From proto to planet: the evolution of planets born through gravitational instability

The gravitational instability (GI) theory of planet formation describes how planets are born via the fragmentation of young circumstellar accretion discs into order 1-10 Jupiter mass protoplanets. In this thesis I examine the various competing process that act on these protoplanets as they evolve, accrete and migrate into their final configurations.

In Chapter three I describe a population synthesis model of GI that I interfaced with state-ofthe-art protoplanet contraction simulations. I combine these results with Radial Velocity (RV) and Direct Imaging (DI) observations to constrain the modern GI theory, finding that GI must occur in tens of percent of systems if it formed the gas giants inside 5 AU. Chapter four details 3D smoothed particle hydrodynamics (SPH) simulations that I ran in order to investigate the accretion of metallic pebbles onto GI protoplanets. I show that migrating protoplanets efficiently accrete all pebbles in the 0.3-300 millimetre size range, potentially increasing protoplanet metallicity to 10% provided that 30-50% of metals in young discs are locked into pebbles. In Chapter five I extend these simulations in order to resolve protoplanet internal structure and to study the formation of rocky cores. I find that thermal feedback from core growth is able to completely disrupt protoplanets, leaving behind a population of 1-10 Earth mass rocky cores at tens of AU in young discs. In this thesis I show that if disc fragmentation is common, if gas accretion is limited (due to high opacity or protoplanet feedback) and if a population of pebble size grains (greater than 0.1 millimetre) exist in young discs, then GI is able to form essentially all configurations of planetary systems. Future work should aim to better understand these assumptions in order to constrain to what extent GI may work alongside the core accretion (CA) theory of planet formation.