The debate between the methods of pebble and planetesimal accretion for the formation of planets has yet to be resolved. Both of these scenarios have been looked into individually and now this work looks to combine them along with investigating distinguishing parameters between the scenarios.
The main aim of this work is to investigate the effect of planetesimal formation and accretion on the composition of gas giants, an answer the question of whether the composition of a planet can constrain its formation pathway.
We perform semi-analytical 1D simulations of protoplanetary disks, including the treatment of viscous evolution and heating, pebble drift, and simple chemistry to simulate the growth of planets from planetary embryos to gas giants through pebble, planetesimal, and gas accretion as they migrate through the disk. The chemical composition of the planet's atmosphere is traced in order to look for parameters that may constrain its formation scenario.
Our simulation confirms, as presented in Bitsch 2021, that the composition of the planetary gas atmosphere is dominated by the accretion of gas vapor that is enriched by inward drifting evaporating pebbles. Including planetesimal formation hinders this enhancement, as many pebbles are locked into planetesimals and cannot evaporate. This results in a dramatic drop of the accreted heavy elements both in the planetesimal formation and accretion case, demonstrating that planetesimal accretion needs to be inefficient in order to explain planets with high heavy element content. The pebble accretion scenario can explain super-solar C/H and O/H abundances, while the planetesimal formation and accretion scenarios present a general depletion of most of the elemental species. We observe a remarkable drop in the volatile-to-refractory ratio in the planetesimal accretion case, due to a large number of refractories being accreted through planetesimals, which may constrain the formation pathway of the planet.