserved on Mars, such as meteors and auroras.[140] A transit of theEarth as seen from Mars will occur on November 10, 2084.[155] Thereare also transits of Mercury and transits of Venus, and the moonsPhobos and Deimos are of sufficiently small angular diameter that theirpartial "eclipses" of the Sun are best considered transits (see Transit ofDeimos from Mars).[156] [157]

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Viewing

Animation of the apparent retrograde motion of Mars in 2003 as seen from Earth

Because the orbit of Mars is eccentricits apparent magnitude at oppositionfrom the Sun can range from −3.0 to−1.4. The minimum brightness ismagnitude +1.6 when the planet is inconjunction with the Sun.[8] Marsusually appears a distinct yellow,orange, or reddish color; the actualcolor of Mars is closer to butterscotch,and the redness seen is just dust in theplanet's atmosphere; considering thisNASA's Spirit rover has taken picturesof a greenish-brown, mud-coloredlandscape with blue-grey rocks andpatches of light red colored sand.[158]

When farthest away from the Earth, itis more than seven times as far fromthe latter as when it is closest. Whenleast favorably positioned, it can belost in the Sun's glare for months at atime. At its most favorable times—at15- or 17-year intervals, and alwaysbetween late July and late September—Mars shows a wealth of surface detail to a telescope. Especially noticeable,even at low magnification, are the polar ice caps.[159]

As Mars approaches opposition it begins a period of retrograde motion, which means it will appear to movebackwards in a looping motion with respect to the background stars. The duration of this retrograde motion lasts forabout 72 days, and Mars reaches its peak luminosity in the middle of this motion.[160]

Closest approaches

Relative

The point Mars’ geocentric longitude is 180° different from the Sun's is known as opposition, which is near the timeof closest approach to the Earth. The time of opposition can occur as much as 8½ days away from the closestapproach. The distance at close approach varies between about 54[161] and about 103 million km due to the planets'elliptical orbits, which causes comparable variation in angular size.[162] The last Mars opposition occurred onJanuary 29, 2010. The next one will occur on March 3, 2012 at a distance of about 100 million km.[163] The averagetime between the successive oppositions of Mars, its synodic period, is 780 days but the number of days between thedates of successive oppositions can range from 764 to 812.[164]

As Mars approaches opposition it begins a period of retrograde motion, which makes it appear to move backwards ina looping motion relative to the background stars. The duration of this retrograde motion is about 72 days.

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Absolute, around the present time

Mars oppositions from 2003–2018, viewed from above the ecliptic with the Earthcentered

Mars made its closest approach to Earth andmaximum apparent brightness in nearly60,000 years, 55,758,006 km(0.372719 AU), magnitude −2.88, on 27August 2003 at 9:51:13 UT. This occurredwhen Mars was one day from oppositionand about three days from its perihelion,making Mars particularly easy to see fromEarth. The last time it came so close isestimated to have been on September 12, 57617 BC, the next time being in 2287.[165]

This record approach was only very slightlycloser than other recent close approaches.For instance, the minimum distance onAugust 22, 1924 was 0.37285 AU, and theminimum distance on August 24, 2208 willbe 0.37279 AU.[107]

An email sent during the close approach in2003 has, in succeeding years, repeatedlyspawned hoax emails saying that Mars willmake its closest approach for thousands ofyears, and will look as big as the Moon.[166]

Historical observationsThe history of observations of Mars is marked by the oppositions of Mars, when the planet is closest to Earth andhence is most easily visible, which occur every couple of years. Even more notable are the perihelic oppositions ofMars which occur every 15 or 17 years, and are distinguished because Mars is close to perihelion, making it evencloser to Earth.The existence of Mars as a wandering object in the night sky was recorded by the ancient Egyptian astronomers andby 1534 BCE they were familiar with the retrograde motion of the planet.[167] By the period of the Neo-BabylonianEmpire, the Babylonian astronomers were making regular records of the positions of the planets and systematicobservations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of thezodiac, every 79 years. They also invented arithmetic methods for making minor corrections to the predictedpositions of the planets.[168] [169]

In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicatingthe planet was farther away.[170] Ptolemy, a Greek living in Alexandria,[171] attempted to address the problem of theorbital motion of Mars. Ptolemy's model and his collective work on astronomy was presented in the multi-volumecollection Almagest, which became the authoritative treatise on Western astronomy for the next fourteencenturies.[172] Literature from ancient China confirms that Mars was known by Chinese astronomers by no later thanthe fourth century BCE.[173] In the fifth century CE, the Indian astronomical text Surya Siddhanta estimated thediameter of Mars.[174]

During the seventeenth century, Tycho Brahe measured the diurnal parallax of Mars that Johannes Kepler used to make a preliminary calculation of the relative distance to the planet.[175] When the telescope became available, the diurnal parallax of Mars was again measured in an effort to determine the Sun-Earth distance. This was first

Mars 75

performed by Giovanni Domenico Cassini in 1672. The early parallax measurements were hampered by the qualityof the instruments.[176] The only occultation of Mars by Venus observed was that of October 13, 1590, seen byMichael Maestlin at Heidelberg.[177] In 1610, Mars was viewed by Galileo Galilei, who was first to see it viatelescope.[178] The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomerChristiaan Huygens.[179]

Martian "canals"

Map of Mars by Giovanni Schiaparelli

Mars sketched as observed by Lowell sometime before 1914. (South top)

Map of Mars from Hubble Space Telescope as seen near the 1999 opposition. (North top)

By the 19th century, the resolution of telescopes reached a level sufficient for surface features to be identified. InSeptember 1877, a perihelic opposition of Mars occurred on September 5. In that year, Italian astronomer GiovanniSchiaparelli used a 22 cm telescope in Milan to help produce the first detailed map of Mars. These maps notablycontained features he called canali, which were later shown to be an optical illusion. These canali were supposedlylong straight lines on the surface of Mars to which he gave names of famous rivers on Earth. His term, which means"channels" or "grooves", was popularly mistranslated in English as "canals".[180] [181]

Influenced by the observations, the orientalist Percival Lowell founded an observatory which had a 300 and 450 mmtelescope. The observatory was used for the exploration of Mars during the last good opportunity in 1894 and thefollowing less favorable oppositions. He published several books on Mars and life on the planet, which had a greatinfluence on the public.[182] The canali were also found by other astronomers, like Henri Joseph Perrotin and LouisThollon in Nice, using one of the largest telescopes of that time.[183] [184]

The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martiansummer) in combination with the canals lead to speculation about life on Mars, and it was a long held belief thatMars contained vast seas and vegetation. The telescope never reached the resolution required to give proof to anyspeculations. As bigger telescopes were used, fewer long, straight canali were observed. During an observation in1909 by Flammarion with a 840 mm telescope, irregular patterns were observed, but no canali were seen.[185]

Even in the 1960s articles were published on Martian biology, putting aside explanations other than life for theseasonal changes on Mars. Detailed scenarios for the metabolism and chemical cycles for a functional ecosystemhave been published.[186]

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It was not until spacecraft visited the planet during NASA's Mariner missions in the 1960s that these myths weredispelled. The results of the Viking life-detection experiments started an intermission in which the hypothesis of ahostile, dead planet was generally accepted.[187]

Some maps of Mars were made using the data from these missions, but it was not until the Mars Global Surveyormission, launched in 1996 and operated until late 2006, that complete, extremely detailed maps of the martiantopography, magnetic field and surface minerals were obtained.[188] These maps are now available online, forexample, at Google Mars.

In culture

Martian geological features, such as the "Face onMars", sometimes trigger facial pareidolia

Mars is named after the Roman god of war. In different cultures, Marsrepresents masculinity and youth. Its symbol, a circle with an arrowpointing out to the upper right, is also used as a symbol for the malegender.

Mars has been featured in science fiction media, and one theme isintelligent "Martians", responsible for the speculated "canals" and"faces" on the planet. Another theme is Mars being a future colony ofthe Earth, or human expedition.The many failures in Mars exploration probes resulted in a satiricalcounter-culture blaming the failures on an Earth-Mars "BermudaTriangle", a Mars "Curse", or a "Great Galactic Ghoul" that feeds onMartian spacecraft.[127]

Intelligent "Martians"

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An 1893 soap ad playing on the popular idea thatMars was populated.

The popular idea that Mars was populated by intelligent Martiansexploded in the late 19th century. Schiaparelli's "canali" observationscombined with Percival Lowell's books on the subject put forward thestandard notion of a planet that was a drying, cooling, dying worldwith ancient civilizations constructing irrigation works.[189]

Many other observations and proclamations by notable personalitiesadded to what has been termed "Mars Fever".[190] In 1899 whileinvestigating atmospheric radio noise using his receivers in hisColorado Springs lab, inventor Nikola Tesla observed repetitive signalsthat he later surmised might have been radio communications comingfrom another planet, possibly Mars. In a 1901 interview Tesla said:

It was some time afterward when the thought flashed uponmy mind that the disturbances I had observed might bedue to an intelligent control. Although I could not deciphertheir meaning, it was impossible for me to think of them ashaving been entirely accidental. The feeling is constantlygrowing on me that I had been the first to hear the greeting of one planet to another.[191]

Tesla's theories gained support from Lord Kelvin who, while visiting the United States in 1902, was reported to havesaid that he thought Tesla had picked up Martian signals being sent to the United States.[192] Kelvin "emphatically"denied this report shortly before departing America: "What I really said was that the inhabitants of Mars, if there areany, were doubtless able to see New York, particularly the glare of the electricity."[193]

In a New York Times article in 1901, Edward Charles Pickering, director of the Harvard College Observatory, saidthat they had received a telegram from Lowell Observatory in Arizona that seemed to confirm that Mars was tryingto communicate with the Earth.[194]

Early in December 1900, we received from Lowell Observatory in Arizona a telegram that a shaft oflight had been seen to project from Mars (the Lowell observatory makes a specialty of Mars) lastingseventy minutes. I wired these facts to Europe and sent out neostyle copies through this country. Theobserver there is a careful, reliable man and there is no reason to doubt that the light existed. It wasgiven as from a well-known geographical point on Mars. That was all. Now the story has gone the worldover. In Europe it is stated that I have been in communication with Mars, and all sorts of exaggerationshave spring up. Whatever the light was, we have no means of knowing. Whether it had intelligence ornot, no one can say. It is absolutely inexplicable.[194]

Pickering later proposed creating a set of mirrors in Texas, intended to signal Martians.[195]

In recent decades, the high resolution mapping of the surface of Mars, culminating in Mars Global Surveyor,revealed no artifacts of habitation by 'intelligent' life, but pseudoscientific speculation about intelligent life on Marscontinues from commentators such as Richard C. Hoagland. Reminiscent of the canali controversy, somespeculations are based on small scale features perceived in the spacecraft images, such as 'pyramids' and the 'Face onMars'. Planetary astronomer Carl Sagan wrote:

Mars has become a kind of mythic arena onto which we have projected our Earthly hopes and fears.[181]

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Martian tripod illustration from the1906 French edition of The War of

the Worlds by H.G. Wells.

