Composition of giant planets: The roles of pebbles and planetesimals

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One of the current challenges of planet formation theory is to explain the enrichment of observed exoplanetary atmospheres. While past studies have focused on scenarios where either pebbles or planetesimals are the main drivers of heavy element enrichment, here we combine the two approaches to understand whether the composition of a planet can constrain its formation pathway. We study three different formation scenarios: pebble accretion, pebble accretion with planetesimal formation inside the disc, and combined pebble and planetesimal accretion. We used the CHEMCOMP code to perform semi-analytical 1D simulations of protoplanetary discs, including viscous evolution, pebble drift, and simple chemistry to simulate the growth of planets from planetary embryos to gas giants as they migrate through the disc, while simultaneously tracking their composition. Our simulations confirm that the composition of the planetary atmosphere is dominated by the accretion of gas vapour enriched by inward-drifting and evaporating pebbles. Including planetesimal formation hinders this enrichment because many pebbles are locked into planetesimals and cannot evaporate and enrich the disc. This results in a dramatic drop in accreted heavy elements in the cases of planetesimal formation and accretion, demonstrating that planetesimal formation needs to be inefficient in order to explain planets with high heavy element content. On the other hand, accretion of planetesimals enhances the refractory component of the atmosphere, leading to low volatile-to-refractory ratios in the atmosphere, in contrast to the majority of pure pebble simulations. However, low volatile-to-refractory ratios can also be achieved in the pure pebble accretion scenario if the planet migrates all the way into the inner disc and accretes gas that is enriched in evaporated refractories. Distinguishing these two scenarios requires knowledge about the planet’s atmospheric C/H and O/H ratios, which are much higher in the pure pebble scenario compared to the planetesimal formation and accretion scenario. This implies that a detailed knowledge of the composition of planetary atmospheres could help to distinguish between the different formation scenarios.