Upload
vuonganh
View
214
Download
0
Embed Size (px)
Citation preview
Observational constraints from the Solar System
and fromExtrasolar Planets
Lecture Universität Heidelberg WS 11/12Dr. Christoph Mordasini Partially based on script by Prof. W. Benz
Lecture 1 Part I
Mentor Prof. T. Henning
Lecture overview
1. Introduction
2. Planet formation paradigm
3. Structure of the Solar System
4. The surprise: 51 Peg b
5. Detection techniques: radial velocity, transits, direct imaging, (microlensing, timing, astrometry)
6. Properties of extrasolar planets: mass, distance, eccentricity distributions, metallicity effect, mass-radius diagram, ...
Galaxies, stars and planets
10 billion galaxies
100 billion stars
how many planets?
How many harbor life?
How frequent? Life?First generation of human beings with technology to answer this.
Important questions - Planet formation: From dust to planets
10-100 million10 μm
years
- Planet evolution: Habitability
Mars
Venus
Earth
Moon
Terrestrial planets in the solar system: similar initial conditions very different outcome.
The characterization also of exoplanets has just started.
How?
Sun Stars
Herschel’s 1789
For many centuries
Solar System Exoplanets
La Silla Obs. ESO
For a decade
Cloud collapseHertzsprung RusselNuclear FusionStellar Mass Funct.
Formation in disksCollisionsGas accretionMigration
Life Extraterrestrial Life ?
Darwin ESA
In a decade ?
AstrobiologyHabitable ZoneBiomarkersComplex Life
Ways to understanding
Core
Accretion
Gravitational
Instability
Planet formation: The paradigm
- remote observations- in-situ measurements- sample returns- laboratory analysis- theoretical modeling
Party line
A satisfactory theory should explain the formation of planets in the solar system as well as around other stars.
Minority line
dust
planetesimals protoplanets
in presence of gas in absence of gas
giant impacts
terrestrial planets
107 years
giantplanets
Sta
r &
pro
topl
anet
ary
disk
migration
type I type II
107 years
108 years
dynamical re-arrangement
Planet formation: Sequential picture
Planet formationInitial conditions, task and orders of magnitude
Task•follow the evolution of the gas and dust for a period of about 100 Million years.
Orders of magnitude to remember•Msun ~2 1033 g•MJ ~ 2 1030 g ~ 1/1000 Msun ~ 318 ME
•ME ~ 6 1027 g•RJ ~ 7.19 109 cm ~ 1/10 Rsun
•RE ~ 6.4 108 cm ~ 1/10 RJ
•AU ~1.5 1013 cm•Lsun ~ 3.8 1033 erg/s
Initial condition•disk of dust and gas orbiting a new born star•total mass of the disk: ~1-10 % of stellar mass•total mass of dust: ~2% of mass of gas
dust(μm)
planetesimals(∼km)
protoplanets
Earth-sized(∼1000 km)
gas giants(∼10000 km)
105-107
years107-108
years
dust sticking
Self.Gravity
runaway growth
oligarchicgrowth
runaway gas accretion
late stagesgiant impacts
size
time104-105 years
Challengesin planet formation
Difficulty: -huge dynamical rage in size/mass- dynamical range in time: 100 million orbital timescales-lots of physics involved, changing over time: gravity, drag, hydrodynamics, radiation transfer, magnetic fields,..- non-linearities (runaway growth)-feedback mechanism (grav. scattering)
Solar systemSystem architecture
Orbital data major planets
Note•Sun has 99.96% of the mass, but only 0.6% of the angular momentum. Solar Prot ~25 d.•LJ/Ltot: 0.61, Lsaturn/Ltot: 0.25•Jupiter is dominating the dynamics. Important during formation (small mars, Asteroids)•mostly circular orbits, all prograde (same rotation direction as the sun)•nearly co-planar orbits: formation in a disk•spacing: Titius-Bode law an=aMercury+0.3 2n-1 n=1,2,...: Orbital stability in Hill units
Inner systemOuter system
Asteroids
Rocky planets
gas giants
ice giants
Solar systemSystem architecture II
Minor bodies
Asteroids•rocky composition, some with significant water content•a few 100’000 known.•total mass 1/30 of lunar mass (1 lunar mass ~1/81 ME): not a destroyed planet.•26 with diameters larger than 200 km. Largest: Ceres 900 km.•2.2 AU < a < 3.2 AU for 95%: between Mars and Jupiter •existence of families (groups with similar orbits and reflectance properties)•All prograde, most have e<0.3 and i<25 deg.•leftovers from formation phase: important obs. constraint on e.g. migration.