The depiction of Mars in fiction has been stimulated by its dramatic red color andby nineteenth century scientific speculations that its surface conditions not onlymight support life, but intelligent life.[196] Thus originated a large number ofscience fiction scenarios, among which is H. G. Wells' The War of the Worlds,published in 1898, in which Martians seek to escape their dying planet byinvading Earth. A subsequent US radio adaptation of The War of the Worlds onOctober 30, 1938 by Orson Welles was presented as a live news broadcast, andbecame notorious for causing a public panic when many listeners mistook it forthe truth.[197]

Influential works included Ray Bradbury's The Martian Chronicles, in whichhuman explorers accidentally destroy a Martian civilization, Edgar RiceBurroughs' Barsoom series, C. S. Lewis' novel Out of the Silent Planet(1938),[198] and a number of Robert A. Heinlein stories before themid-sixties.[199]

Author Jonathan Swift made reference to the moons of Mars, about 150 years before their actual discovery by AsaphHall, detailing reasonably accurate descriptions of their orbits, in the 19th chapter of his novel Gulliver's Travels.[200]

A comic figure of an intelligent Martian, Marvin the Martian, appeared on television in 1948 as a character in theLooney Tunes animated cartoons of Warner Brothers, and has continued as part of popular culture to the present.[201]

After the Mariner and Viking spacecraft had returned pictures of Mars as it really is, an apparently lifeless andcanal-less world, these ideas about Mars had to be abandoned and a vogue for accurate, realist depictions of humancolonies on Mars developed, the best known of which may be Kim Stanley Robinson's Mars trilogy.Pseudo-scientific speculations about the Face on Mars and other enigmatic landmarks spotted by space probes havemeant that ancient civilizations continue to be a popular theme in science fiction, especially in film.[202]

The theme of a Martian colony that fights for independence from Earth is a major plot element in the novels of GregBear as well as the movie Total Recall (based on a short story by Philip K. Dick) and the television series Babylon 5.Some video games also use this element, including Red Faction and the Zone of the Enders series. Mars (and itsmoons) were also the setting for the popular Doom video game franchise and the later Martian Gothic.

TimelineTimeline based on one by NASA Goddard Space Flight Center, as of 2011[203]

Mission Launch Note

Marsnik 1 (Mars 1960A) 10 October 1960 Attempted Mars flyby (launch failure)

Marsnik 2 (Mars 1960B) 14 October 1960 Attempted Mars flyby (launch failure)

Sputnik 22 24 October 1962 Attempted Mars flyby

Mars 1 1 November 1962 Mars flyby (contact lost)

Sputnik 24 4 November 1962 Attempted Mars lander

Mariner 3 5 November 1964 Attempted Mars flyby

Mariner 4 28 November 1964 Mars flyby

Zond 2 30 November 1964 Mars flyby (contact lost)

Zond 3 18 July 1965 Lunar flyby, Mars test vehicle

Mariner 6 25 February 1969 Mars flyby

Mariner 7 27 March 1969 Mars flyby

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Mars 1969A 27 March 1969 Attempted Mars orbiter (launch failure)

Mars 1969B 2 April 1969 Attempted Mars orbiter (launch failure)

Mariner 8 8 May 1971 Attempted Mars flyby (launch failure)

Cosmos 419 10 May 1971 Attempted Mars orbiter, lander

Mars 2 19 May 1971 Mars orbiter, attempted lander

Mars 3 28 May 1971 Mars orbiter, lander

Mariner 9 30 May 1971 Mars orbiter

Mars 4 21 July 1973 Mars flyby (attempted orbiter)

Mars 5 25 July 1973 Mars orbiter

Mars 6 5 August 1973 Mars lander (contact lost)

Mars 7 9 August 1973 Mars flyby (attempted lander)

Viking 1 20 August 1975 Mars orbiter and lander

Viking 2 9 September 1975 Mars orbiter and lander

Phobos 1 7 July 1988 Attempted Mars orbiter, Phobos landers

Phobos 2 12 July 1988 Mars orbiter, attempted Phobos landers

Mars Observer 25 September 1992 Attempted Mars orbiter (contact lost)

Mars Global Surveyor 7 November 1996 Mars orbiter

Mars 96 16 November 1996 Attempted Mars orbiter, landers

Mars Pathfinder 4 December 1996 Mars lander and rover

Nozomi (Planet-B) 3 July 1998 Mars orbiter

Mars Climate Orbiter 11 December 1998 Attempted Mars orbiter

Mars Polar Lander 3 January 1999 Attempted Mars lander

Deep Space 2 (DS2) 3 January 1999 Attempted Mars penetrators

2001 Mars Odyssey 7 April 2001 Mars orbiter

Mars Express 2 June 2003 Mars orbiter and lander

Spirit (MER-A) 10 June 2003 Mars rover

Opportunity (MER-B) 7 July 2003 Mars rover

Mars Reconnaissance Orbiter 10 August 2005 Mars orbiter

Phoenix 4 August 2007 Mars Scout lander

Fobos-Grunt 8 November 2011 Phobos lander

Yinghuo-1 8 November 2011 Mars orbiter

Mars Science Laboratory 26 November 2011 Mars rover

MAVEN 18 November 2013 (planned) Mars Scout mission orbiter

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Notesa. Best fit ellipsoidb. There are many serpentinization reactions. Olivine is a solid solution between forsterite and fayalite whose

general formula is . The reaction producing methane from olivine can be written as:Forsterite + Fayalite + Water + Carbonic acid → Serpentine + Magnetite + Methane , or (in balanced form):

→

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External links• Mars (http:/ / www. dmoz. org/ Science/ Astronomy/ Solar_System/ Planets/ Mars/ / ) at the Open Directory

Project• Mars Exploration Program (http:/ / mars. jpl. nasa. gov/ )• On Mars: Exploration of the Red Planet 1958–1978 (http:/ / history. nasa. gov/ SP-4212/ on-mars. html) from the

NASA History Office.• Mars Unearthed (http:/ / www. marsunearthed. com/ )—Comparisons of terrains between Earth and Mars• Be on Mars (http:/ / dualmoments. com/ marsrovers/ index. html)—Anaglyphs from the Mars Rovers (3D)• Mars articles in Planetary Science Research Discoveries (http:/ / www. psrd. hawaii. edu/ Archive/ Archive-Mars.

html)• Geody Mars (http:/ / www. geody. com/ ?world=mars)—World's search engine that supports NASA World Wind,

Celestia, and other applications• Mars Society (http:/ / www. marssociety. org/ )—The Mars Society, an international organization dedicated to the

study, exploration, and settlement of Mars.• NASA/JPL OnMars WMS Server for Mars Data (http:/ / onmars. jpl. nasa. gov/ )—Work as Google Earth client

overlays• New Papers about Martian Geomorphology (http:/ / ice. tsu. ru/ index. php?option=com_content&

view=category& layout=blog& id=24& Itemid=92)Media• Computer Simulation of a flyby through Mariner Valley (http:/ / www. maniacworld. com/ mars_mariner_valley.

htm)• Movie of Mars (http:/ / sos. noaa. gov/ videos/ Mars. mov) at National Oceanic and Atmospheric Administration• Flight Into Mariner Valley (http:/ / themis. asu. edu/ valles_video/ )—NASA/JPL/Arizona State University 3D

flythrough of Valles Marineris• Mars (http:/ / www. astronomycast. com/ astronomy/ episode-52-mars/ ) Astronomy Cast episode #52, includes

full transcript• 15 Amazing Pictures of the Red Planet (http:/ / www. huffingtonpost. com/ jim-bell/ pictures-of-mars_b_791652.

html#198680) – slideshow at The Huffington Post

Cartographic resources• Gazeteer of Planetary Nomenclature—Mars (USGS) (http:/ / planetarynames. wr. usgs. gov/ jsp/ SystemSearch2.

jsp?System=Mars)• PDS Map-a-planet (http:/ / pdsmaps. wr. usgs. gov/ PDS/ public/ explorer/ html/ marspick. htm)• Viking Photomap (http:/ / planetologia. elte. hu/ terkep/ mars-viking-en. pdf)• MOLA (topographic) map (http:/ / planetologia. elte. hu/ terkep/ mars-mola-en. pdf)• 3D maps of Mars in NASA World Wind (http:/ / www. worldwindcentral. com/ wiki/ Mars)• Google Mars (http:/ / www. google. com/ mars/ )—Interactive image of Mars• Ralph Aeschliman's Online Atlas of Mars (http:/ / ralphaeschliman. com/ id30. htm)

Upsilon Andromedae d 88

Upsilon Andromedae d

Upsilon Andromedae d

Extrasolar planet List of extrasolar planets

Parent star

Star Upsilon Andromedae A

Constellation Andromeda

Right ascension (α) 01h 36m 47.8s

Declination (δ) +41° 24′ 20″

Apparent magnitude (mV) 4.09

Distance 44.0 ± 0.1 ly(13.49 ± 0.03 pc)

Spectral type F8V

Mass (m) 1.28 M☉

Radius (r) 1.480 ± 0.087 R☉

Temperature (T) 6074 ± 13.1 K

Metallicity [Fe/H] 0

Age 3.3 Gyr

Orbital elements

Semimajor axis (a) 2.54 ± 0.15 AU(~380 Gm)

~188 mas

Periastron (q) 1.88 ± 0.18 AU(~282 Gm)

Apastron (Q) 3.19 ± 0.28 AU(~478 Gm)

Eccentricity (e) 0.258 ± 0.032

Orbital period (P) 1290.1 ± 8.4 d(~3.532 y)

Inclination (i) 23.8 ± 1[1] °

Argument ofperiastron

(ω) 279 ± 10°

Time of periastron (T0) 2,448,827 ± 30 JD

Semi-amplitude (K) 63.4 ± 1.5 m/s

Physical characteristics

Mass (m) 10.25[1] MJ

Discovery information

Discovery date April 15, 1999

Discoverer(s) Butler, Marcy et al.

Detection method Radial velocity

Upsilon Andromedae d 89

Discovery site California and CarnegiePlanet Search

 USA

Discovery status Published

Other designations

50 Andromedae d, Upsilon Andromedae Ad

Database references

Extrasolar PlanetsEncyclopaedia

data [2]

SIMBAD data [3]

Upsilon Andromedae d is an extrasolar planet orbiting the Sun-like star Upsilon Andromedae A. Its discovery inApril 1999 by Geoffrey Marcy and R. Paul Butler made Upsilon Andromedae the first star (other than the pulsarPSR 1257+12 and the Sun) to be known to host a multiple-planet planetary system. Upsilon Andromedae d is thethird planet from its star in order of distance and the outermost known planet in its planetary system.

