1. Timescales for Giant Planet Formation Dave Stevenson Caltech Harvard, May 17, 2004

2. The Important Questions How did our giant planets (J,S,U &N)form? How long did it take? How is this expressed in the structure of the system (dynamics, interiors, satellites, etc.)? Is that mechanism of giant planet formation universal or could it vary from system to system? (Extrasolar planets.) What do we need to make further progress?

3. Here are the Main Points Most likely mechanism of Jupiter & Saturn formation is the core accretion model. Uranus & Neptune may have formed similarly (but without the additional gas inflow) For J & S, the timescale may be set by the gas accretion. For U&N, the timescale may be set by core accretion. This requires core accretion before the nebula is eliminated (5 Ma or even more?) Supported by current structure models May not apply to giant planet formation in other systems…. Disk instability model attractive for hot Jupiters.

4. Some Significant Facts Jupiter & Saturn are mostly gas. (So they must have formed in the presence of a nebula). Jupiter may have a dense core & Saturn almost certainly has a dense core. Both are enriched in heavy elements throughout. [Ar/H] = 3 x solar in Jupiter, implying delivery of T= 40K material. In situ formation of large satellites. Uranus and Neptune exist! And largely formed while some nebula was around, because they have several Earth masses of gas.This gas could only come from the nebula

5. J,S U,N Gas (H 2,He) Ice (mainly H 2 O) Rock (silicates, oxides, met. Fe) E LI Line of cosmic ice & rock condensate (variable gas) e.g., Ganymede

6. Gas Ice Rock M/M J =1 M/M J =10 -2 M/M J =10 -4 Increasing mass LI E J sJ SG subJupiters Superganymedes Not represented in our solar system Extrasolars

7. (Tristan Guillot)

8. This shows the heavy element abundance in the four major planets and estimated uncertainties A major source of uncertainty is in the equations of state. ( Guillot)

9. D 2 Hugoniots Discrepancy is large and important to giant planet models Latest (Sandia) data support less severe compression

10. Some Time Scales Collapse from interstellar medium ~ few 10 5 yrs. Disk orbital times; dynamical times ~10-1000yrs. “Viscous” evolution times~10 6 -10 7 yr. Orbital migration (disk interaction) ~10 5 -10 6 yr Runaway Accretion of solid bodies & oligarchic growth ~10 6 -10 7 yr Loss of nebula gas ~5 x 10 6 yr but what does this really mean? Rapid phase of gas accretion ~1000yr. Solid body growth by orbit crossing (inner solar system)~10 7 -10 9 yr

11. Boss, 2002

13. Common Viewpoint Heavy element core  Nucleated instability Absence of Heavy element core  Gaseous Instability

14. Correct Viewpoint Heavy element core  Nucleated Gaseous instability instability and core rainout Absence of Heavy element core  Gaseous Nucleated Instability Instability and core dredge up But do you get an acceptable core? But is this efficient?

15. Nucleated Instability model (“Standard” Case) Pollack et al, 1996 Embryo formation (runaway) Embryo isolation Rapid gas accretion Truncated by gap formation

16. Or is this 1.5 -3 Ma? Or is this 2- 4 Ma? The gas accretion could take place even when the nebula is only 10% (or even 1%?) of the so-called minimum nebula

17. Gas Expected for Uranus & Neptune Progressive increase in the hydrostatically bound nebula as the solid body grows Essentially instantaneous Amount depends on opacity and is only logarithmically sensitive to the nebula density! It is compatible with observation

18. Pros and Cons - The Nucleated Instability There can be no doubt that solid cores can form: Existence of Uranus and Neptune

19. Pros and Cons - The Nucleated Instability There can be no doubt that solid cores can form: Existence of Uranus and Neptune But do they form fast enough so that massive gas accretion takes place?

20. Pros and Cons - The Nucleated Instability There can be no doubt that solid cores can form: Existence of Uranus and Neptune Saturn (at least) has a core that agrees with the theory. But do they form fast enough so that massive gas accretion takes place?

