Planet Formation: Formation of Brown Dwarfs and Massive Planets

1. Planet Formation:Formation of Brown Dwarfs and Massive Planets Åke Nordlund Centre for Star and Planet Formation and Niels Bohr Institute University of Copenhagen

2. Centre for Star and Planet FormationUniversity of Copenhagen

3. Overview • Brown Dwarfs and Massive Planets • What’s the difference? • Brown Dwarf Formation • Turbulent fragmentation • Interrupted accretion • Planet Formation • Core accretion • Gravitational instability • Cosmochemical evidence  new and severe constraints  new paradigm for planet formation

4. Brown Dwarfs and Massive Planets • What’s the difference? • Two alternatives: • Ask the IAU • Look at the physics • Are there differences in physics? • Planets orbit BDs (not vice versa ;-) • Massive planets are generally not formed the way BDs are • Stars use up all of X+Y+Z = 1 • Massive Planets settle mostly for the Z...

5. Brown Dwarf Formation • Star formation (vastly better understood than planet formation!) • Formation of GMCs in the Galaxy • de Avillez et al 2004-2010 • Formation of MCs in GMc • Kritsuk et al 2010 • Formation of stars in MCs • Padoan & Nordlund 2010 Brown Dwarfs are marked with black dots, more massive stars with white dots. 10003 cells, Mach 9 supersonic MHD-turbulence Padoan & Nordlund (2010)

6. Brown Dwarf Formation • Brown Dwarfs form the same way other stars form • Turbulent fragmentation in cold molecular gas • BD mass fragments are exceedingly numerous, but ... • ... only a tiny fraction are dense enough to collapse into BDs • Successful fragments are confined by very large dynamic pressure (smooth convergence + shock); see PPV review (Whitworth et al 2006 • As for all stars, high speed and low surrounding density stops further accretion • ”Escape” of star from pre-stellar core & surroundings • Brown Dwarfs and Massive Planets • Similar structure, but two modes of formation • direct, gravitational, by turbulent compression • indirect, assisted by (rapid!) core formation • Some massive ’planets’ in excentric, non-aligned orbits may form through the indirect (BD-like) mode

7. Adaptive Mesh Refinementsimulations can cover 10 pc – 10 AU range • Padoan, Kritsuk, Norman (2005)

8. Zooming in on disks: Note that the ’envelope’ is highly structured Padoan, Kritsuk, Norman (2005)

9. Zooming in on disks: Note that the ’envelope’ is highly structured Mass accreting from the ’envelope’ is not likely to be distributed in a smooth and symmetric fashion Padoan, Kritsuk, Norman (2005)

10. Planet Formation: Issues • Core Accretion • Barely fast enough in the SS; Jupiter (and Saturn?) • Very difficult to explain Jupiter’s abundance pattern • Much too slow at current Uranus & Neptune • Enter Nice model... • Much too slow for wide orbit M-dwarf gas giants • Hello Nice? • Excentric and non-coplanar orbits • ?? • Type I migration • arbitrary (and large) pre-factors

11. Planet Formation: Issues • Core Accretion • Barely fast enough in the SS; Jupiter (and Saturn?) • Very difficult to explain Jupiter’s abundance pattern • Much too slow at current Uranus & Neptune • Enter Nice model... • Much too slow for wide orbit M-dwarf gas giants • Hello Nice? • Gravitational Collapse • Does it work? What about cooling time scales? • conflicting results • What about the metallicity correlation?

12. Planet Formation: Major IssueWhen and How was Z separated from X+Y? • Current Paradigm: • Start out with planetisimals + some remnant gas • So, separation must have happened earlier!? • Let’s back up to that time then: • Gas + dust in a disk • Can gas and dust be separated? • Yes, easily! • Just read Weidenshilling (1977) • Unfortunately, the Sun devours the Z, leaves an X+Y disk

13. Enter Johansen, Boley, et al ... • Turbulence • driven by the magneto-rotational-instability (Johansen et al) • or by gravitational instabilities (Boley et al, see particularly arXiv:1005.2624) • Coupled, gas & dust dynamics • Turbulence creates vortices; concentrates dust • Turbulence + streaming-instabilities; concentrates dust • Locally extreme dust-to-gas ratio  gravitational collapse • Voila, planetesimals!

14. Planet Formation: Issues • Constraints from cosmo-chemistry • Live 26Al and 60Fe was present in the early SS • Allows only a few My from SN-injection to SS-inclusion • Isotopes – including the live ones – were initially homogeneously distributed • Shown by correlation 26Al – 54Cr • Bulk portions of the SS solids were subjected to ”thermalprocessing”, which sublimated some solids, including the (silicate?) ’carrier’ of 26Al • Caused major heterogeneity of 26Al

15. Cosmo-chemistry; 26Al revisited CAI = Calcium Aluminum Inclusions AOA= Amoeboid Olivine Aggregates The CAI and AOA inclusions have condensed out of a dense, 26Al rich gas phase

16. The key importance of 26Al • 26Al is radio-active, with a 750.000 yr half-life • Can be used as a very accurate clock, if initially uniform • It was present in the early Solar System • Enough to melt bodies larger than about 50-100 km • Need this bodies to form quickly! • It originates in ordinary supernovae • Was transported to the early Solar System in a few Myr • Now (in press) has shown to be subjected to ”thermal processing” (T > 1500 K) in the early SS

