Life in the Universe: Extra-solar planets

DEPARTMENT OF PHYSICS AND ASTRONOMY

Life in the Universe:Life in the Universe:Extra-solar planetsExtra-solar planets

Dr. Matt BurleighDr. Matt Burleighwww.star.le.ac.uk/~mbuwww.star.le.ac.uk/~mbu

Dr. Matt Burleigh 3677: Life in the Universe

3677 Timetable3677 Timetable

• Today and Tuesday 11am: MB Extrasolar Today and Tuesday 11am: MB Extrasolar planetsplanets

• Then Mark Sims (Life in the solar system)Then Mark Sims (Life in the solar system)


ContentsContents

• Methods for detectionMethods for detection– Doppler “wobble”Doppler “wobble”– TransitsTransits– Direct ImagingDirect Imaging

• CharacterisationCharacterisation– StatisticsStatistics– Implications for formation scenariosImplications for formation scenarios


Useful reading / web sitesUseful reading / web sites

• Extra-solar planets encyclopaediaExtra-solar planets encyclopaedia• California & Carnegie Planets SearchCalifornia & Carnegie Planets Search• How stuff works planet-hunting pageHow stuff works planet-hunting page

– Includes lots of animations & graphicsIncludes lots of animations & graphics

• JPL planet finding pageJPL planet finding page– Look at the science & multimedia gallery pagesLook at the science & multimedia gallery pages


What is a planet?What is a planet?

• International Astronomical Union definition –International Astronomical Union definition –

– An object orbiting a star An object orbiting a star • But see later this lecture…But see later this lecture…

– Too small for dueterium fusion to occurToo small for dueterium fusion to occur• Less than 13 times the mass of JupiterLess than 13 times the mass of Jupiter

– Formation mechanism?Formation mechanism?• Forms from a circumstellar diskForms from a circumstellar disk

– Lower mass limit – IAU decided that Pluto should Lower mass limit – IAU decided that Pluto should be downgraded!be downgraded!


A brief history of extra-solar planetsA brief history of extra-solar planets• In the 16th century the Italian philosopher In the 16th century the Italian philosopher

Giordano Bruno said that the fixed stars are Giordano Bruno said that the fixed stars are really suns like our own, with planets going really suns like our own, with planets going round themround them

• 1991 Radio astronomers Alex Wolszczan & Dale 1991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar Frail discovered planets around a pulsar PSR1257+12PSR1257+12– Variations in arrival times of pulses suggests Variations in arrival times of pulses suggests

presence of three or more planetspresence of three or more planets– Planets probably formed from debris left after Planets probably formed from debris left after

supernova explosionsupernova explosion

• 1995 Planet found around nearby Sun-like star 1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & 51 Peg by Swiss astronomers Michel Mayor & Didier Queloz using the “Doppler Wobble” Didier Queloz using the “Doppler Wobble” methodmethod– Most successful detection method by far, but Most successful detection method by far, but

other methods like transits are now very other methods like transits are now very successfulsuccessful

– 405 exoplanets 405 exoplanets in totalin total found to date by all found to date by all methodsmethods


Planet Hunting: The Radial Velocity TechniquePlanet Hunting: The Radial Velocity Technique(“Doppler Wobble”)(“Doppler Wobble”)

• Star + planet orbit common centre of Star + planet orbit common centre of gravitygravity

• As star moves towards observer, As star moves towards observer, wavelength of light shortens (is blue-wavelength of light shortens (is blue-shifted)shifted)

• Light red-shifted as star moves awayLight red-shifted as star moves away

• Measure:Measure:

ee = ( = (ee) / ) / ee = v = vrr / c / cobserved wavelength, observed wavelength, ee=emitted wavelength=emitted wavelength

377 planets detected by Doppler 377 planets detected by Doppler Wobble inc. 38 multiple systemsWobble inc. 38 multiple systems


M* from spectral type


Doppler Wobble Method: SummaryDoppler Wobble Method: Summary• Precision of current surveys is now 1m/s:Precision of current surveys is now 1m/s:

– Jupiter causes Sun’s velocity to vary by 12.5m/sJupiter causes Sun’s velocity to vary by 12.5m/s– All nearby, bright Sun-like stars are good targetsAll nearby, bright Sun-like stars are good targets

• Lots of lines in spectra, relatively inactiveLots of lines in spectra, relatively inactive

– Smallest planet found by this method is Smallest planet found by this method is ~2~2MMearthearth

• Length of surveys limits distances planets have been found Length of surveys limits distances planets have been found from starsfrom stars– Earliest surveys started 1989Earliest surveys started 1989– Jupiter (5AU from Sun) takes 12 yrs to orbit SunJupiter (5AU from Sun) takes 12 yrs to orbit Sun– Saturn takes 30 yearsSaturn takes 30 years

• Would remain undetectedWould remain undetected

• Do not see planet Do not see planet directlydirectly


Doppler Wobble Method: SummaryDoppler Wobble Method: Summary

• Since measure K (= vSince measure K (= v* * sin i), not vsin i), not v** directly, only know directly, only know

mass in terms of the orbital inclination imass in terms of the orbital inclination i• Therefore only know the planet’s Therefore only know the planet’s minimum minimum mass, mass, MM sin sin ii

– If i=90If i=90oo (eclipsing or (eclipsing or transitingtransiting) then know mass exactly) then know mass exactly

i=90i=9000

Orbital Orbital planeplane

ii00

Orbital Orbital planeplane


TransitsTransits

• Planets observed at inclinations near 90Planets observed at inclinations near 90o o will transit their host stars will transit their host stars


TransitsTransits

• AssumingAssuming– The whole planet passes in front of the starThe whole planet passes in front of the star– And ignoring limb darkening as negligibleAnd ignoring limb darkening as negligible

• Then the depth of the eclipse is simply the ratio of the planetary and Then the depth of the eclipse is simply the ratio of the planetary and stellar disk areas:stellar disk areas:– i.e. i.e. f / ff / f** = = RRpp

22 / / RR**2 2 = (R = (Rp p / R/ R**))22

• We measure the change in magnitude We measure the change in magnitude m, and obtain the stellar m, and obtain the stellar radius from the spectral type radius from the spectral type – Hence by converting to flux we can measure the planet’s radiusHence by converting to flux we can measure the planet’s radius– Rem. Rem. m = mm = mtransittransit – m – m* * = 2.5 log (f = 2.5 log (f* * / f/ ftransittransit))

• (smaller number means brighter)(smaller number means brighter)


TransitsTransits

Example: first known transiting planet HD209458bExample: first known transiting planet HD209458b m = 0.017 magsm = 0.017 mags

– So (fSo (f* * / f/ ftransittransit) = 1.0158, i.e. ) = 1.0158, i.e. f=1.58%f=1.58%

– From the spectral type (G0) R=1.15RFrom the spectral type (G0) R=1.15Rsunsun

– So using So using f / ff / f** = (R = (Rp p / R/ R**))2 2 and setting fand setting f**=100%=100%

– Find RFind Rpp=0.145R=0.145Rsunsun

– Since RSince Rsunsun=9.73R=9.73RJ J thenthen

– RRpp = 1.41R = 1.41RJJ


TransitsTransits

• HD209458b more:HD209458b more:– From Doppler wobble method know From Doppler wobble method know MM sin sin ii = 0.62M = 0.62MJJ

– Transiting, hence assume i=90Transiting, hence assume i=90oo, so , so MM=0.62M=0.62MJJ

– Density = 0.29 g/cmDensity = 0.29 g/cm33

• c.f. Saturn 0.69 g/cmc.f. Saturn 0.69 g/cm33 – HD209458b is a gas giant! HD209458b is a gas giant!