DiscoveryLike the majority of known extrasolar planets, Upsilon Andromedae d was detected by measuring variations in itsstar's radial velocity as a result of the planet's gravity. This was done by making precise measurements of theDoppler shift of the spectrum of Upsilon Andromedae A. At the time of discovery, Upsilon Andromedae A wasalready known to host one extrasolar planet, the hot Jupiter Upsilon Andromedae b; however, by 1999, it was clearthat the inner planet could not explain the velocity curve.In 1999, astronomers at both San Francisco State University and the Harvard-Smithsonian Center for Astrophysicsindependently concluded that a three-planet model best fit the data.[4] The two new planets were designated UpsilonAndromedae c and Upsilon Andromedae d.

Orbit and massLike the majority of long-period extrasolar planets, Upsilon Andromedae d revolves around its star in an eccentricorbit, more eccentric than that of any of the major planets in our solar system (including Pluto).[5] The orbit'ssemimajor axis puts the planet in the habitable zone of Upsilon Andromedae A.[6]

To explain the planet's orbital eccentricity, some have proposed a close encounter with a (now lost) outer planet ofUpsilon Andromedae A. The encounter would have moved Upsilon Andromedae d into an eccentric orbit closer tothe star and ejected the outer planet from the system. Subsequently gravitational perturbations from UpsilonAndromedae d moved the inner planet Upsilon Andromedae c into its present eccentric orbit.[7] If so the rogue planetwould have had to eject immediately; it is unclear how likely this situation might be. Other models are possible.[8]

A limitation of the radial velocity method used to detect Upsilon Andromedae d is that the orbital inclination isunknown, and only a lower limit on the planet's mass can be obtained. However, by combining radial velocitymeasurements from ground-based telescopes with astrometric data from the Hubble Space Telescope, astronomershave determined the orbital inclination as well as the actual mass of Upsilon Andromedae d, which is about 10.25times the mass of Jupiter.[1]

Preliminary astrometric measurements suggest the orbit of Upsilon Andromedae d may be inclined at 155.5° to theplane of the sky.[9] However, these measurements were later proved useful only for upper limits;[10] worthless forHD 192263 b and probably 55 Cancri c, and contradict even the inner planet u And b's inclination of >30°. Themutual inclination between c and d meanwhile is 29.9 degrees.[1]

Upsilon Andromedae d 90

Characteristics

Artist's impression of Upsilon Andromedae d, portrayed as aclass II planet with water vapor clouds, as seen from a

hypothetical large moon with liquid water

Given the planet's high mass, it is likely that it is a gas giantwith no solid surface and surface gravity of over 25 times thatof Earth. Since the planet has only been detected indirectlythrough observations of its star, properties such as its radius,composition, and temperature are unknown.

Upsilon Andromedae d lies in the habitable zone of UpsilonAndromedae A as defined both by the ability for an Earthlikeworld to retain liquid water at its surface and based on theamount of ultraviolet radiation received from the star.[6]

Simulations suggest that even on eccentric orbits, terrestrialplanets may be able to support liquid water throughout theyear.[11] This suggests that if sufficiently large moons ofUpsilon Andromedae d exist, they may be able to support extraterrestrial life.

References[1] Barbara E. McArthur et al. (2010). "New Observational Constraints on the υ Andromedae system with data from the Hubble Space Telescope

and Hobby-Eberly Telescope" (http:/ / hubblesite. org/ pubinfo/ pdf/ 2010/ 17/ pdf. pdf) (PDF). The Astrophysical Journal 715 (2):1203–1220. Bibcode 2010ApJ...715.1203M. doi:10.1088/0004-637X/715/2/1203. .

[2] http:/ / exoplanet. eu/ planet. php?p1=Ups+ And& p2=d[3] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Ups+ And+ d[4] Butler et al.; Marcy, Geoffrey W.; Fischer, Debra A.; Brown, Timothy M.; Contos, Adam R.; Korzennik, Sylvain G.; Nisenson, Peter; Noyes,

Robert W. (1999). "Evidence for Multiple Companions to υ Andromedae" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 526/ 2/ 916/ 40403.html). The Astrophysical Journal 526 (2): 916–927. Bibcode 1999ApJ...526..916B. doi:10.1086/308035. .

[5] Butler, R. et al. (2006). "Catalog of Nearby Exoplanets" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 646/ 1/ 505/ 64046. html). TheAstrophysical Journal 646 (1): 505–522. arXiv:astro-ph/0607493. Bibcode 2006ApJ...646..505B. doi:10.1086/504701. . ( web version (http:/ /exoplanets. org/ planets. shtml))

[6] Buccino, A. et al. (2006). "Ultraviolet Radiation Constraints around the Circumstellar Habitable Zones". Icarus 183 (2): 491–503.arXiv:astro-ph/0512291. Bibcode 2005astro.ph.12291B. doi:10.1016/j.icarus.2006.03.007.

[7] Ford, E. et al. (2005). "Planet-planet scattering in the upsilon Andromedae system" (http:/ / simbad. u-strasbg. fr/ cgi-bin/ cdsbib?2005Natur.434. . 873F). Nature 434 (7035): 873–876. arXiv:astro-ph/0502441. Bibcode 2005Natur.434..873F. doi:10.1038/nature03427.PMID 15829958. .

[8] Rory Barnes; Richard Greenberg (2008). "Extrasolar Planet Interactions". arXiv:0801.3226v1 [astro-ph].[9] Han et al.; Black, David C.; Gatewood, George (2001). "Preliminary Astrometric Masses for Proposed Extrasolar Planetary Companions"

(http:/ / www. iop. org/ EJ/ article/ 1538-4357/ 548/ 1/ L57/ 005774. html). The Astrophysical Journal 548 (1): L57–L60.Bibcode 2001ApJ...548L..57H. doi:10.1086/318927. .

[10] Pourbaix, D. and Arenou, F. (2001). "Screening the Hipparcos-based astrometric orbits of sub-stellar objects". Astronomy and Astrophysics372 (3): 935–944. arXiv:astro-ph/0104412. Bibcode 2001A&A...372..935P. doi:10.1051/0004-6361:20010597.

[11] Williams, D., Pollard, D. (2002). "Earth-like worlds on eccentric orbits: excursions beyond the habitable zone" (http:/ / journals. cambridge.org/ action/ displayAbstract?fromPage=online& aid=105145). International Journal of Astrobiology (Cambridge University Press) 1 (01):61–69. Bibcode 2002IJAsB...1...61W. doi:10.1017/S1473550402001064. .

91

Appendix

Habitable zone

An example of a system, based on stellarluminosity for predicting the location of the

habitable zone around types of stars

In astronomy and astrobiology, the habitable zone is the region arounda star where a planet of Earth-like size, composition and atmosphericpressure can maintain liquid water on its surface.[1] Since liquid wateris essential for all known forms of life, planets in this zone areconsidered the most promising sites to host extraterrestrial life(although exotic forms of life that do not require liquid water mightpossibly exist in different kinds of environments). Alternate terms forthe habitable zone are "HZ", "life zone," "ecosphere," and "GoldilocksZone."[2]

The phrase "habitable zone" is sometimes used more generally todenote various regions that are considered favorable to life in some way. One prominent example is the Galactichabitable zone. Such concepts are inferred from the empirical study of conditions favorable for life on Earth. Ifdifferent kinds of habitable zones are considered, their intersection is the region considered most likely to containlife. To avoid ambiguity, the habitable zone as defined in the first paragraph above may then be referred to as the"circumstellar habitable zone."

The location of planets and natural satellites (moons) within its parent's star's habitable zone (and a near circularorbit) is but one of many criteria for planetary habitability. The term "Goldilocks planet" is used for any planet thatis located within the CHZ[3] [4] although when used in the context of planetary habitability the term implies terrestrialplanets with conditions roughly comparable to those of the Earth (i.e. an Earth analog). The name originates from thestory of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones thatare too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".Likewise, a planet following this Goldilocks Principle is one that is neither too close nor too far from a star to ruleout liquid water on its surface. While only about a dozen planets have been confirmed in the habitable zone, theKepler spacecraft has identified a further 54 candidates and current estimates indicate that there are "at least 500million" such planets in the Milky Way.[5]

Habitable zones, however, are not stable. Over the life of a star, the nature of the zone moves and changes.[6]

Astronomical objects located in the zone are typically close in proximity to their parent star and as such moreexposed to adverse effects such as damaging tidal forces and solar flares. Combined with galactic habitability, theseand many other exclusionary factors reinforce a contrasting theory of interstellar "dead zones" where life cannotexist, supporting the Rare Earth Hypothesis.Some planetary scientists suggests that habitable zone theory may prove limiting in scope and overly simplistic.There is growing support for equivalent zones around stars where other elements (such as methane and ammonia)could exist in stable liquid forms. Astrobiologists theorise that these environments could be conducive to alternativebiochemistry.[7] Additionally there is probably an abundance of potential habitats outside of the habitable zonewithin subsurface oceans of extraterrestrial liquid water. It may follow for similar oceans consisting of ammonia ormethane.[8]

The habitable zone is used in the Active Search for Extra-Terrestrial Intelligence as a means of selecting target stars for the transmission of interstellar radio messages (IRMs). It is supposed that should intelligence extraterrestrial life

Habitable zone 92

exists elsewhere in the universe, that it would most likely be found in a habitable zone.

Circumstellar habitable zoneBeyond the outer edge of the habitable zone, a planet will be too cold to sustain liquid water on its surface. Anywater present will freeze. A planet closer to its star than the inner edge of the habitable zone will be too hot. Anywater present will boil away or be lost into space entirely. Liquid water is considered important because carboncompounds dissolved in water form the basis of all Earthly life, so watery planets are good candidates to supportsimilar carbon-based biochemistries.Theoretical determinations of the habitable zone are based on empirical observation of the habitability of the Earthand its orbit within Solar System. Various complications must be taken into account, such as the greenhouse effectand changing albedo due to clouds.

Solar System Range estimationEstimates for the habitable zone within our own solar system range from 0.725 (the aphelion of planet Venus fallswithin this range) to 3.0 astronomical units (including the complete orbits of the planet Mars and dwarf planetCeres), based on various scientific models.Estimation is made difficult due to a number of factors, notwithstanding the planetary habitability of the Earth anddifferences observed in other planetary bodies within the the Solar System's zone (the Earth being the only SolarSystem body known to have substantial surface water).

Inner edge Outer edge References Notes

0.725 AU 1.24 AU Dole 1964[9] Used optically thin atmospheres and fixed albedos.

0.95 AU 1.01 AU Hart et al. 1978, 1979[10] stars K0 or later cannot have HZs

0.95 AU 3.0 AU Fogg 1992[11] Used Carbon cycles.

0.95 AU 1.37 AU Kasting et al. 1993[12]

– 1%–2% fartherout

Budyko 1969[13] ... and Earth would have global glaciation.

– 1%–2% fartherout

Sellers 1969[14] ... and Earth would have global glaciation.

– 1%–2% fartherout

North 1975[15] ... and Earth would have global glaciation.

4%–7%closer

– Rasool & DeBurgh1970[16]

... and oceans would never have condensed.

– – Schneider and Thompson1980[17]

disagreed with Hart.

– – Kasting 1991[18]

– – Kasting 1988[19] Water clouds can shrink HZ as they counter GHG effect with higher albedos.

