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A Unitary Model for the Formation of Terrestrial Planets and (Rocky) Close-In Super-Earths A

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A Unitary Model for the Formation of Terrestrial Planets and (Rocky) Close-In Super-Earths A A unitary model for the formation of terrestrial planets and (rocky) close-in super-Earths A. Morbidelli (OCA, Nice), M.Lambrechts, S.Jacobson, A.Johansen, A.Izidoro, S.Raymond, B.Bitsch Planet populations Because of their unveiled rocky nature, SuperEarths are often considered as scaled-up versions of the Solar System’s terrestrial planets. Is this correct? Jupiter Saturn Neptune SuperEarths Uranus Earth Owen and Wu, 2017 Data from Fulton et al., 2017 Venus ? Mars Dressing et al., 2015 K11f Mercury K11b WHAT IS THE EARTH? Tform=40-120 My • Not just a 1 ME planet @ 1 AU • Long accretion timescale • Late Moon-forming event • Protracted core formation • Assemblage from Mars-mass embryos Tform= 2-4 My • The small mass of the embryos explain why they did not migrate close to the Sun WHAT ABOUT PLANETS MORE MASSIVE THAN EARTH? More massive final planets -> more mass available Planetary embryos are expected to grow by pebble accretion. So, more mass available -> higher pebble flux We have done simulations of pebble accretion featuring: • Pebble accretion, migration, mutual interactions (scattering, merging) • An initial system of Moon-mass planetesimals • Gas disk decaying as exp(-t/1My) from MMSN-like density • Pebble-flux decaying as exp(-t/1My) from different initial fluxes • Pebble Stokes number: 3x10-3 Integrated pebble flux = 38 ME Integrated pebble flux = 114 ME Integrated pebble flux = 190 ME Total final mass ~ 1ME Total final mass ~ 9ME Total final mass ~ 20-30 ME Inner disk’s edge Inner disk’s edge Inner disk’s edge SuperEarths formed within disk’s lifetime No Earth-mass planets formed within disk’s lifetime No Earth-mass planets formed within disk’s lifetime Discontinuities mark merging events accretion -> strong migration accretion -> some migration Final distribution of planetary systems, at the end of gas-disk lifetime High pebble flux: Large migration pb. flux = 340 M Feedback of migration on accretion E Different sims. -> different results pb. flux = 190 ME pb. flux = 114 ME Low pebble flux: Small migration Solar System embryos Ordered accretion Different sims. -> same results pb. flux = 38 ME Beyond the gas-disk lifetime: Instabilities! Beyond the gas-disk lifetime: Instabilities! Instability causes multiple merging events of embryos. This leads to planets up to 4 ME on a timescale much longer than the disk’s lifetime. Analog to Earth formation in the Solar System Beyond the gas-disk lifetime: Instabilities! Beyond the gas-disk lifetime: Instabilities! Instabilities break resonant chains (Izidoro et al., 2017). Systems spread. Merging events possible. Reproduces Kepler observations well in terms of period-ratio distribution Summary of final systems • Overlapping mass-range • Overlapping distances from host star Migration-dominated formation mode For individual planets, it may not be easy to say which way they formed Best diagnostic: the presence of a H-rich atmosphere Earth-like • planets formed in the migration dominated mode acquire a formation mode large mass within the lifetime of the disk, so they can accrete primitive atmospheres • They can suffer a few post-gas giant impacts but these are ineffective in removing atmospheres (Schlichting & Mukhopadhyay, 2018) • Exception: evaporation for strongly irradiated planets But the planetary systems formed in the different regimes are clearly distinct! CONCLUSIONS We have produced a unitary model for the formation of terrestrial planets and rocky super-Earths • The pebble mass-flux is the key parameter -> see next slide • Two different formation regimes • Two distinct final populations These results, and the observed distribution of extrasolar planets Earth suggest that we have not Venus Mars discovered truly terrestrial planets yet Mercury What sets the pebble mass-flux? Unclear! Certainly: • Disk mass • Metallicity • Size But it can be much more complicated than that: • Fraction of pebble sequestered planetesimal formation • Possibility to generate “late pebbles” as debris • Apparition of pressure bumps (e.g. giant planet formation) blocking the pebble flux Not just the mass-flux matters but also the pebble accretion efficiency, which depends on: • Pebbles’ stokes number (dependent on pebble size and gas density) • Size of the embryos’ seeds • Scale height of pebbles layer (depends on disk’s turbulence and pebbles Stokes number) In the case of the Solar System, it is likely that the early formation of Jupiter (<1My; Kruijer et al., 2017) was the key factor limiting the pebble mass flux.
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