TransitsTransits

• For an edge-on orbit, transit duration is For an edge-on orbit, transit duration is given by:given by:

t = (PRt = (PR**) / () / (a)a) • Where P=period in days, a=semi-major axis of Where P=period in days, a=semi-major axis of

orbitorbit

• Probability of transit (for random orbit)Probability of transit (for random orbit)– PPtransittransit= R= R** / a / a

– For Earth (P=1yr, a=1AU), For Earth (P=1yr, a=1AU), PPtransittransit=0.5%=0.5%

– But for close, “hot” Jupiters, But for close, “hot” Jupiters, PPtransittransit==10%10%

– Of course, relative probability of detecting Of course, relative probability of detecting Earths is lower since would have to observe Earths is lower since would have to observe for up to 1 yearfor up to 1 year


TransitsTransits

• AdvantagesAdvantages– Easy. Can be done with small, cheap telescopesEasy. Can be done with small, cheap telescopes

• E.g. WASPE.g. WASP

– Possible to detect low mass planets, including “Earths”, Possible to detect low mass planets, including “Earths”, especially from space (Kepler mission, 2008)especially from space (Kepler mission, 2008)

• DisadvantagesDisadvantages– Probability of seeing a transit is lowProbability of seeing a transit is low

• Need to observe many stars simultaneouslyNeed to observe many stars simultaneously

– Easy to confuse with starspots, binary/triple systemsEasy to confuse with starspots, binary/triple systems– Needs radial velocity measurements for confirmation, massesNeeds radial velocity measurements for confirmation, masses


Super WASPSuper WASP

• Wide Angle Search for Planets Wide Angle Search for Planets (by transit method)(by transit method)

• First telescope located in La First telescope located in La Palma, second in South AfricaPalma, second in South Africa

• Operations started May ‘04Operations started May ‘04• Data stored and processed at Data stored and processed at

Leicester Leicester • >30 new planets detected!>30 new planets detected!• www.superwasp.orgwww.superwasp.org• www.wasp.le.ac.www.wasp.le.ac.ukuk


Super WASPSuper WASP

• SuperWASP monitors about 1/4 SuperWASP monitors about 1/4 of the sky from each siteof the sky from each site

• That means millions of stars, That means millions of stars, every night!every night!


Direct detectionDirect detection

• Imaging = spectroscopy = physics: Imaging = spectroscopy = physics: composition & structurecomposition & structure

• DifficultDifficult• Why? Why?

– Stars like the Sun are billions of times brighter than Stars like the Sun are billions of times brighter than planetsplanets

– Planets and stars lie very close together on the skyPlanets and stars lie very close together on the sky• At 10pc Jupiter and the Sun are separated by 0.5”At 10pc Jupiter and the Sun are separated by 0.5”


Direct detectionDirect detection

• Problem 1:Problem 1:– Stars bright, planets faintStars bright, planets faint

• Solution:Solution:– Block starlight with a coronagraphBlock starlight with a coronagraph

• Problem 2:Problem 2:– Earth’s atmosphere distorts starlight, reduces Earth’s atmosphere distorts starlight, reduces

resolutionresolution• Solution:Solution:

– Adaptive optics, Interferometry – difficult, Adaptive optics, Interferometry – difficult, expensiveexpensive

– Or look around very young and/or intrinsically faint Or look around very young and/or intrinsically faint stars (not Sun-like)stars (not Sun-like)


First directly imaged planet?First directly imaged planet?

• 2M1207 in TW Hya 2M1207 in TW Hya associationassociation

• Discovered at ESO VLT in Discovered at ESO VLT in ChileChile

• 25M25Mjupjup Brown dwarf + 5M Brown dwarf + 5Mjup jup “planet”“planet”

• Distance ~55pc Distance ~55pc • Very young cluster ~10M Very young cluster ~10M

yearsyears• Physical separation ~55AUPhysical separation ~55AU• A brown dwarf is a failed A brown dwarf is a failed

starstar– Can this really be called a Can this really be called a

planet? planet? – Formation mechanism may Formation mechanism may

be crucial!be crucial!