– – Ramanathan and Collins1991[20]

GHG effect IR trapping is greater than water cloud albedo cooling, and Venus wouldhave to have started "Dry."

– – Lovelock 1991[21]

– – Whitemire et al. 1991[22]

Habitable zone 93

Extrasolar extrapolationAstronomers use apparent magnitude, luminosity and stellar flux along with the inverse square law to calculatehabitable zones for stars. The "center" of the HZ is defined as the distance that an exoplanet would have to be fromits parent star in order to receive the right amount of energy from the star to maintain liquid water. For example a starwith 25% of the luminosity of the Sun will have a CHZ centered at about 0.50 AU, while a star with twice the Sun'sluminosity will have a CHZ centered at about 1.4 AU.

Further habitability requirements

Flare radiation

Small stars such as red dwarfs produce much more dangerous stellar flare activity than a star the size of the Sun. Theflares would blast planets in the liquid-water-zone of red dwarfs with radiation. See Habitability of red dwarfsystems#Variability.

Tidal forces

Stars smaller than the Sun have liquid-water-zones much closer to the star so planets would experience larger tideswhich could remove axial tilt, resulting in a lack of seasons. This would lead to much colder poles and a much hotterequator, and over time the planet's water would be boiled away. It is also possible that the planet's day could besynchronized with its year, causing one-half of the planet to permanently face the star and the other half to bepermanently frozen.[23]

Galactic habitable zoneThe location of a planetary system within a galaxy must also be favorable to the development of life, and this has ledto the concept of a galactic habitable zone (GHZ),[24] [25] although the concept has been challenged.[26]

Planetary habitability theory suggests star systems favourable to life should be located close enough to the galacticcenter for sufficient levels of heavy elements to form rocky (terrestrial). (This may not preclude life existing on gasgiants or gaseous planets[27] which may be more common elsewhere, however life on gas giants (like Jupiter andSaturn) is currently considered less likely.)[28] On the other hand, the planetary system must be far enough from thegalactic center it would not be affected by dangerous high-frequency radiation, which would damage anycarbon-based life. A way for life to evolve despite these opposing requirements is that the Sun may have originatednearer the center but have migrated outwards.[29] .Also, most of the stars in the galactic center are old, unstable, dying stars, meaning few or no stars form in thegalactic center.[30] Some types of spiral galaxies in later time periods have been depleted of gas in dust in regionsnear to the galactic center, resulting in minimal new star formation in those parts of the galaxy. Because terrestrialplanets form from the same types of nebulae as stars, it can be reasoned if stars cannot form in the galactic center,terrestrial planets cannot, either.In our galaxy (the Milky Way), the GHZ is currently believed to be a slowly expanding region approximately 25,000light years (8 kiloparsecs) from the galactic core and some 6,000 light years (2 kiloparsecs) in width, containing starsroughly 4 billion to 8 billion years old. Other galaxies differ in their compositions, and may have a larger or smallerGHZ – or none at all (see: elliptical galaxy).

Habitable zone 94

Finding habitable zone planets and moonsGoldilocks planets are of key interest to researchers looking either for existing (and possibly intelligent) life or forfuture homes for the human race.[31]

The Drake equation, which attempts to estimate the likelihood of non-terrestrial intelligent life, incorporates a factor(ne) for the average number of life-supporting planets in a star system with planets. The discovery of extrasolarGoldilocks planets helps to refine estimates for this figure. Very low estimates would contribute to the Rare Earthhypothesis, which posits that a series of extremely unlikely events and conditions led to the rise of life on Earth.High estimates would reinforce the Copernican mediocrity principle, in that large numbers of Goldilocks planetswould imply that Earth is not especially exceptional.Finding Earth-sized Goldilocks planets is a key part of the Kepler Mission, which uses a space telescope (launchedon 7 March 2009 UTC) to survey and compile the characteristics of habitable-zone planets.[32] As of April 2011,Kepler has discovered 1,235 possible planets, with 54 of those candidates located within the Goldilocks zone.[33]

Discoveries in the zone

Artist's impression of Upsilon Andromedae d, portrayed as aclass II planet with water vapor clouds, as seen from a

hypothetical large moon with liquid water

The majority of planets within our planet huntingneighbourhood are located within the GHZ, therefore thesearch for "habitable" planets has focused on data indicating aplanet's position in the Goldilocks zone. The majority of theseplanets found have been gas giants, however more recentlysmaller Super-Earths and possible terrestrial planets have beendetected in the zone.

Early discoveries: Gas giants with possible moons

Although the extrasolar planet 70 Virginis b (discovered in1996) was initially nicknamed "Goldilocks" because it wasthought to be within the star's CHZ, it is now believed to becloser to its sun making it far too warm to be "just right" forlife, analogous to Venus thus it is not a Goldilocks planet.[34]

16 Cygni Bb (discovered in 1996) is a large gas giant with an eccentric orbit that was found to spend some of its timeinside the habitable zone. However the orbit means it would experience extreme seasonal effects. Despite this,simulations suggest that an Earth-like moon would be able to support liquid water at its surface over the course of ayear.[35]

Gliese 876 b (discovered in 1998) and Gliese 876 c (discovered in 2001) are both gas giants discovered in thehabitable zone around Gliese 876 although thought not to be watery may possibility have habitable moons existing inorbit.[36]

Upsilon Andromedae d (discovered in 1999) is another gas giant discovered in the habitable zone considered largeenough for the possibility of water clouds and watery moons.[37]

HD 28185 b (discovered April 4, 2001) is a gas giant was found to orbit entirely within its star's habitable zone[38]

[39] and has a low orbital eccentricity, comparable to that of Mars in our solar system.[40] However like earlierdiscoveries it is a gas giant. Tidal interactions suggest that HD 28185 b could harbor Earth-mass satellites in orbitaround it for many billions of years.[41] Such moons, if they exist, may be able to provide a habitable environment,though it is unclear whether such satellites would form in the first place.[42]

55 Cancri f (discovered in 2005), a Jupiter like gas giant exoplanet, orbits and also resides within the yellow dwarf star companion of 55 Cancri binary star systems habitable zone.[43] While conditions upon this massive and dense

Habitable zone 95

planet are not conducive to the formation of water or for that matter biological life as we know it, the potential existsfor a system of satellite moons to be orbiting the planet and thus transiting through this zone and being conducive forbiological development.

Recent breakthroughs: Super-Earths and terrestrials

The Gliese 581 system (first discovered in 2005) has a set of slightly oversized terrestrial planets mirroring our ownsolar system's. The third planet, planet c (discovered in 2007), is expected to be analogous to Venus's position(slightly too close), the fourth planet g (unconfirmed as of Oct. 2010) to the Earth/Goldilocks position, and the fifthplanet d (discovered in 2007) to the Mars position. Planet d may be too cold, but unlike Mars, it is several timesmore massive than Earth and may have a dense atmosphere to retain heat. One caveat with this system is that itorbits a red dwarf, probably resulting in most of the issues regarding habitability of red dwarf systems, such as all theplanets likely being tidally locked to the star.GJ 1214 b (December 16, 2009), though just outside of the habitable zone, does provide indications of being anocean planet, meaning it is believed to be an extrasolar planet of the superearth variety, surrounded by a deep liquidocean of water.On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planetcandidates, including 54 that may be in the "Habitable Zone."[44] [45] [46] [47] Six candidates (KOI 326.01, KOI701.03, KOI 268.01, KOI 1026.01, KOI 854.01, KOI 70.03) in the "Habitable Zone" are listed as smaller than twicethe size of Earth,[47] although the one which got the most attention as "Earth-size" (KOI 326.01) turns out to be infact much larger.[48] A September 2011 study by Muirhead et al reports that a re-calibration of estimated radii andeffective temperatures of several dwarf stars in the Kepler sample yields six additional Earth-sized candidates withinthe habitable zones of their stars: KOI 463.01, KOI 1422.02, KOI 947.01, KOI 812.03, KOI 448.02, KOI1361.01.[49] Based on these latest Kepler findings, astronomer Seth Shostak estimates that "within a thousandlight-years of Earth" there are "at least 30,000 of these habitable worlds."[50] Also based on the findings, the KeplerTeam estimates "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitablezone.[5]

HD 85512 b (discovered in 2011) is believed to be an Earth-like planet in the HD 85512 system.Kepler-22b, confirmed December 5, 2011.[51] , one of the first likely terrestrial planets detected in the habitable zoneof a sun-like main sequence star using the transit method. Kepler 22-b is a super-Earth (2.4 times the size of Earth).With an atmosphere, the estimated surface temperature is around 22 degrees Celsius (-11 without an atmosphere).

Criticism• The concept of a habitable zone is criticized by Ian Stewart and Jack Cohen in their book Evolving the Alien, for

two reasons: the first is that the hypothesis assumes alien life has the same requirements as terrestrial life; thesecond is that, even assuming this, other circumstances may result in suitable planets outside the "habitable zone".For instance, Jupiter's moon Europa is thought to have a subsurface ocean with an environment similar to the deepoceans of Earth. The existence of extremophiles (such as the tardigrades) on Earth makes life on Europa seemmore plausible, despite the fact that Europa is not in the presumed CHZ. Astronomer Carl Sagan believed that lifewas also possible on the gas giants, such as Jupiter itself. A discovery of any form of life in such an environmentwould expose these hypothetical restrictions as too conservative. Life can evolve to tolerate extreme conditionswhen the relevant selection pressures dictate, and thus it is not necessary for them to be "just right".[52]

• Differing levels of volcanic activity, lunar effects, planetary mass, and even radioactive decay may affect theradiation and heat levels acting on a planet to modify conditions supporting life. And while it is likely that Earthlife could adapt to an environment like Europa's, it is far less likely for life to develop there in the first place, or tomove there and adapt without advanced technology. Therefore, a planet that has moved away from a habitablezone is more likely to have life than one that has moved into it.[53]

Habitable zone 96

• Scientists describe extensive computer simulations in the Astrophysical Journal[54] that show that, at least ingalaxies similar to our own Milky Way, stars such as the Sun can migrate great distances, thus challenging thenotion that parts of these galaxies are more conducive to supporting life than other areas.[55]

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[30] Solstation - Habitable (http:/ / www. solstation. com/ habitable. htm)[31] Joe Palca. "'Goldilocks' Planet's Temperature Just Right For Life" (http:/ / www. npr. org/ templates/ story/ story. php?storyId=130215192).

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[36] Sudarsky, David et al. (2003). "Theoretical Spectra and Atmospheres of Extrasolar Giant Planets" (http:/ / iopscience. iop. org/ 0004-637X/588/ 2/ 1121/ fulltext). The Astrophysical Journal 588 (2): 1121–1148. arXiv:astro-ph/0210216. Bibcode 2003ApJ...588.1121S.doi:10.1086/374331. .

