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
What’s the difference? Two alternatives: 1. Ask the IAU 2. 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...
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 1000 3 cells, Mach 9 supersonic MHD-turbulence Padoan & Nordlund (2010) Brown Dwarfs are marked with black dots, more massive stars with white dots.
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
Mass accreting from the ’envelope’ is not likely to be distributed in a smooth and symmetric fashion
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
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?
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
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!
Constraints from cosmo-chemistry Live 26 Al and 60 Fe 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 26 Al – 54 Cr Bulk portions of the SS solids were subjected to ”thermal processing”, which sublimated some solids, including the (silicate?) ’carrier’ of 26 Al Caused major heterogeneity of 26 Al
The CAI and AOA inclusions have condensed out of a dense, 26 Al rich gas phase CAI = Calcium Aluminum Inclusions AOA= Amoeboid Olivine Aggregates
26 Al 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
From the reproducibility btw samples, the time scale of formation of the first solids in the Solar System was only a few thousand yrs! 27
The conclusion from this correlation is that 26Al was initially homogeously distrubuted, but suffered thermal processing
Cf. Johansen & Lacerda (2010) ”Pebble accretion” onto planetesimals ”Doubles the mass in less than 150 years”
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
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?!
The heavy elements in all planets accreted first ”Pebble accretion” (Johansen et al) ”Core assist plus gas capture” (Boley et al) Atmospheres (gas contents) are secondary 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
Hot (~1000 K) atmospheres in hot (or warm) pp-disk Scale heights ~ radii at R ~ 10-100 R 0 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 ~ 10 5 pp-disk Mars: surface pressure ~ 10 2 pp-disk Jupiter: only case that retains significant H Saturn: looses more H+He Uranus, Neptune: loose most of the H+He
Rapid planet formation Satisfies cosmo-chemistry constraints Planetary atmospheres & water Consistent with all SS 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)
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... Spacings are clearly approximately logarithmic (including in the SS), but the number of Hill radii seems superficially to have nothing to do with it However, if the total (XY+Z) initial mass is used, all gaps are similar, in terms of Hill radii!
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
Planet formation; cosmo-chemical evidence Major fraction of SS mass underwent ”thermal processing” Planetesimals must have formed very 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