1. Planet Formation O V E R V I E W Jack J. Lissauer - NASA Ames

2. Constraints: Physics & Chemistry –Newton’s laws –Standard chemistry –Interstellar chemistry –High-pressure physics Observations: Meteorites to Extrasolar Planets –Our Solar System Dynamics Planetary composition Meteorites Geology –Other Stars Molecular clouds Star formation Circumstellar disks Extrasolar planets

3. Models: Solar Nebula Theory & Planetesimal Hypothesis –Protoplanetary Disks - formation & evolution –Formation of planetesimals –Planetesimals to terrestrial planets –Accumulation of giant planet gaseous envelopes –Disappearance of protoplanetary disks

4. Our Solar System Dynamics –Planetary orbits nearly circular & coplanar –Spacing increases with distance from Sun –All giant planets have satellite systems –Planetary rings close to planets Compositions –Largest bodies most gas-rich –Rocky bodies near Sun, icy bodies farther out –Elemental/isotopic abundances similar (except volatiles) –Meteorites - active heterogeneous environment Planetary Geology: Cratering Record –Far more small bodies in 1 st 800 Myr than today

5. Jupiter Saturn Uranus Neptune Pluto

7. Murchison (Australia) CM2 Carbonaceous Chondrite Fall 1969 Sep 28 Photo: Jackie Beckett © AMNH 2003

10. Planet Formation: Solar System Constraints Orbital motions –Flat, prograde, low eccentricity –Spins mostly prograde, low obliquity Age –Primitive meteorites 4.56 Gyr –Moon rocks 3 - 4.4 Gyr –Earth rocks < 4 Gyr Sizes & densities of planets –Largest planets most gas-rich Asteroid & Comets –Asteroid belt: Small objects, low total mass –Kuiper belt: Larger version of asteroid belt (?) –Oort cloud

11. Planet Formation: Solar System Constraints II Satellite systems –Regular systems: Rings, small moons, larger moons –Irregular satellites: Outer orbits, many retrograde Meteorites –Very rapid cooling of chondrules and CAIs –Isotopic uniformity of meteorites, but exceptions –Interstellar grains –Differentiation of some small bodies Planetary atmospheres –Giant planets dominated by accreted gasses –Terrestrial planets dominated by volatilized solids Planetary surfaces: variation in cratering rates Angular momentum distribution

12. Circumstellar Disks Young Stars –Evidence: IR excesses, rotation curves, proplyd images –Radii tens to hundreds of AU ( even larger for massive stars) –Typical mass ~ 0.01 - 0.1 M Sun –Lifetime (substantial dust) < 10 Myr –Some show evidence for gaps, inner holes Main Sequence Stars –Second generation debris disks - unseen parent bodies –Low mass, gas poor –More prominent around younger stars, but substantial variations –Some show evidence for gaps, inner holes

13. Extrasolar Planets ~ 1% of sunlike stars have planets more massive than Saturn within 0.1 AU –Some of these planets known to be gas giants –Models suggest these planets migrated inwards ~ 7% of sunlike stars have planets more massive than Jupiter within 2 AU –Some of these planets have very eccentric orbits At least a few % of sunlike stars have Jupiter-like (0.5 - 2 M J, 4 AU 20% do not Observable planets are more common around higher metallicity stars Binary stars can host planets

14. Solar Nebula Theory (Kant 1755, LaPlace 1796) The Planets Formed in a Disk in Orbit About the Sun Explains near coplanarity and circularity of planetary orbits Disks are believed to form around most young stars Theory: Collapse of rotating molecular cloud cores Observations: Proplyds,  Pic, IR spectra of young stars Predicts planets to be common, at least about single stars

15. Solar Nebula/Protoplanetary Disk Minimum mass solar nebula –Planets masses, augmented to solar composition –~ 0.02 M o – Minimum mass disk not viable for the Solar System Infall –Shock front Disk dynamics –Magnetic torques –Gravitational torques –Viscous torques Disk chemistry –Equilibrium condensation –Kinetic inhibition Clearing

16. Planetesimal Hypothesis (Chamberlain 1895, Safronov 1969) Planets Grow via Binary Accretion of Solid Bodies Massive Giant Planets Gravitationally Trap H 2 + He Atmospheres Planetesimals and condensation sequence explain planetary composition vs. mass General; for planets, asteroids, comets, moons Can account for Solar System; predicts diversity

18. Dust -> Terrestrial Planets  m - cm: Dust settles towards midplane of disk; sticks, grows. Chondrule & CAI formation?? cm - km: Two possibilities: continued sticking or gravitational instabilities km - 10,000 km: Binary collisions - runaway growth; isolation; giant impacts

19. Growth From 1 km Planetesimals at 0.4 AU (Barnes, Quinn, Lissauer & Richardson 2006) 100 Orbits

20. Runaway Growth (Rory Barnes et al. 2006) 357 Orbits

21. Runaway Growth Gravitational encounters important for bodies > 1 km. Close encounters alter trajectories of bodies. Equipartition of energy determines random velocities. Random velocities determine growth rate.

22. Oligarchic Growth

23. Embryos to Terrestrial Planets Sun-Jupiter-Saturn (Chambers 2001)

24. Terrestrial Planets: Masses & Orbits Mergers continue until stable configuration reached Fewer planets usually more stable, even though planets are larger Resonances (commensurabilities in orbital periods) destabilize system Stable configurations need to last billions of years Giant impacts & chaos imply diversity

25. Planet Formation in Binary Star Systems > 50 % stars are in multiple star systems 19 planets known in multiple star systems What is the effect of a stellar companion on the planet formation processes?