[37] Williams, D., Pollard, D. (2002). "Earth-like worlds on eccentric orbits: excursions beyond the habitable zone" (http:/ / journals. cambridge.org/ action/ displayAbstract?fromPage=online& aid=105145). International Journal of Astrobiology (Cambridge University Press) 1 (01):61–69. Bibcode 2002IJAsB...1...61W. doi:10.1017/S1473550402001064. .

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[42] Canup, R., Ward, W. (2006). "A common mass scaling for satellite systems of gaseous planets". Nature 441 (7095): 834 – 839.doi:10.1038/nature04860. PMID 16778883. http:/ / www. nature. com/ nature/ journal/ v441/ n7095/ abs/ nature04860. html.

[43] http:/ / www. scientificamerican. com/ " Exoplanets in the habitable zone"[44] "NASA Finds Earth-size Planet Candidates in Habitable Zone, Six Planet System" (http:/ / kepler. nasa. gov/ news/ nasakeplernews/ index.

cfm?FuseAction=ShowNews& NewsID=98). NASA (http:/ / www. nasa. gov/ ). 2011-02-02. . Retrieved 2011-02-02.[45] Borenstein, Seth (2 February 2011). "NASA spots scores of potentially livable worlds" (http:/ / msnbc. msn. com/ id/ 41387915/ ns/

technology_and_science-space). MSNBC News. . Retrieved 2011-02-02.[46] Overbye, Dennis (2 February 2011). "Kepler Planet Hunter Finds 1,200 Possibilities" (http:/ / www. nytimes. com/ 2011/ 02/ 03/ science/

03planet. html). New York Times. . Retrieved 2011-02-02.[47] Borucki, William J.; Koch, David G; Basri, Gibor; Batalha, Natalie; Brown, Timothy M.; et. al. (1 February 2011). "Characteristics of

planetary candidates observed by Kepler, II: Analysis of the first four months of data". arXiv:1102.0541 [astro-ph.EP].[48] Grant, Andrew (8 March 2011`). "Exclusive: "Most Earth-Like" Exoplanet Gets Major Demotion—It Isn’t Habitable" (http:/ / blogs.

discovermagazine. com/ 80beats/ 2011/ 03/ 08/ exclusive-most-earth-like-exoplanet-gets-major-demotion�it-isnt-habitable/ ). 80beats (http:// blogs. discovermagazine. com/ 80beats). Discover Magazine. . Retrieved 2011-03-09.

[49] http:/ / arxiv. org/ abs/ 1109. 1819[50] Shostak, Seth (3 February 2011). "A Bucketful of Worlds" (http:/ / www. huffingtonpost. com/ seth-shostak/

a-bucketful-of-worlds_b_817921. html). Huffington Post. . Retrieved 2011-02-03.[51] BBC NEWS, "Kepler 22-b: Earth-like planet confirmed" 12/5/2011 http:/ / www. bbc. co. uk/ news/ science-environment-16040655[52] Evolving the Alien by Ian Stewart and Jack Cohen[53] Planets for Man (http:/ / rand. org/ pubs/ commercial_books/ 2007/ RAND_CB183-1. pdf)[54] Rok Roškar, Victor P. Debattista, Thomas R. Quinn, Gregory S. Stinson, and James Wadsley, Riding the Spiral Waves: Implications of

Stellar Migration for the Properties of Galactic Disks, Astrophysical Journal Letters, Volume 684, Number 2, 2008 September 10 (http:/ /arxiv. org/ abs/ 0808. 0206v1)

[55] Immigrant Sun: Our Star Could be Far from Where It Started in Milky Way (http:/ / newswise. com/ articles/ view/ 544325/ ) Newswise,Retrieved on September 15, 2008.

External links• The Habitable Zone Gallery (http:/ / www. hzgallery. org/ )• The Encyclopedia of Astrobiology, Astronomy and Spaceflight (Habitable Zone) (http:/ / www. daviddarling.

info/ encyclopedia/ H/ habzone. html)• The Encyclopedia of Astrobiology, Astronomy and Spaceflight (Galactic Habitable Zone) (http:/ / www.

daviddarling. info/ encyclopedia/ G/ galactic_habitable_zone. html)• "Stars and Habitable Planets" at SolStation (http:/ / www. solstation. com/ habitable. htm)• Nikos Prantzos (2006). "On the Galactic Habitable Zone". Space Science Reviews 135: 313–322.

arXiv:astro-ph/0612316. Bibcode 2008SSRv..135..313P. doi:10.1007/s11214-007-9236-9.• Swiss Scientist: Search for Life Next (http:/ / www. washingtonpost. com/ wp-dyn/ content/ article/ 2007/ 04/ 25/

AR2007042501974. html)

Habitable zone 98

• Stephen H. Dole and Isaac Asimov's Planets for Man (http:/ / www. rand. org/ pubs/ commercial_books/ 2007/RAND_CB183-1. pdf) Defining what environment is habitable for people. (copyright 1964)

• NASA: The Goldilocks Zone (http:/ / science. nasa. gov/ headlines/ y2003/ 02oct_goldilocks. htm)• Interstellar Real Estate: Location, Location, Location – Defining the Habitable Zone (http:/ / btc. montana. edu/

ceres/ astrobiology/ files/ HabitableZone. htm)• Definition of "goldilocks" connoting "moderate characteristics" and examples referring to planets dating to 1935.

(http:/ / www. doubletongued. org/ index. php/ dictionary/ goldilocks/ )• "Exoplanet Habitable Zone Candidates: exoplanets in terms of their historical chances for residing in the habitable

zone" (http:/ / www. planetarybiology. com/ hz_candidates/ ). www.planetarybiology.com.• "Exoplanets in relation to host star's current habitable zone" (http:/ / www. planetarybiology. com/

exoexplorer_planets/ ). www.planetarybiology.com.• "exoExplorer: a free Windows application for visualizing exoplanet environments in 3D" (http:/ / www.

planetarybiology. com/ exoexplorer/ ). www.planetarybiology.com.• "Calculating the location of habitable zone at zero age main sequence (ZAMS)" (http:/ / www. planetarybiology.

com/ downloads/ astronomical_circumstances_06. pdf). www.planetarybiology.com. Retrieved 2008-11-10.• Shiga, David (2008-11-19). "Why the universe may be teeming with aliens" (http:/ / www. newscientist. com/

article/ mg20026831. 600-why-the-universe-may-be-teeming-with-aliens. html?full=true). Space. NewScientist.Retrieved 2009-11-19.

• Seager, Sara; Ford; Turner (2002). "Characterizing Earth-like planets with Terrestrial Planet Finder".arXiv:astro-ph/0212551 [astro-ph].

• "Kepler Mission to Hunt for Earth-like Planets" (http:/ / science. nasa. gov/ headlines/ y2009/ 20feb_kepler. htm).Science@NASA. February 20, 2009. Retrieved 2010-03-31.

• Simmonsa et al. "The New Worlds Observer: a mission for high-resolution spectroscopy of extra-solar terrestrialplanets" (http:/ / newworlds. colorado. edu/ info/ documents/ NewWorldsObserver2004. pdf). New Worlds.Retrieved 2010-03-31.

• Cockell et al. (2009). "Darwin—an experimental astronomy mission to search for extrasolar planets" (http:/ /www. springerlink. com/ content/ c18m4q1487204x85/ fulltext. pdf). Springerlink. pp. 435–461.doi:10.1007/s10686-008-9121-x.

• Atkinson, Nancy (March 19, 2009). "JWST Will Provide Capability to Search for Biomarkers on Earth-likeWorlds" (http:/ / www. universetoday. com/ 2009/ 03/ 19/jwst-will-provide-capability-to-search-for-biomarkers-on-earth-like-worlds/ ). Universe Today. Retrieved2010-03-31.

• "PlanetQuest: Mission" (http:/ / planetquest. jpl. nasa. gov/ missions/ simPQMission. cfm). SIM PlanetQuest.Retrieved 2010-03-31.

Article Sources and Contributors 99

Article Sources and ContributorsEarth analog  Source: http://en.wikipedia.org/w/index.php?oldid=464339968  Contributors: Bearcat, BlueEarth, Cjmclark, EvenGreenerFish, Fierokato, Glennwells, Lowellian, Qoncept, RJHall,Serendipodous, Sharemind, Trulystand700, Xybernauts, 2 anonymous edits

16 Cygni Bb  Source: http://en.wikipedia.org/w/index.php?oldid=464329633  Contributors: Aldaron, Aldebaran66, BlueEarth, Bryan Derksen, Chaos syndrome, Chermundy, Colonies Chris,Cuddlyopedia, EvenGreenerFish, FKmailliW, Harbingerdawn, Icalanise, James McBride, Kheider, Ling.Nut, Materialscientist, Mike s, NuclearVacuum, Poppy, Rich Farmbrough, Rjwilmsi,Telescopi, TenPoundHammer, Tyrogthekreeper, Xenni, Zimriel, 5 anonymous edits

23 Librae b  Source: http://en.wikipedia.org/w/index.php?oldid=464329678  Contributors: Aldaron, BlueEarth, Bryan Derksen, EvenGreenerFish, Hurricane Devon, Icalanise, Jolielegal, Mike s,Rjwilmsi, Telescopi, The Mysterious El Willstro, Tyrogthekreeper, 2 anonymous edits

55 Cancri f  Source: http://en.wikipedia.org/w/index.php?oldid=464329747  Contributors: A8UDI, Anarchist42, Bender235, Bensin, BlueEarth, Borgx, Bryan Derksen, Chermundy, Csmiller,Cuddlyopedia, Dana boomer, Darklilac, Drtony999, EvenGreenerFish, GabrielVelasquez, GwydionM, Headbomb, Icalanise, J. Langton, James McBride, Kastchei, Kheider, Kugellager,Materialscientist, Mike s, Murgh, Mzmadmike, Nergaal, Nibios, Nickshanks, NuclearVacuum, Ohconfucius, Rainbow87, Rich Farmbrough, Tabletop, Telescopi, TenPoundHammer, TheMysterious El Willstro, 13 anonymous edits

