Formation of Terrestrial-like Systems with Pebble Accretion

Abstract:

Terrestrial planet formation concludes, after the protoplanetary gas disc dissipates, in a giant impact phase where protoplanets with masses similar to Mars -or larger- collide to form the final planetary system. It remains however unclear how these protoplanets form in a gas disc that only survives for a few Myr. The formation of such Mars-like bodies by planetesimal accretion, at Mars-like orbits, exceeds the gas disc lifetime, even when assuming a planetesimal population with a collective mass exceeding several Earth masses (Walsh & Levison, 2019).

Here we model the complete growth evolution, from planetesimal to planet, using a new GPU-accelerated N-body code (based on Genga, Grimm & Stadel 2014) that includes how protoplanets can accrete pebbles. Observations of young protoplanetary discs show the presence of tens to hundreds of Earth masses of such mm-sized pebbles drifting towards the terrestrial zone. There, planetary embryos can accelerate their growth by efficiently sweeping up these pebbles, because gas drag increases their accretion cross section (Lambrechts et al 2019).

Two initial planetesimal seed distributions are explored: a narrow 0.1 AU annulus at the location of Mars, or a more broad planetesimal ring between Mercury and Ceres-like orbits. We find that, for both cases, an initial populations of Ceres-mass seed planetesimals with a collective mass of only ~ 2 Mars masses are sufficient to allow protoplanets to grow to Mars mass, or even larger, depending the pebble flux (between 20 and 100 Earth mass per Myr) and the gas disc lifetime.

Larger-than-Mars embryos migrate towards 1AU. In this way an orbital configuration can be reached with a few near-Earth mass planets around 1 AU and smaller Mars-mass objects on wider orbits (and in also closer orbits for the wide ring initial condition), reminiscent of the Solar System terrestrial planets.

This scenario implies that the bulk composition of the terrestrial protoplanets will be dominated by -typically late- accreted pebbles. This opens up new avenues to understand the composition of Solar System terrestrial planets, which appear to be in line with recent cosmochemical results arguing that the Earth grew to near completion within a few Myr (Schiller et al 2018).