17. From the reproducibility btw samples, the time scale of formation of the first solids in the Solar System wasonlya few thousand yrs! Measurement technnique 27

18. Evidence from 26Al (Larsen, … Bizzarro 2010) The conclusion from this correlation is that 26Al was initially homogeously distrubuted, but suffered thermal processing

19. Returning to Planetesimal Formation:Consider ’Pebble Accretion’ • Cf. Johansen & Lacerda (2010) • ”Pebble accretion” onto planetesimals • ”Doubles the mass in less than 150 years”

20. My Question: Why stop there?! • Cf. Johansen & Lacerda (2010) • ”Pebble accretion” onto planetesimals • ”Doubles the mass in less than 150 years” • Why should it stop there? • Technical reasons: • Periodic box  constrained growth (Johansen) • Planetesimal Hill radius not resolved (Boley) • No physical reasons: • Hill radius keeps growing: volume proportional to mass • Unlike the end of run-away growth; excitation does not kill it

21. New Paradigm: Change of Focus • Primordial solar system (SS) gas has X+Y+Z=1 • Planets are gravitationally bound objects which have retained Z , but not much X+Y • Jupiter managed to keep about 1/3 • Other planets kept only Z + light atmosphere • When and How did the separation btw X+Y and Z take place?! • Before planet formation?? • Or, as part of planet formation?!

22. New Paradigm: Hypotheses • The heavy elements inallplanets accreted first • ”Pebble accretion” (Johansen et al) • ”Core assist plus gas capture” (Boley et al) • Atmospheres (gas contents) aresecondary consequences • Formed in initially near-hydrostatic equilibrium with hot proto-planetary gas disk • Disk gas density falls quickly, atmospheres cool and retract • Consistent atmosphere properties; estimated from core masses and orbits

23. Block Diagrams Gravitational Collapse Core Accretion New paradigm XYZ - Gravity XYZ - Gravity Z / XY - Separation Z / XY - Separation Z / XY - Separation Z - Accretion Z - Accretion Z - Accretion Migration Gas – Fractionation Gas - Accretion Gas - Accretion Gas - Fractionation

24. Atmospheric Fractionation • Hot (~1000 K) atmospheres in hot (or warm) pp-disk • Scale heights ~ radii at R ~ 10-100 R0 • Thermal N2-speed smaller than escape speed there • Thermal H2-speed larger than escape speed there • Leads to H-fractionation when disk pressure falls • Semi-quantitative estimates: • Mercury: total loss • Venus, Earth: surface pressure ~ 105 pp-disk • Mars: surface pressure ~ 102 pp-disk • Jupiter: only case that retains significant H • Saturn: looses more H+He • Uranus, Neptune: loose most of the H+He

25. New Paradigm: Solutions • Rapid planet formation • Satisfies cosmo-chemistry constraints • Planetary atmospheres & water • Consistent with allSS planet atmospheres • Jupiter’s abundance pattern • ’Lost’ (or did not capture) 2/3 of X+Y • Heavy fractination of Venus, Earth & Mars atmospheres • Long forgotten and neglected constraint (Pepin 1991)

26. Spacings are clearly approximately logarithmic (including in the SS), but the number of Hill radii seems superficially to have nothing to do with it New Paradigm: SolutionsLog-spacing of planet orbits • Approximate log-spacing of plants • Gregory (1715) • Titius (1766) • Bode (1772) • Hayes & Tremain (1998) • Poveda & Lara (2008) • Lovis et al (2010) • Consider the no. of Hill radii ... However, if the total (XY+Z) initial mass is used, all gaps are similar, in terms of Hill radii!

27. Summary; Brown Dwarfs • Brown Dwarfs can form the same way other stars form • Turbulent fragmentation in cold molecular gas • BD mass fragments are exceedingly numerous, but ... • ... only a tiny fraction are dense enough to collapse into BDs • Successful fragments are confined by very large dynamic pressure (smooth convergence + shock) • Brown Dwarfs and Massive Planets • Similar structure, but two modes of formation • direct, gravitational, by turbulent compression • indirect, assisted by (rapid!) core formation • Some massive ’planets’ in excentric, non-aligned orbits may form through the indirect (BD-like) mode

28. Summary; Planets • Planet formation; cosmo-chemical evidence • Major fraction of SS mass underwent ”thermal processing” • Planetesimals musthaveformedvery quickly (few thousand years) • Isotopic differences must be quickly ”locked” into planets • Planet formation; new paradigm • Focus on separation of initial X+Y+Z=1 into dust (keep) + gas (throw) • Concentrate on brief period when mass ratio (disk/star) peaks • Consider GI-saturated, self-regulating disks (Boley et al) • Consider streaming instabilities (Johansen et al) • Gas giant cores and rocky planets form quickly by ”pebble accretion” • Core mass & gas disk evolution controls acquisition of atmospheres • Estimates consistent with SS rocky planets, gas giants, and ice giants • Explains Jupiter’s abundance pattern • Earth’s and Venus’ fractionation pattern • Rapid planet formation  ’locking’ of minerals and isotope patterns

29. Thanks for your attention!