Gliese 581 c  Source: http://en.wikipedia.org/w/index.php?oldid=464261757  Contributors: 44Dume, 7yl4r, 84user, A Softer Answer, A7x, Aboyall, Acalamari, Adam McMaster, AdamM,Agathoclea, Ahruman, Alansohn, Aldaron, Allstarecho, Altmany, AndrewBuck, AndrewRT, AndyHuston, Angr, Aranae, Aranherunar, Arb, Argo Navis, Arrenlex, ArthurWeasley, Astor14,Atlasdude2, Attilios, Aurelia19, BMF81, Baba O RLY, Barbara Shack, Bartschlabs, Belegor, Ben Ram, BenWoodruff, Bender235, Benhocking, Beyond silence, BlaiseFEgan, BlueEarth,Bluebird47, BlytheG, Bob A, BobGreenwade, Boxaa35aa, Brainpower111, Brighterorange, Bryan Derksen, C.Fred, CRGreathouse, Campaigner80, Camw, Cbdorsett, Cbrown1023, Chaossyndrome, Chase me ladies, I'm the Cavalry, Chermundy, Chocolateboy, Chrisdicknson, Chtito, Chuck Sirloin, CieloEstrellado, Ckatz, CloudNine, Coelacan, CommonsDelinker, Conor Wright,Cop 663, Cremepuff222, Crobichaud, Cyclopia, DARTH SIDIOUS 2, DDima, DJ Sturm, Danilot, Danjel, Darry2385, Dave101, Davin, Dawnseeker2000, Dbachmann, Debivort, Djscottish,Dorftrottel, Dr Henry Draper, Dr.Franich, Dr.kwan, DrBob, DragonflySixtyseven, Dub8lad1, Długosz, Echtner, EdGl, Edward, Egumtow, Elb2000, Electron9, Elliskev, Email4mobile,Enaidmawr, Ernest lk lam, Eschbaumer, Esseh, Evand, EvenGreenerFish, Feezo, Fletcher, Fram, Francis Tyers, Fusion7, Fæ, GabrielVelasquez, Gaius Cornelius, GalaxiaGuy, Garion96,Garrytowns, Gekritzl, Gene Nygaard, Get123, Giftlite, Gilliam, Gioto, Glycerinester, Grika, Gspr, Gu1dry, Gyrobo, Hawkania, Headbomb, Hellisp, Henrykus, HereToHelp, Hermitage17,Hibernian, Hiberniantears, HighSimSim, HumphreyW, Hussain24, Icalanise, Iisthphir, Imagehave, Innonexess, Interchange88, J. Langton, J.delanoy, JQF, Jacfed5, Jacob Lundberg, Jammus,JayMan, Jcmenal, Jcrook1987, Jefffire, Jeffmedkeff, Jepaan, JerryFriedman, JesseRafe, John, John254, Jonel, Jordan.Kreiger, JorisvS, Joseph Solis in Australia, JoshuaZ, Jpsowin, Jrockley,Jugbug, Jyril, KGasso, KGyST, Kablammo, Katalaveno, Kazooou, Kbrown69, Kdmichel, Kendrick7, Kenyon, Kevin Nelson, Killing Vector, King Hildebrand, King Vegita, Korean alpha forknowledge, Krunchy, Kuru, Kwamikagami, L Kensington, L1A1 FAL, LERK, Lantzy, Latitude0116, Lazlonaruto, Leftus, Lemonflash, Liftarn, Logical Premise, Lord mrazon, Lucianomendez,Luna Santin, M999j, MECU, MER-C, Madhava 1947, Majoreditor, Makeemlighter, Mandavi, Marcelo Pinto, Marhawkman, Markus Kuhn, Marskell, MartinLing, Mattym129, Maury Markowitz,Maxaddict, Mdf, Meggar, Mejor Los Indios, Mic of orion, Michaelbusch, Michaelritchie200, Mike s, MisterSheik, Mithridates, Mixojapan, Mkhbgpa, MonteChristof, Mr Stephen, MrZap, Msp0,Mxn, N-true, NO NO NO E-Z-GO!, NatureA16, Nedlum, Neo-Jay, Nergaal, Newseditor, Nexcet, Nikopoley, Nimwe, Nomadhacker, Noobeditor, Novangelis, NuclearVacuum, NuclearWarfare,Oblivious, OhanaUnited, Oliphaunt, Optakeover, Ourai, Ouzo, Owlbuster, Oxymoron83, Pafcool2, Pagrashtak, Patricius Augustus, PedroPVZ, Peter Karlsen, Pharos, Phil1988, Pinkville, Piotrus,Pmanderson, Pnutbutter123, Pokrajac, Potatoswatter, PresN, Proxima Centauri, Qaletaqa, Quasso, Qurq, R'n'B, RWyn, Rdfox 76, Reaper X, Redtigerxyz, Refsworldlee, Reswik, Reyk, RichFarmbrough, Rich.lewis, Ricnun, Rjwilmsi, Rmhermen, Robma, RockMFR, Rursus, Russoc4, SGGH, Saforrest, Saintrain, Sarenne, Schmalls, SchmuckyTheCat, Serendipodous, Sethnk, Shadowdemon, Shawn81, Shijualex, Signa727, SimonATL, Sirex98, Skatebiker, Skizzik, Slashme, Slipperyweasel, Snagglepuss, Somebody500, Sophie means wisdom, Soumyasch, Spacepotato,Spitfire, Spytox, Stephenchou0722, Stereo, Str1977, Stygian Shade, Swid, Sylvain1972, Tabletop, Tainter, TeaDrinker, TehamazingSpiderMan, Telescopi, Temurjin, Teveten, Tgun-mk2,Th1rt3en, The Dark, The Land, The Mysterious El Willstro, The Thing That Should Not Be, The freddinator, TheOtherSiguy, Themanwithoutapast, Thingg, Thrig, Thue, Tom,Transcendentalstate, Turangalila, Tuttt, Tyrogthekreeper, UBeR, Unexpect, Usul.riddle, Utcursch, Vagary, Vaniac, Vanished User 0001, VanishedUser314159, Vegasprof, Versus22, Viriditas,Visor, Vorash2000, Vsst, W1k13rh3nry, Wafulz, Wasted Sapience, Waywardhorizons, Wentideas, Wereon, Whereizben, WikiCantona, Wikisteff, Winslo2260, Wintran, Wolfger, WolfmanSF,Woohookitty, Writtenright, Xiaphias, YanA, Ykliu, Ylai, Zazaban, Zimriel, Zntrip, Zzyzx11, Александр Мотин, 529 anonymous edits

Gliese 581 d  Source: http://en.wikipedia.org/w/index.php?oldid=464365001  Contributors: 1234r00t, 2over0, A7x, Acalamari, Aldaron, Alex contributing, Andrewkantor, Angr, Arb, ArdentV,Arthur Rubin, Ashmoo, Azazelvis, B.Lameira, Bender235, Bensin, Blmpxcvd, BlueEarth, Cessator, Chaos syndrome, Chermundy, Cpl Syx, DavidSJ, Davin, Dead Fish Jr, Debivort, Dr HenryDraper, Drapik, Drilnoth, Emesee, Epbr123, EvenGreenerFish, Everyking, Fosnez, Fusion7, GabrielVelasquez, Gadren, Gilgamesh 42, Gioto, Glycerinester, Gwen Gale, Halojedi20, Headbomb,Hiberniantears, HumphreyW, Icalanise, IvoShandor, J. Langton, JerryFriedman, Jmturner, Jonathunder, JorisvS, Jrockley, Justsomerunner, Jyril, KJG2007, Kevin Nelson, Kheider, Kurihaya,Kwamikagami, Malljaja, Mejor Los Indios, Mer78749, Mightymights, Mimihitam, Modify, Murgh, Nergaal, Newone, Ngorongoro, Northgrove, NuclearVacuum, Pallab1234, Pauli133, PeteyParrot, Petri Krohn, Pine, PresN, Proxima Centauri, Quantanew, Rajczek, Rich.lewis, Ricnun, RodC, Rogermw, Rory096, Ruohao, Rursus, Russell.Sancto, Scewing, Shouriki, Spacepotato,Susurrus, Telescopi, The Anome, Themanwithoutapast, Tom soldier, Tyler, Tyrogthekreeper, Vaniac, Vyznev Xnebara, Wjfox2005, Writtenright, Wurdnurd, Yliannos, Zedla, Zerbu, Zimriel, 87anonymous edits

Gliese 581 g  Source: http://en.wikipedia.org/w/index.php?oldid=464340313  Contributors: Acalamari, Aceofhearts1968, Alan Roddis, Aldaron, Alpha Ralpha Boulevard, Alyeska2112,Anypodetos, Art LaPella, Athenean, Autonova, Avarince, Bdell555, Beaber, Bender235, Bensin, BlueEarth, Bongwarrior, Breawycker, C628, CJISBEAST, Cbdorsett, Chasrob, ChiZeroOne,Cliff, Coekon, Colonies Chris, CommonsDelinker, Courcelles, Cuaxdon, DH85868993, Daniel Case, Danust, Darth Wombat, Darthvalium, DidgeGuy, Dlorp, Dmarquard, Doodledevil,Dougofborg, Dr.Magellan, Drbogdan, Drtony999, Ecofuninthesun, Edoe, Electro101, Email4mobile, EvenGreenerFish, Ferinex, GabrielVelasquez, Gekritzl, Giancarlo Rossi, Gimelthedog,Glacialfox, Glenn, Graywyvern, Gtking619, Gwen Gale, Hammersoft, Harbingerdawn, Headbomb, Hibernian, Hiberniantears, Horologium, Icalanise, JElliots, JQF, Jeffq, Jillids, JorisvS,Joshrulzz, Kanags, Katherine, Kbrose, Kevin Nelson, Kevlar67, Koppapa, Ktr101, Lajsikonik, Lectonar, Liftarn, Lorax2000, Maasje, Macarenses, Martin Cash, Matt The Tuba Guy,MayFlowerNorth, Medeis, Micru, Mtu, Murgh, Nanobear, Narayanese, Nathan Johnson, Neumannk, Nivenus, NocturneNoir, NuclearWarfare, Olegwiki, Ouro, Panchurret, Petersam, Philg88,Piotrus, Pol098, Quantanew, RJHall, Rjwilmsi, Roentgenium111, Ryulong, SDC, SaltedCracker, Seansinc, Seminkjole, SlowJog, SolarMcPanel, Sopher99, Spacepotato, Spy007au, Stackedaktor,Stephan Schulz, Styath, Surajt88, Tbhotch, Terra Novus, The shaggy one, Thingg, ThomasStrohmann, Tigerghost, Tinss, Tomruen, Trekphiler, Treybien, Trulystand700, Viriditas, Vrenator,Waldir, Wikipelli, William M. Connolley, WolfmanSF, YOKOTA Kuniteru, Yug, Zerbu, Гатерас, 126 anonymous edits

Gliese 876 b  Source: http://en.wikipedia.org/w/index.php?oldid=464329799  Contributors: Aldaron, Alpha Centaury, AndrewBuck, Benhocking, BlueEarth, Bryan Derksen, CarloscomB, Chaossyndrome, Chermundy, Cuddlyopedia, EvenGreenerFish, Fotaun, Fusion7, Headbomb, Hurricane Devon, Icalanise, JohnVanVliet, Joshua Scott, Kozuch, Mike s, Nehrams2020, Nick,NuclearVacuum, PresN, Retired username, Rjwilmsi, Telescopi, TomMadigan, Zimriel, 4 anonymous edits

Gliese 876 c  Source: http://en.wikipedia.org/w/index.php?oldid=464329835  Contributors: Aldaron, Benhocking, BlueEarth, Bryan Derksen, CarloscomB, Chaos syndrome, Chermundy,Cuddlyopedia, Eluchil404, EvenGreenerFish, Headbomb, Hurricane Devon, Icalanise, JohnVanVliet, Joshua Scott, Kozuch, Materialscientist, Mike s, Nehrams2020, NuclearVacuum, PresN,Proxima Centauri, Retired username, Rjwilmsi, Telescopi, 3 anonymous edits