First directly imaged planetary systemFirst directly imaged planetary system• Last year 3 planets imaged Last year 3 planets imaged

around the star HR8799around the star HR8799• 130 light years away (40pc)130 light years away (40pc)• Three planets at 24, 38 and 68AU Three planets at 24, 38 and 68AU

separationseparation– In comparison, Jupiter is at 5AU In comparison, Jupiter is at 5AU

and Neptune at 30AUand Neptune at 30AU• Masses of 7Mjup, 10Mjup and Masses of 7Mjup, 10Mjup and

10Mjup10Mjup• Young: 60Myr Young: 60Myr

– Earth is ~4.5GyrEarth is ~4.5Gyr


Fomalhaut (alpha Piscis Austrini)Fomalhaut (alpha Piscis Austrini)• One of the brightest stars in One of the brightest stars in the southern skythe southern sky

• Long known to have a dusty Long known to have a dusty debris diskdebris disk

• Shape of disk suggested Shape of disk suggested presence of planetpresence of planet

• 2M2Mjupjup planet imaged by HST planet imaged by HST

inside disk inside disk

• 200Myr old200Myr old

• Like early solar systemLike early solar system


Direct detection: White DwarfsDirect detection: White Dwarfs

• White dwarfs are the end state of stars like the SunWhite dwarfs are the end state of stars like the Sun– What will happen to the solar system in the future?What will happen to the solar system in the future?

• WDs are 1,000-10,000 times fainter than Sun-like WDs are 1,000-10,000 times fainter than Sun-like starsstars– contrast problem reducedcontrast problem reduced

• Outer planets should survive evolution of Sun to Outer planets should survive evolution of Sun to white dwarf stage, and migrate outwards white dwarf stage, and migrate outwards – more easily resolvedmore easily resolved

• Over 100 WD within 20pcOver 100 WD within 20pc– At 10pc a separation of 100AU = 10” on skyAt 10pc a separation of 100AU = 10” on sky

• At Leicester we are searching for planets around At Leicester we are searching for planets around nearby WD with 8m telescopes and the Spitzer nearby WD with 8m telescopes and the Spitzer space telescopespace telescope


Direct detection: White DwarfsDirect detection: White Dwarfs

• No planets yet, just brown dwarfsNo planets yet, just brown dwarfs– Currently limited to finding planets >5MCurrently limited to finding planets >5M jupjup

– Not very commonNot very common– May have to wait for JWSTMay have to wait for JWST

• But have found some WDs are But have found some WDs are surrounded by dust and gas diskssurrounded by dust and gas disks– Remains of small rocky planets and Remains of small rocky planets and

asteroids that strayed too close to WD asteroids that strayed too close to WD – Ripped apart by tidal forcesRipped apart by tidal forces– Can study composition of extra-solar Can study composition of extra-solar

terrestrial bodies!terrestrial bodies!


What we know about What we know about

extra-solar planetsextra-solar planets

• 405 planets now found405 planets now found• 38 multiple systems38 multiple systems• 62 transiting planets62 transiting planets• Unexpected population Unexpected population

with periods of 2-4 days: with periods of 2-4 days: “hot Jupiters”“hot Jupiters”

• Planets with orbits like Planets with orbits like Jupiter discovered Jupiter discovered (eg (eg 55 Cancri d)55 Cancri d)

• Smallest planet: Smallest planet: CoRoT-7b - 1.7RCoRoT-7b - 1.7Rearthearth


Extra-solar planet period distributionExtra-solar planet period distribution

• Notice the “pile-up” Notice the “pile-up” at periods of 2-4 at periods of 2-4 days / 0.04-0.05AUdays / 0.04-0.05AU

• The most distant The most distant planets discovered planets discovered by radial velocities by radial velocities so far are at 5-6AUso far are at 5-6AU

• Imaging surveys Imaging surveys finding very wide finding very wide orbit planetsorbit planets


““Hot Jupiter” planetsHot Jupiter” planets

• Doppler Wobble and transit surveys Doppler Wobble and transit surveys find many gas giants in orbits of 2-4 find many gas giants in orbits of 2-4 daysdays– cf Mercury’s orbit is 80 dayscf Mercury’s orbit is 80 days

• Surveys are Surveys are biasedbiased towards finding towards finding themthem– Larger Doppler Wobble signalLarger Doppler Wobble signal– Greater probability of transitGreater probability of transit

• These planets are heated to >1000These planets are heated to >1000ooF F on “day” sideon “day” side– And are “tidally locked” like the MoonAnd are “tidally locked” like the Moon– Causes extreme weather conditionsCauses extreme weather conditions