Solar systemSystem architecture III
Minor bodies cont.
Trans-Neptunian Objects (TNO) and Kuiper Belt objects (KBO)•icy composition, not much altered (slow evolution). Low albedo (<coal).•estimated 70’000 with diameter >100 km. Larger than typical asteroids.•located beyond Neptune: 30 AU< a < 70 AU.•3 classes:
•classical KBO: 42-47 AU, mean eccentricity ~ 0.07 (small), i < 30 deg.•scattered KBO: large e, total M 0.5-1.5 ME ,source of short period comets, perihel at ~35 AU•Plutinos: 3:2 resonance with Neptune, as Pluto, 0.1<e<0.34, 0<i<2 deg.
Oort Cloud•hypothetical spherical cloud surrounding the sun, extending out 100’000 AU.•Source of long period comets.•Not (yet) directly observed.•Weak gravitationally bound: effect of passing stars.•Objects scattered outwards during planet formation.
Solar systemPhysical properties
Physical data major planets
•Stars: burn hydrogen: M>~75 MJ
•Brown dwarfs: burn deuterium ~13<M/MJ<75•Planet definition (IAU 2006) :A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
Approximately to scale
Solar systemPhysical properties II
Composition terrestrial planets
•Inner structure determination: observations (seismic waves, gravitational moments, surface temperature and abundances) combined with modeling.•Terrestrial planets: Iron core, silicate mantle.
•Size of core vs mantle varies: impact history•Earth: core 1/3, mantle 2/3 (in mass). Close to chondritic (primitive meteorites) composition
Earth
Solar systemPhysical properties III
Composition giants
•X=Hydrogen, Y=Helium, Z=”Metals”•Solar composition (primordial): X0 0.71, Y0 0.27, Z0 0.015•The gas giant planets (Jupiter, Saturn) are clearly enriched compared to solar composition. Expected Jupiter solar: 4.8 ME, Saturn solar: 1.4 ME. This is much less than the inferred values. They didn’t form like the sun from the same collapsing cloud. Important constraint•The ice giants consist of ~25% rock, ~60-79% ice, and ~5-15% H/He
Significant uncertainties: equation of state (EOS) of H/He under extreme p and T badly known.
Amount of metals [ME]MJ=~318 ME, MS=~95 ME
Possible J,S compositions Guillot 1999
Historical perspective
Selected discoveries in the Solar System•until 1600 only six planets were known: Mercury, Venus, Earth, Mars, Jupiter and Saturn. Extensively studied since antiquity. •Aristarchus from Samos (270 BC): heliocentric system.•beginning of 17th century: discoveries of satellites of Jupiter and Saturn by Galilei (1564-1642), Huygens (1629-1659) and Cassini (1625-1712).•1781 discovery of Uranus by William Herschel •1846 discovery of Neptune by Johann Galle. Neptune was first theoretically predicted by John Adams and Urbain Le Verrier who studied the perturbations of the orbit of Uranus.•1930 discovery of Pluto by Clyde Tombaugh•1978 discovery of Charon, Pluto’s moon by James Christy•1992 discovery of the first TNO object (QB1) by Jane Luu and Jewitt
Herschel’s big telescope
Historical perspective IISome early formation theories•Rene Decartes (1594-1650)
•space is filled with a universal substance. Planets form in vortices which form at locations of least motions•secondary vortices form around the vortices which make the moons.