HD 23127 b  Source: http://en.wikipedia.org/w/index.php?oldid=464329995  Contributors: BlueEarth, Epinedo, EvenGreenerFish, Icalanise, Mike s, NuclearVacuum, Rjwilmsi, Telescopi

HD 28185 b  Source: http://en.wikipedia.org/w/index.php?oldid=464330043  Contributors: ArgGeo, B.Lameira, Babajobu, Bananas21ca, Benhocking, Bensin, BlueEarth, Bryan Derksen,CBC11, Chaos syndrome, Colonies Chris, EvenGreenerFish, GabrielVelasquez, Gene Nygaard, Girls gone docile, Hermógenes Teixeira Pinto Filho, Hurricane Devon, Icalanise, Jeff G.,Jswhitten, Kcordina, Koavf, Kozuch, Maf654321, Materialscientist, Mike s, Nehrams2020, Nick, Nickshanks, Night Gyr, Northgrove, NuclearVacuum, O RLY?, OverlordQ, Reyk, Rjwilmsi,Samsara, Spacepotato, StAnselm, SunCreator, Systemofadown823, Telescopi, Wafulz, X96lee15, 21 anonymous edits

HD 37124 b  Source: http://en.wikipedia.org/w/index.php?oldid=462310450  Contributors: BlueEarth, C.Fred, Censor, EvenGreenerFish, Headbomb, Icalanise, Lizard6543, Mike s,NuclearVacuum, Pok148, Rjwilmsi, Telescopi, Woohookitty, Zil, 2 anonymous edits

HD 69830 d  Source: http://en.wikipedia.org/w/index.php?oldid=464329938  Contributors: 123dylan456, A7x, B.Lameira, BlueEarth, Chermundy, EvenGreenerFish, Gfoley4, Icalanise,Jc-S0CO, Kheider, Lazlonaruto, Mike s, Newone, NuclearVacuum, Opacic, PresN, Rjwilmsi, Rpyle731, Ruslik0, Telescopi, TheOtherSiguy, Wind50, Xanzzibar, 19 anonymous edits

HD 85512 b  Source: http://en.wikipedia.org/w/index.php?oldid=464364873  Contributors: Aleksa Lukic, ArdWar, Bender235, BlueEarth, Chermundy, Dc987, DiverDave, EvenGreenerFish,GeirThomasAndersen, Harley peters, Headbomb, Icalanise, Khayman, Koppapa, Mightymights, Mokgen, Mortense, Newone, Nishanoire, Porturology, RL0919, Serge925, Starkiller88, Surajt88,Trulystand700, Woohookitty, 33 anonymous edits

Kepler-22b  Source: http://en.wikipedia.org/w/index.php?oldid=464393720  Contributors: Article editor, Bender235, Caltas, Ciphers, Dmytro, Electron9, EvenGreenerFish, Fanyavizuri, Galinkin, George100, Gravitophoton, GwydionM, Hangfromthefloor, Hibernian, HladnorukiLuka, Hunnjazal, Jamsta, Jan Baykara, Kanags, KirtZJ, Leicu, MathewDill, Meggaluvva,

Article Sources and Contributors 100

Mycroft.Holmes, OberRanks, Orlady, Ottawakismet, Raksidelic, Roentgenium111, Samcashion, Sigmundur, Spartan S58, Up Tack, Wer900, Гатерас, 49 anonymous edits

Mars  Source: http://en.wikipedia.org/w/index.php?oldid=464386705  Contributors: -- April, -xwingsx-, 0x6D667061, 10metreh, 11warnjames, 12345tyuio, 129.128.164.xxx, 17Drew, 1exec1, 84user, A. B., A.T.M.Schipperijn, A314268, A3RO, ABTU, AMK1211, AOEU, ARC Gritt, AWESOME-Odude, Aaaa123, Aaarrrggh, Aardark, Aaronbwsm, Abductive, Acalamari, Ace45954, Aces lead, Action Jackson IV, Adam Bishop, Adamwankenobi, Adashiel, Addshore, Aelffin, Aeon1006, Aeonx, Aeusoes1, Agent003, AghastAmok, Agordhandas12788, Ahoerstemeier, Aillema, Aim Here, Airplaneman, AjaxSmack, Ajraddatz, Akademy, Akira112, Aksi great, Alai, Aldo12xu, Aldude14, Ale jrb, AlefZet, Alegoo92, Alex.muller, AlexiusHoratius, Alexvickers, Alexwcovington, Allstarecho, AllyUnion, AlphaEta, Alrasheedan, Altonbr, Alvez3, American2, AmiDaniel, Amorymeltzer, Anchoress, Anders Feder, Andonic, Andrew Hampe, Andrew the coolest, Andrewpmk, Android Mouse, Andy Johnston, Andy M. Wang, Andy120290, AndySmith84, Angie023, Angmering, AnimAlu, Animum, Anixea, Anonymous Dissident, Answerthis, Antandrus, Anthony, Anton Mravcek, Antonio Lopez, Apexfreak, Apollo2011, ApostleJoe, Aquilosion, Aranherunar, Arctic.gnome, Ariel., Arivu jevi, Arj, Arjun01, Arno, Arsonal, Art LaPella, ArthurWeasley, Ascidian, Ashmoo, Aster2, Asteron, Astrobhadauria, Astrophil, Astudent, Athenean, Aubadaurada, Aude, Audin, Aussie895, Avenged Eightfold, AvicAWB, Avoided, Avram Fawcett, Awolf002, Ayleuss, Azzjiggla, B, B.d.mills, B2012ball21, BD2412, BGManofID, BRG, Babbler, BaconBoy914, Bacteria, Bad Astronomer, Badanedwa, Baldhur, Bamf, BanyanTree, Bart133, BatteryIncluded, BeaverWithChainsaw, Bedford, Beerdola, Begoon, Beland, Belar, Belekvor, Ben MacDui, Benbest, Bender235, Benhocking, Bennish, Beowulf king, Berek, Bergsten, Bernard192, BerserkerBen, BetoCG, Bg, Bhadani, Bigboss sb, Bigfrozenhead, BilCat, Bilboon, BillC, Birkett, Bjf, Bkell, Blaablaa, Black Sword, BlaiseFEgan, Blast Ulna, BlindEagle, BlongerBros, Bloodredrover, Bloodshedder, Blu Aardvark, BlueEarth, Bluhbluhbluh94, Bob Castle, Bob rulz, Bob the Rabbid, Bob the ducq, Bobblewik, Bobet, Bobith2, Bobo192, Bongwarrior, Boomchakalaka2, Borgx, Borislav, BovineBeast, Bovineboy2008, Braaropolis, BradDeharder, Bradeos Graphon, Bradlegar, Branddobbe, Brandmeister, Brandmeister (old), Breno, Brian0918, Brighterorange, Brion VIBBER, Broberds, Bruce1ee, Bryan Derksen, Bryan.dollery, Bschlee, Bubba73, Bullzeye101, Bummerdude62, Bungle, Burmiester, C0nanPayne, CFCF, CFLeon, CKA3KA, Cactus Guru, Cadwaladr, Caggy, Caknuck, Calamarain, Calaschysm, Caltas, CalviNet, Calypsos, Cameron Dewe, Can't sleep, clown will eat me, Canadian-Bacon, CanadianCaesar, CanisRufus, Cantus, CapitalR, Capt Jim, Captain Disdain, Carapar999, Caribbean H.Q., Carre, Carrionluggage, Casliber, Catgut, Causa sui, Cbsteffen, CecilWard, Cenarium, Centrx, Ceyockey, Cfassett, Challisrussia, Chanting Fox, CharlesC, Charlesblack, CharlieZeb, Charvest, Chensiyuan, Chetvorno, ChicXulub, Chihuahua22, Chiros Sunrider, Chmee2, Chodorkovskiy, Choptube, Chowbok, Chris Mounce, Chrisjj, Chrislk02, ChristTrekker, Christian List, Christopher Cooper, Chriswiki, Chuckchuckerson, Chunky Rice, ChunkySoup, Civil Engineer III, Cjkporter, Ckatz, Claytopia3, Closedmouth, Cmapm, Coelacan, ComaDivine, CommonsDelinker, Conversion script, CosineKitty, Cosmium, Cosmoskramer, Courcelles, Coyo, Cpollak, Crazycomputers, Crazynas, Crd721, Crescentnebula, Crip22, Cromag, Crosscountry511, Crystallina, Cs302b, Curps, Cww, Cyan, Cyktsui, Czj, D. 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Upsilon Andromedae d  Source: http://en.wikipedia.org/w/index.php?oldid=464329875  Contributors: Aldaron, Benhocking, BlueEarth, Bryan Derksen, CarloscomB, Chaos syndrome,Chermundy, Cuddlyopedia, EvenGreenerFish, Headbomb, IVAN3MAN, Icalanise, John Belushi, Judgesurreal777, Jyril, Lucianomendez, Lugnuts, Materialscientist, Mike s, Nbound,Nehrams2020, Nergaal, NuclearVacuum, PresN, Rjwilmsi, Spacepotato, Telescopi, The Mysterious El Willstro, Where, Why2200, WilyD, Zimriel, 10 anonymous edits

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Image Sources, Licenses and Contributors 102