Extra-solar planet mass distribution Extra-solar planet mass distribution • Mass distribution peaks at 1-Mass distribution peaks at 1-

2 x mass of Jupiter2 x mass of Jupiter• Lowest mass planet so far: Lowest mass planet so far:

5.5xM5.5xMEarthEarth

• Super-Jupiters (>few MSuper-Jupiters (>few MJupJup) )

are not commonare not common– Implications for planet Implications for planet

formation theories?formation theories?– Or only exist in number at Or only exist in number at

large separation?large separation?– Or exist around massive Or exist around massive

stars?stars?


Eccentricity vs semi-major axis

obse

rvat

iona

l bia

s

extra-solar planets

solar system planets

:

- large distribution of e (same as close binary stars)

-

- but some planets in circular orbits do exist far away from star

- the planets in our own system have small eccentricities ie STABLE

- planets close to the star are tidally circularized

What we know about extra-solar planetsWhat we know about extra-solar planets

most extra-solar planets are in much more eccentric orbits than the giant planets in the solar system


Results of the Planet Hunting surveysResults of the Planet Hunting surveys

• Of 2000 stars surveyed Of 2000 stars surveyed – 5% have gas giants between 5% have gas giants between

0.02AU and 5AU0.02AU and 5AU– 10% may have gas giants in 10% may have gas giants in

wider orbitswider orbits– <1% have Hot Jupiters<1% have Hot Jupiters

• How many have How many have Earths…..?Earths…..?


What about the stars themselves?What about the stars themselves?

• Surveys began by targeting sun-like Surveys began by targeting sun-like stars (spectral types F, G and K)stars (spectral types F, G and K)

• Now extended to M dwarfs (<1 MNow extended to M dwarfs (<1 Msunsun) and ) and subgiants (>1.5Msubgiants (>1.5Msunsun))– Subgiants are the descendants of A starsSubgiants are the descendants of A stars

• Incidence of planets is greatest for late Incidence of planets is greatest for late F starsF stars– F7-9V > GV > KV > MVF7-9V > GV > KV > MV

• Stars that host planets appear to be on Stars that host planets appear to be on average more metal-richaverage more metal-rich

• More massive stars tend to have more More massive stars tend to have more massive planetsmassive planets


MetallicityMetallicityThe abundance of elements heavier

than He relative to the Sun

• Overall, ~5% of solar-like stars have radial velocity –detected JupitersOverall, ~5% of solar-like stars have radial velocity –detected Jupiters• But if we take metallicity into account:But if we take metallicity into account:

– >20% of stars with 3x the metal content of the Sun have planets>20% of stars with 3x the metal content of the Sun have planets

– ~3% of stars with 1/3~3% of stars with 1/3rdrd of the Sun’s metallicity have planets of the Sun’s metallicity have planets • Does this result imply that planets more easily form in metal-rich environments?Does this result imply that planets more easily form in metal-rich environments?

– If so, then maybe planet hunters should be targeting metal-rich starsIf so, then maybe planet hunters should be targeting metal-rich stars– Especially if we are looking for rocky planetsEspecially if we are looking for rocky planets


Planet formation Planet formation scenariosscenarios

• There are two main models which have been proposed to• describe the formation of the extra-solar planets:

– (I) Planets form from dust which agglomerates into cores which then accrete gas from a disc.

– (II) A gravitational instability in a protostellar disc creates a number of giant planets.

• Both models have trouble reproducing both the observed distribution of extra-solar planets and the solar-system.


Accretion onto coresAccretion onto cores

• Planetary cores form through the agglomeration of dust into grains, pebbles, rocks and planetesimals within a gaseous disc

• At the smallest scale (<1 cm) cohesion occurs by non-gravitational forces e.g. chemical processes.

• On the largest scale (>1 km) gravitational attraction will dominate.

• On intermediate scales the process is poorly understood.

• These planetesimals coalesce to form planetary cores

• The most massive cores accrete gas to form the giant planets

• Planet formation occurs over 107 yrs.


Gravitational instabilityGravitational instability

• A gravitational instability requires a sudden change in disc properties on a timescale less than the dynamical timescale of the disc.

