Heavy element enriched atmospheres and where they are born

Abstract

The heavy element content of giant exoplanets, inferred from structure models based on their radius and mass, often exceeds predictions based on classical core accretion. Pebble drift, coupled with volatile evaporation, has been proposed as a possible remedy to this with the level of heavy element enrichment a planet can accrete, as well as its atmospheric composition, being strongly dependent on where in the disc it is forming. We use a planet formation model which simulates the evolution of the protoplanetary disc, accounting for pebble growth, drift and evaporation, and the formation of planets from pebble and gas accretion. The growth and migration of planetary embryos is simulated in 10 different protoplanetary discs which have their chemical compositions matched to the host stars of the planets which we aim to reproduce, providing a more realistic model of their growth than previous studies. The heavy element content of giant exoplanets is used to infer their formation location and thus make a prediction of their atmospheric abundances. We focus here on giants more massive than Saturn, as we expect that their heavy element content is dominated by their envelope rather than their core. The heavy element content of 9 out of the 10 planets simulated is successfully matched to their observed values. Our simulations predict formation in the inner disc regions, where the majority of the volatiles have already evaporated and can thus be accreted onto the planet via the gas. As the majority of the planetary heavy element content originates from water vapour accretion, our simulations predict a high atmospheric O/H ratio in combination with a low atmospheric C/O ratio, in general agreement with observations. For certain planets, namely WASP-84b, these properties may be observable in the near future, offering a method of testing the constraints made on the planet's formation.