Image Sources, Licenses and ContributorsImage:The Earth seen from Apollo 17.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:The_Earth_seen_from_Apollo_17.jpg  License: Public Domain  Contributors: NASA. Phototaken by either Harrison Schmitt or Ron Evans (of the Apollo 17 crew).File:Mars Earth Comparison.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_Earth_Comparison.png  License: Public Domain  Contributors: Edward, RHorning, Sebman81,Tony Wills, UrhixidurImage:AncientMars.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:AncientMars.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: IttizFile:Titan multi spectral overlay.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Titan_multi_spectral_overlay.jpg  License: Public Domain  Contributors: NASA/JPL/Space ScienceInstituteFile:Planet Kepler-22b.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Planet_Kepler-22b.jpg  License: Public Domain  Contributors: Jan Baykara, Kanags, Tintin the reporterFile:TerraformedVenus.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:TerraformedVenus.jpg  License: GNU Free Documentation License  Contributors: Original uploader wasIttiz at en.wikipediaImage:16 Cyg B b rv.pdf  Source: http://en.wikipedia.org/w/index.php?title=File:16_Cyg_B_b_rv.pdf  License: Attribution  Contributors: This file was uploaded by w:User:JamesMcBrideJames McBride.File:Flag of the United States.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_the_United_States.svg  License: Public Domain  Contributors: AnomieImage:16CygBbOrbit.svg  Source: http://en.wikipedia.org/w/index.php?title=File:16CygBbOrbit.svg  License: Public Domain  Contributors: Original uploader was Chaos syndrome aten.wikipediaImage:16 Cygni sunset.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:16_Cygni_sunset.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: NuclearVacuumImage:Planet_23_Librae_b.png  Source: http://en.wikipedia.org/w/index.php?title=File:Planet_23_Librae_b.png  License: GNU Free Documentation License  Contributors: Tyrogthekreeper(talk)) created this work entirely by myself.Image:196222main exoplanet-final.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:196222main_exoplanet-final.jpg  License: Public Domain  Contributors: NASA/JPL-CaltechImage:55 Cnc f rv.pdf  Source: http://en.wikipedia.org/w/index.php?title=File:55_Cnc_f_rv.pdf  License: Attribution  Contributors: This file was uploaded by w:User:James McBrideJamesMcBride.File:PlanetQuest-55Cancri-f.png  Source: http://en.wikipedia.org/w/index.php?title=File:PlanetQuest-55Cancri-f.png  License: Public Domain  Contributors: Planet Quest: New Worlds Atlas(Tweaks by Kevin Heider)Image:ESO - The Planetary System in Gliese 581 (by).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESO_-_The_Planetary_System_in_Gliese_581_(by).jpg  License: unknown Contributors: ESOFile:Flag of Chile.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_Chile.svg  License: Public Domain  Contributors: SKoppFile:Exoplanet Comparison Gliese 581 c.png  Source: http://en.wikipedia.org/w/index.php?title=File:Exoplanet_Comparison_Gliese_581_c.png  License: unknown  Contributors: AldaronImage:GJ581orbits.svg  Source: http://en.wikipedia.org/w/index.php?title=File:GJ581orbits.svg  License: Public Domain  Contributors: IcalaniseImage:Gliese 581 d-v1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Gliese_581_d-v1.jpg  License: GNU Free Documentation License  Contributors: Debivort at en.wikipediaImage:Planet Gliese 581 d.png  Source: http://en.wikipedia.org/w/index.php?title=File:Planet_Gliese_581_d.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors:Tyrogthekreeper (talk)Image:Gliese 581 g.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Gliese_581_g.jpg  License: Public Domain  Contributors: ZerbuFile:GJ581orbits Vogt2010.svg  Source: http://en.wikipedia.org/w/index.php?title=File:GJ581orbits_Vogt2010.svg  License: Public Domain  Contributors: IcalaniseFile:Gliese 581 system compared to solar system.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Gliese_581_system_compared_to_solar_system.jpg  License: Public Domain Contributors: Zina Deretsky, National Science FoundationFile:KeckTwilight-hi.png  Source: http://en.wikipedia.org/w/index.php?title=File:KeckTwilight-hi.png  License: Public Domain  Contributors: JPL / CaltechImage:Gliese-876 b.png  Source: http://en.wikipedia.org/w/index.php?title=File:Gliese-876_b.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:JohnVanVlietFile:Gliese876Orbits.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Gliese876Orbits.svg  License: Public Domain  Contributors: IcalaniseImage:ConceptJKV-Gliese876-c.png  Source: http://en.wikipedia.org/w/index.php?title=File:ConceptJKV-Gliese876-c.png  License: Creative Commons Attribution-Sharealike 3.0 Contributors: User:JohnVanVlietImage:HD 23127 b.png  Source: http://en.wikipedia.org/w/index.php?title=File:HD_23127_b.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:EpinedoFile:Flag of Australia.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Flag_of_Australia.svg  License: Public Domain  Contributors: Anomie, MifterImage:Moon of HD 28185 b.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Moon_of_HD_28185_b.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:NuclearVacuumImage:HD 69830 and HD 69830 d.png  Source: http://en.wikipedia.org/w/index.php?title=File:HD_69830_and_HD_69830_d.png  License: unknown  Contributors: CelestiaImage:HD85512bWithHZ.svg  Source: http://en.wikipedia.org/w/index.php?title=File:HD85512bWithHZ.svg  License: Creative Commons Zero  Contributors: IcalaniseImage:Kepler-22b_System_Diagram.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Kepler-22b_System_Diagram.jpg  License: Public Domain  Contributors: Hibernian, Phatom87File:Planet_Kepler-22b.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Planet_Kepler-22b.jpg  License: Public Domain  Contributors: Jan Baykara, Kanags, Tintin the reporterFile:Mars symbol.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_symbol.svg  License: Public Domain  Contributors: , redesigned byFile:Mars Valles Marineris.jpeg  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_Valles_Marineris.jpeg  License: Public Domain  Contributors: Avala, Bricktop, Common Good,Homonihilis, Hunyadym, MGA73, Njardarlogar, Ruslik0, TheDJ, Yarl, Ævar Arnfjörð Bjarmason, 2 anonymous editsImage:Speakerlink.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Speakerlink.svg  License: Creative Commons Attribution 3.0  Contributors: Woodstone. Original uploader wasWoodstone at en.wikipediaFile:Olympus Mons alt.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Olympus_Mons_alt.jpg  License: Public Domain  Contributors: Image by NASA, modifications by SeddonFile:Tharsis Tholus block.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Tharsis_Tholus_block.JPG  License: Public Domain  Contributors: Jim Secosky modified NASA image..Original uploader was Jimmarsmars at en.wikipediaFile:Nasa mars opportunity rock water 150 eng 02mar04.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Nasa_mars_opportunity_rock_water_150_eng_02mar04.jpg  License:Public Domain  Contributors: NASA/JPL/US Geological SurveyImage:Mars NPArea-PIA00161 modest.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_NPArea-PIA00161_modest.jpg  License: Public Domain  Contributors:NASA/JPL/USGSImage:South_Polar_Cap_of_Mars_during_Martian_South_summer_2000.jpg  Source:http://en.wikipedia.org/w/index.php?title=File:South_Polar_Cap_of_Mars_during_Martian_South_summer_2000.jpg  License: Public Domain  Contributors: NASA/JPL/MSSSFile:MarsTopoMap-PIA02031 modest.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:MarsTopoMap-PIA02031_modest.jpg  License: Public Domain  Contributors:NASA/JPL-Caltech.File:Victoria Crater, Cape Verde-Mars.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Victoria_Crater,_Cape_Verde-Mars.jpg  License: Public Domain  Contributors: Marsen:Opportunity roverFile:Magnify-clip.png  Source: http://en.wikipedia.org/w/index.php?title=File:Magnify-clip.png  License: Public Domain  Contributors: User:Erasoft24File:Mars caves from NASA orbiters.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_caves_from_NASA_orbiters.jpg  License: Public Domain  Contributors: NASA JPLUSGS ASU NAUFile:Mars atmosphere.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_atmosphere.jpg  License: Public Domain  Contributors: Original uploader was Alkuin at de.wikipediaFile:Martian Methane Map.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Martian_Methane_Map.jpg  License: Public Domain  Contributors: Michael Mumma, TrentSchindler/NASA

Image Sources, Licenses and Contributors 103

File:2005-1103mars-full.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:2005-1103mars-full.jpg  License: Public domain  Contributors: ComputerHotline, Fred J, Herbythyme, Ies,Kallerna, Lars Lindberg Christensen, Pringles, Roomba, Ruslik0, Tryphon, Wst, 15 anonymous editsFile:Marsorbitsolarsystem.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Marsorbitsolarsystem.gif  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:LookangImage:ThePlanets Orbits Ceres Mars PolarView.svg  Source: http://en.wikipedia.org/w/index.php?title=File:ThePlanets_Orbits_Ceres_Mars_PolarView.svg  License: Creative CommonsAttribution-ShareAlike 3.0 Unported  Contributors: User:EurocommuterImage:ThePlanets Orbits Ceres Mars.svg  Source: http://en.wikipedia.org/w/index.php?title=File:ThePlanets_Orbits_Ceres_Mars.svg  License: Creative Commons Attribution-ShareAlike 3.0Unported  Contributors: User:EurocommuterImage:Phobos colour 2008.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Phobos_colour_2008.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech/University ofArizonaImage:Deimos-MRO.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Deimos-MRO.jpg  License: Public Domain  Contributors: NASA/JPL-caltech/University of ArizonaImage:Mars Viking 21i093.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_Viking_21i093.png  License: Public domain  Contributors: "Roel van der Hoorn (Van der Hoorn)"Image:Mars Viking 11h016.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_Viking_11h016.png  License: Public domain  Contributors: "Roel van der Hoorn (Van derHoorn)"Image:Soviet Union-1972-Stamp-0.16. Mars 3.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Soviet_Union-1972-Stamp-0.16._Mars_3.jpg  License: Public Domain  Contributors:Augiasstallputzer, Editor at Large, EugeneZelenko, Michael RomanovImage:MER Spirit Lander Pan Sol16-A18R1 br2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:MER_Spirit_Lander_Pan_Sol16-A18R1_br2.jpg  License: Public Domain Contributors: Bryan Derksen, Li-sung, 1 anonymous editsImage:PhoenixSolarPanelandRoboticArm.png  Source: http://en.wikipedia.org/w/index.php?title=File:PhoenixSolarPanelandRoboticArm.png  License: Public Domain  Contributors: NASAJPL. Original uploader was Arjun G. Menon at en.wikipediaImage:15-ml-06-phobos2-A067R1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:15-ml-06-phobos2-A067R1.jpg  License: Public Domain  Contributors: Bryan Derksen,ComputerHotline, Kristaga, Li-sung, RHorningFile:Apparent retrograde motion of Mars in 2003.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Apparent_retrograde_motion_of_Mars_in_2003.gif  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: Eugene Alvin Villar (seav)File:Mars oppositions 2003-2018.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_oppositions_2003-2018.png  License: Public domain  Contributors: Tom Ruen (talk)Image:Karte Mars Schiaparelli MKL1888.png  Source: http://en.wikipedia.org/w/index.php?title=File:Karte_Mars_Schiaparelli_MKL1888.png  License: Public Domain  Contributors:unkownImage:Lowell Mars channels.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Lowell_Mars_channels.jpg  License: Public Domain  Contributors: Badseed, Boleslav1, Hiphats,Marcok, 姫宮南, 1 anonymous editsImage:Mars HST Mollweide map 1999.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_HST_Mollweide_map_1999.png  License: Public Domain  Contributors: NASA.File:Mars face.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mars_face.png  License: Public Domain  Contributors: NASA / JPL / Malin Space Science SystemsFile:Kirks Soap Yerkes Mars.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Kirks_Soap_Yerkes_Mars.jpg  License: Public Domain  Contributors: Further attribution given withinto "American Institute of Physics, Niels Bohr Library". 1893 ad its self is attributed in text to an unnamed Chicago newspaper.File:War-of-the-worlds-tripod.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:War-of-the-worlds-tripod.jpg  License: Public Domain  Contributors: Illustration by Alvim Corréa.Original uploader was Bblackmoor at en.wikipediaFile:UpsilonAndromedae D moons.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:UpsilonAndromedae_D_moons.jpg  License: Creative Commons Attribution-Sharealike 3.0 Contributors: LucianomendezFile:Habitable zone - HZ.png  Source: http://en.wikipedia.org/w/index.php?title=File:Habitable_zone_-_HZ.png  License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: Habitable_zone-en.svg: Chewie derivative work: Ignacio javier igjav (talk)

License 104

LicenseCreative Commons Attribution-Share Alike 3.0 Unported//creativecommons.org/licenses/by-sa/3.0/