21. Pros and Cons - The Nucleated Instability There can be no doubt that solid cores can form: Existence of Uranus and Neptune Saturn (at least) has a core that agrees with the theory. But do they form fast enough so that massive gas accretion takes place? A weak test, especially since so much heavy material is delivered aside from the core.

22. Pros and Cons - The Nucleated Instability There can be no doubt that solid cores can form: Existence of Uranus and Neptune Saturn (at least) has a core that agrees with the theory. Specificity of published models is artificial; shorter timescales are possible But do they form fast enough so that massive gas accretion takes place? A weak test, especially since so much heavy material is delivered aside from the core.

23. Pros and Cons - The Nucleated Instability There can be no doubt that solid cores can form: Existence of Uranus and Neptune Saturn (at least) has a core that agrees with the theory. Specificity of published models is artificial; shorter timescales are possible But do they form fast enough so that massive gas accretion takes place? A weak test, especially since so much heavy material is delivered aside from the core. More models needed

24. Disk Instabilities An example from the work of Alan Boss There is little doubt that such instabilities can arise in a model. Need  gas G  /  c > a few Easier, early on than later, but depends on mass redistribution within the disk. Might depend on finite amplitude disturbances

25. Gaseous protoplanet Ice/rock particles Contraction & settling on ~10 3 yr

26. What’s Wrong with this Picture? 1.As you approach the state where core mass ~ included gas mass, further “rainout” causes gas to compress (to support the pressure) and heat adiabatically. 2.Large gravitational energy release Consequence: evaporation Dense ice & rock core Dense non-ideal mix of ice, gas & rock. Comparable mass fractions. “Usual core” Predicted “core” for instability model

27. Pros and Cons - The Gas Instability This process is fast!

28. Pros and Cons - The Gas Instability This process is fast! You don’t even know for sure if it happens! Depends on the rate at which you approach instability, etc.

29. Pros and Cons - The Gas Instability This process is fast! Core rainout can satisfy the need for a core You don’t even know for sure if it happens! Depends on the rate at which you approach instability, etc.

30. Pros and Cons - The Gas Instability This process is fast! Core rainout can satisfy the need for a core You don’t even know for sure if it happens! Depends on the rate at which you approach instability, etc. May not work; may not have the right mass

31. Pros and Cons - The Gas Instability This process is fast! Core rainout can satisfy the need for a core Compatible with extrasolar planets You don’t even know for sure if it happens! Depends on the rate at which you approach instability, etc. May not have the right mass

32. Pros and Cons - The Gas Instability This process is fast! Core rainout can satisfy the need for a core Compatible with extrasolar planets You don’t even know for sure if it happens! Depends on the rate at which you approach instability, etc. May not have the right mass Still need to make Uranus and Neptune

34. This parameter That parameter Gas instability Nucleated instability NO GIANT PLANETS GIANT PLANETS

35. The Galilean satellites were the first planetary system (other than our own) to be discovered. We know a lot about it ….. But the effort to understand it’s origin has been small compared to understanding the solar nebula (at least until recently)

36. Inflow may Cause Compact disk Canup & Ward (2002),from Lubow(1999)

37. Accretion Disk Good Features May be a natural outcome of the accumulation of a giant planet Control is from the “outside” Bad Features Origin of viscosity is unclear Permissive model (you can get whatever you want!) Cooling & contraction Viscous spreading of gas Solids & gas infall

38. Suggested Chronology dM/dt time 10 -2 M  /yr Rapid gas accretion Declining accretion as nebula gap develops; onset of satellite formation ~10 6 yr Usual gas inflow from nebula, which accelerates as mass increases. Gas fills the Roche lobe but then contraction & cooling allows a protoJupiter (~700K, 2R J ) Last stages of inflow at much lower fluxes (one MMSN =0.02M J per 10 6 yr)

39. Repeating the Main Points Most likely mechanism of Jupiter & Saturn formation is the core accretion model. Uranus & Neptune may have formed similarly (but without the additional gas inflow) For J & S, the timescale may be set by the gas accretion. For U&N, the timescale may be set by core accretion. This requires core accretion before the nebula is eliminated (5 Ma or even more?) Supported by current structure models May not apply to giant planet formation in other systems…. Disk instabilty model attractive for hot Jupiters.