•Georges L. L. Buffon (1707-1804)•catastrophe hypothesis: a huge comet hits the sun and ejects material which form the planet. Conceptually similar to the giant hypothesis for Earth’s moon
•Immanuel Kant (1724-1804)•nebula hypothesis (building on similar early work of Emanuel Swedenborg).•nebula composed of gas and dust is flattened by rotation, particles are colliding, loose energy and drift to the center to form the sun•planets form out of local density enhancements which orbit the sun.
•Pierre Simon de Laplace (1749-1829)•planets are formed during the contraction of the sun.•the sun ejects rings of material which cool and form planets.
Swedenborg Kant Laplace
Science, 267, 360 (January 1995)
Oops
Knowledge is evolving. What is believed correct today can turn up wrong tomorrow!
Planet formation theoryState of the art t<1995. Only one example to study..
Nature, 378, 355 (October 6, 1995)
confirmation by Marcy & Butler(October 12, 1995)
A giant planet with a 4.15 days period!
The discovery
G2 IV, d=15 pc, 5.49 mag• First planet mass object in orbit around a solar like star: 51 Pegasi b.
• Very different from theoretical expectations:
• a = 0.052 AU
• P = 4.23 days• M sin i = 0.468 MJ
• Such planets are now called “Hot Jupiters” or Pegasi planets / Pegasids.
Spektrometer ELODIEObservatoire deHaute-Provence193 cm Teleskop
The wake-up call
• About 0.5 -1 % of sun like planets have such a hot Jupiter (as we know now).
Mayor & Queloz
ApJ, 241, 425 (October 1, 1980)
discovered 15 yearsearlier... by theorists!
Migration: was not new after all
Current status
692 planetsCandidates detected by radial velocity or astrometry524 planetary systems640 planets76 multiple planet systems
Transiting planets171 planetary systems184 planets14 multiple planet systems
Candidates detected by microlensing12 planetary systems13 planets 1 multiple planet systems
Candidates detected by imaging22 planetary systems25 planets1 multiple planet systems
Candidates detected by timing9 planetary systems14 planets4 multiple planet systems
+ 1235 planet candidates from the KEPLER satellite (transit)
Extra-solar planet encyclopedia (http://exoplanet.eu/)9.11.2011
Planet Detection MethodsMichael Perryman, Rep. Prog. Phys., 2000, 63, 1209 (updated April 2007)
[corrections or suggestions please to [email protected]]
Planet Detection Methods
Magneticsuperflares
Accretionon star
Self-accretingplanetesimals
Detectableplanet mass
Pulsars
Slow
Millisec
Whitedwarfs
Radial velocity
Astrometry
Radio
Optical
GroundSpace
Microlensing
PhotometricAstrometric
Space Ground
Imaging
Disks
Reflected/blackbody
Ground
Space
Transits
Miscellaneous
Ground(adaptive
optics)
Spaceinterferometry
(infrared/optical)
Detectionof Life?
Resolvedimaging
MJ
10MJ
ME
10ME
Binaryeclipses
Radioemission
??
3
206 planets(178 systems,
of which 20 multiple)4 planets2 systems
Dynamical effects Photometric signal
2? 1?
Timing(ground)
Timingresiduals
Existing capabilityProjected (10-20 yr)Primary detectionsFollow-up detectionsn = systems; ? = uncertain
11
Freefloating1?
44
Large number of methods, but only few can detect and allow the study of Earth-like planets!
Indirect detection - radial velocityStar and planet move around common center of mass. The stars move also (a little bit).Use optical Doppler effect to measure motion along the line of sight: → measure (periodic) shifts of spectral lines i.e. the stellar radial velocities.