• Planet formation occurs on a timescale of 1000 yrs.

• A number of planets in eccentric orbits may be formed.

• Sudden change in disc properties could be achieved by cooling or by a dynamical interaction.

• Simulations show a large number of planets form from a single disc.

• Only produces gaseous planets – rocky (terrestrial) planets are not formed.

• Is not applicable to the solar system.


Where do the hot Jupiters come from?Where do the hot Jupiters come from?

• No element will condense within ~0.1AU of a star since T>1000KNo element will condense within ~0.1AU of a star since T>1000K• Planets most likely form beyond the “ice-line”, the distance at which Planets most likely form beyond the “ice-line”, the distance at which

ice formsice forms– More solids available for building planetsMore solids available for building planets– Distance depends on mass and conditions of proto-planetary disk, but Distance depends on mass and conditions of proto-planetary disk, but

generally >1AUgenerally >1AU• Hot Jupiters currently at ~0.03-0.04AU cannot have formed thereHot Jupiters currently at ~0.03-0.04AU cannot have formed there

– MigrationMigration: Planets migrate inwards and stop when disk is finally cleared: Planets migrate inwards and stop when disk is finally cleared

• If migration time < disk lifetimeIf migration time < disk lifetime– Planets fall into starPlanets fall into star– Excess of planets at 0.03-0.04AU is evidence of a stopping mechanismExcess of planets at 0.03-0.04AU is evidence of a stopping mechanism

• tides? magnetic cavities? mass transfer?tides? magnetic cavities? mass transfer?

• Large planets will migrate more slowlyLarge planets will migrate more slowly– Explanation for lack of super-Jupiters in close orbitsExplanation for lack of super-Jupiters in close orbits


Hunting for Earth-like planetsHunting for Earth-like planets

• Pace of planet discoveries will continue to increase in next few Pace of planet discoveries will continue to increase in next few yearsyears

• Radial velocity and direct imaging surveys will reveal outer giant Radial velocity and direct imaging surveys will reveal outer giant planets with long periods like our own Solar Systemplanets with long periods like our own Solar System

• Transit surveys will reveal small planets in close orbits to their sunsTransit surveys will reveal small planets in close orbits to their suns• But the greatest goal is the detection of other EarthsBut the greatest goal is the detection of other Earths


Towards other EarthsTowards other Earths

TelescopeTelescope MethodMethod DateDateCorot (Fr)Corot (Fr) TransitsTransits 20072007

Kepler (NASA)Kepler (NASA) TransitsTransits 20082008

GAIA (ESA)GAIA (ESA) AstrometryAstrometry 20122012

SIM (NASA)SIM (NASA) AstrometryAstrometry 2015 (?)2015 (?)

Plato (ESA)Plato (ESA) TransitsTransits 20172017

Darwin (ESA)Darwin (ESA) ImagingImaging 2025+ (?)2025+ (?)

42m E-ELT42m E-ELT ImagingImaging 20182018


KeplerKepler

• Searching for Earths by transit methodSearching for Earths by transit method• Launched last year by NASALaunched last year by NASA• Aims to find an Earth around a Sun-Aims to find an Earth around a Sun-

like star in a one year orbitlike star in a one year orbit• Need three transits to confirmNeed three transits to confirm• So mission lasts at least three years…So mission lasts at least three years…


Towards Other Earths: Habitable ZonesTowards Other Earths: Habitable Zones

• Habitable zone defined as where liquid water existsHabitable zone defined as where liquid water exists• Changes in extent and distance from star according to star’s Changes in extent and distance from star according to star’s

spectral type (ie temperature)spectral type (ie temperature)


Towards Other Earths: BiomarkersTowards Other Earths: Biomarkers

• So we find a planet So we find a planet with the same mass as with the same mass as Earth, and in the Earth, and in the habitable zone:habitable zone:– How can we tell it How can we tell it

harbours life?harbours life?

• Search for biomarkersSearch for biomarkers– WaterWater– OzoneOzone– AlbedoAlbedo

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Life in the Universe: Extra-solar planets