40. Future Work Hydrogen Equation of state. Jupiter Polar orbiter & Probes. Better understanding of the connection between giant planet formation time and terrestrial planets. Systematics for Extrasolar planets. Observations of young systems…when and how does the gas actually leave? More realistic accretion models including orbital migration (the argon problem), etc.

41. The End

42. Back-up Slides

43. Fluid Planets Gas Giants (primarily hydrogen and helium) - Jupiter and Saturn Ice Giants (everything, but including large amounts of H 2 O at high P,T) Uranus and Neptune

44. The Hydrogen Phase diagram Jupiter & Saturn are in the fluid region, possibly crossing a PPT phase transition. Relevant conditions encountered in reverberation shock experiments Helium immiscibility suggested by observation & theory but not well understood.

45. What do we want to Explain (for Jupiter & Saturn)? Consistency with our models of the Giant Planets Size of the System –Mass –Angular momentum budget –Size of individual bodies Compositions & Structures –Temperature and pressure of formation –Processing of the nebular gas can change ice/rock? (CO/CH 4 and N 2 /NH 3 ) –Accretion timescale may affect initial differentiation

46. Some Basic Principles Some of the same basic physics should apply as for planet formation –Dynamics of disks (disk-satellite interaction, gas drag, accumulation of solids, etc.) Satellite systems are compact compared to the solar system –Tidal truncation or even more? Hill sphere ~several hundred planet radii. Satellite system ~ few tens of planet radii –Rapid formation times –Greater role of tides (orbital evolution, internal evolution, etc.) Not all aspects of satellite formation need follow the solar nebula model!

47. The Callisto Constraint Simplest interpretation of Callisto’s gravity is that the interior is only partly differentiated (I/MR 2 =0.355). This requires a “long” accretion time. T surf ~250K.(10 6 yr/  acc ) 1/4 Normal accumulation time ~10,000 yrs or less! Presence of CO 2 suggests low T

48. Constraints from Io and Europa? Io is silicate rich (no evidence for water) Water component for Europa is consistent with hydrated silicates Heat flow on Io may not be simple equilibrium orbital expansion. (Reduces our ability to link to origin but also permits primordial resonance origin) Tidal evolution of Europa (antiquity of an ocean)?

49. Constraint from Ganymede? Fully differentiated - but we don’t know when! Dynamo may constrain history? Perhaps not-all you need is S and 40 K Tidal evolution & previous excitation of eccentricity?

50. What about Amalthea? Very low density (~0.9 g/cc!). May require an icy component even though it doubtless has high porosity Not related to primary satellite formation process? (There are other small satellites in the vicinity). May be accretion from a later transient disk…Testament to the role of impacts.

51. Constraints from the Saturnian System Volatile -rich Titan my be a clue to T,P of origin…or is it partly that the fields & particles environment was less severe than for Ganymede. Extent of differentiation not known- Cassini may tell us. Titan’s orbital eccentricity is intriguing (~0.028 - Evidence of disk-satellite interaction?) Absence of strong evidence for differentiation of Rhea (for example)? Might be a constraint on accretion timescale (but too soon to say). Rhea Titan (fantasy)

52. 1.Spin-out Disk Pollack & Bodenheimer 2.Accretion Disk Many workers (Ruskol, Coradini…) 3.Impact-generated disk Cameron…. 4. Capture & “Co-accretion” (collisions within the Hill sphere) …will not discuss further because clearly not relevant to multiple large regular satellites. Competing Scenarios

53. Giant Impacts? Cartoons indicate the giant impact that may have led to formation of Earth’s moon. Has also been suggested for the moons of Uranus (since Uranus has large obliquity) But not a natural explanation for Jupiter

54. Spin-Out Disk Good Feature May be a natural outcome of the accumulation of a giant planet Bad Features Does not offer a natural way to explain rock at Ganymede & Callisto orbits Material that is shed may evolve too quickly Cooling & contraction

55. Cosmic (~Solar) Abundances