But....- motion of the Sun due to Jupiter: 12 m/s → shift of spectral line by ~50 angstroms or 10 Si atoms
on the CCD → average velocity of cyclist at the Tour de France...- motion of the sun due to Earth: 8 cm/s → difficult to detect because of surface fluctuations
Shape and amplitude of the curve give the Msini (minimal mass), period, eccentricity and T0.
Control room
Instrument: High-precision spectrographLocation: 3.6 m ESO at La Silla Observatory (Chile)Consortium: Universities of Geneva and Bern (CH), Observatoire de Haute Provence (F), Service d'Aéronomie (F), ESO.
Vaccum chamber
The most precise RV instrument:
Telescope
Precision: down to 0.6 m/s.Super-Earth planets in the habitable
zone of K dwarfs.
Progress in ground-based RV detections
Earth-like planet detection from the ground by 2012? → still indirect observations → only close-by planets
51 Peg b
HARPS
Detection probability for a first generation instrument (ELODIE)Instrumental precision =10 m/s
Detection bias RV: The less massive, and the further out, the more difficult to find. Don’t forget when interpreting discoveries!
Mordasini et al. 2009
Transit detection
Jupiter in front of the sun
1% change in luminosity
Earth in front of the sun
0.01% change in luminosity
But... Transits measure radius not mass. Follow-up is necessary to measure mass (by RV).Many false positives (look photometrically like planets, but are not.)
transit detection principle
Simple in theory, difficult in practice.
=>Miniforschungsprojekt at MPIA
(Rp/Rstar)2
Characterization from transits + RV
- radius of planets: From transit measurements
R = 1.27 ± 0.02 RJ
M = 0.63 MJ ρ = 0.40 g/cm3 gaseous planet(Jupiter: 1.34)
↓↓
- mass of planet: From radial velocity measurements
Example HD209458b (first transiting planet, :
Mass-radius relation for extrasolar
planets
After the indirect detection of Hot Jupiters by RV, some doubts persisted about the origin of these observations (Stellar pulsations?). Transits showed unambiguously the planetary origin.
HD209458b: first measured transit
Charbonneau et al. 2000
Transit detection from space•Detection of planets with a radius of only a few Earth radii is very difficult form the ground, due to the noise in the photometric data introduced by the atmosphere.•To detect such planets photometrically, one must go to space.
•Kepler has revolutionized the transit method by finding more than 1200 candidates. •Warning: maybe ~10% are false positives (no RV confirmation)
Kepler candidates (Feb. 2011)
Launch: 2006Launch: 2008
Direct imaging:massive giant planets far out
Very special systems can be imaged from the ground today...far from terrestrial planets!
8 AU from star 6 – 12 MJ
Fomalhaut b M≈ <3 MJ d= 119 AU
Kalas et al. 2008
HR 8799 b,c,d,e: M≈ 5-13 MJd= 15-70 AUDynamical constraints
Marois et al. 2008
Beta Pictoris b M≈ 6-12 MJ d= 8 AU
Reappeared!
Lagrange et al. 2008
Direct detection: (dis)advantages
• Advantages
• Allows physical characterization: Temperature, log g, chemical composition
• Direct detection, no other explanations possible (must exclude background star chance alignment.)
• Disadvantages
• Very difficult, only young objects. Huge brightness contrast, tiny projected separation.
• Measures intrinsic (or reflected) luminosity L. Not mass M. L-M relation is model dependent and very uncertain.
Direct detection: resolution
Difficulty: Resolution
typical numbers: stars ~ 10-100 pc, planet 1 AU → θ = 0.01’’- 0.1” (seeing limits to ~ 0.5”)
Solution:→ use adaptive optics
Direct detection: brightness ratio
visible to near-IRreflected light mid-IR
intrinsic emission
Difficulty: Brightness ratio
Typical numbers: visible: Fplanet / Fstar ≈ 10-9
infrared: Fplanet / Fstar ≈ 10-6
Solution:→ remove star light
- nulling - coronograph
Favorable cases:infrared observationsplanets orbiting less luminous stars→ M dwarfs
young planets→ planet formation