26. Types of Binary Star Systems “Close-Binary” “Wide-Binary”

27. 23.4 AU A B G2 star M = 1.1 Msun K1 star M = 0.91 Msun Disk inclined to binary orbit: i = 0°, 15°, 30°, 45°, 60°, 180° Integration time = 200 Myr - 1 Gyr  Centauri System i

28. RUN 1 (i=0  )  Cen a B = 23.4 AU

29. many numerical experiments are needed to get statistically valid results. Planet formation is c h a ot i c, so

30. RUN 2 (i=0  )  Cen a B = 23.4 AU

31. CB 10b Close Binary a B = 0.2 e B = 0  = 0.5; #2 Simulation includes ‘Jupiter’ & ‘Saturn’ (not plotted)

32. CB 12b Run #1 a B = 0.2 e B = 0.5  = 0.5 Close binary + ‘Jupiter’ & ‘Saturn’

33. CB 14a Run #1 a B = 0.4 e B = 0  = 0.5 Close binary + ‘Jupiter’ & ‘Saturn’

34. CB 14c Run #3 a B = 0.4 e B = 0  = 0.5 Close binary + ‘Jupiter’ & ‘Saturn’

35. Final Planetary Systems: Binaries with Q B = 0.1 AU (30 ° )

36. Q B = 0.3 AU Q B = 0.4 AU

37. Results (Quintana, Lissauer, Chambers, Duncan, Adams): Terrestrial planets can form around short- period binary star systems. Close binaries with small a & low e have planetary statistics similar to Sun-Jupiter-Saturn, Sun-only, and (low i) wide-binary accretion simulations. Fewer planets when a, e are larger, but c h a o s blurs out sharp boundaries. Analogous results for planet formation around each star in wide (q B >> 1 AU) binary systems

39. SHOULD... Giant Planet Formation Models SHOULD... explain the mass distribution: form planets before nebula is dispersed form extrasolar planets gas rich not Jupiter and Saturn are gas rich, but not solar in composition; gas poor Uranus and Neptune are gas poor ; M solid /M  M gas /M  Jupiter 10 - 40 300 Saturn 20 - 30 75 Uranus/Neptune 10 - 15 2 - 3

40. Theories of Giant Planet Formation Core-nucleated growth: Big rocks accumulated gas One model for rocky planets, jovian planets, moons, comets… Explains composition vs. mass Detailed models exist Takes millions of years (depends on M core, atmosphere opacity) Fragmentation during collapse: Planets form like stars M J Separate model for solid bodies; no model for Uranus/Neptune Gravitational instability in disk: Giant gaseous protoplanets

41. Theories of Giant Planet Formation Core-nucleated growth: Big rocks accumulated gas One model for rocky planets, jovian planets, moons, comets… Explains composition vs. mass Detailed models exist Takes millions of years (depends on M core, atmosphere opacity) Fragmentation during collapse: Planets form like stars M J Separate model for solid bodies; no model for Uranus/Neptune Gravitational instability in disk: Giant gaseous protoplanets

42. Theories of Giant Planet Formation Core-nucleated growth: Big rocks accumulated gas One model for rocky planets, jovian planets, moons, comets… Explains composition vs. mass Detailed models exist Takes millions of years (depends on M core, atmosphere opacity) Fragmentation during collapse: Planets form like stars Rapid Binary stars are common Mass gap Opacity-limited fragmentation requires M > 7 M J Separate model for solid bodies; no model for Uranus/Neptune Gravitational instability in disk: Giant gaseous protoplanets

43. Theories of Giant Planet Formation Core-nucleated growth: Big rocks accumulated gas One model for rocky planets, jovian planets, moons, comets… Explains composition vs. mass Detailed models exist Takes millions of years (depends on M core, atmosphere opacity) Fragmentation during collapse: Planets form like stars Rapid Binary stars are common Mass gap Opacity-limited fragmentation requires M > 7 M J Separate model for solid bodies; no model for Uranus/Neptune Gravitational instability in disk: Giant gaseous protoplanets Rapid growth, but cooling limits contraction Difficult to form sufficiently unstable disks (density waves stabilize) No simulations with realistic initial conditions Separate model for solid bodies; no good model for Uranus/Neptune

44. C O R E A C C R E T I O N MODEL  planetesimals accrete into a solid core growing core attracts gas envelope runaway gas accretion with a little more solids no gas accretion ends planet contracts and cools nearby gas accreted ? nearby gas accreted ? tidal truncation ? tidal truncation ? protoplanetary nebula removal ? protoplanetary nebula removal ?

45. Giant Planet Accretion of Solids & Gas Solid core forms like huge terrestrial planet Becomes massive enough to gravitationally trap H 2, He

46. Orbital Evolution Disk-planet interactions –No gap: Migration relative to disk –Gap: Moves with disk –Faster near star - need stopping mechanism Planet-planet scattering –Produces eccentric orbits –Planets well-separated –Some planets ejected

47. Conclusions Planet formation models are developed to fit a very d i v er s e range of data –Meteorites, planetary orbits, composition, circumstellar disks, extrasolar planets Planets form in gas/dust disks orbiting young stars –Most stars form together with such a disk Solid planets grow by pairwise accumulation of small bodies –Massive planets gravitationally trap H 2, He Planets are common, and planetary systems are d i v er s e –Current technologies limit the phase space of observed planetary systems –New technologies will allow observations of more types of extrasolar planets