Heavy-element-enriched atmospheres and where they are born

¹ Department of Physics, University College Cork, Cork, Ireland
² School of Physics, Trinity College Dublin, University of Dublin, Dublin 2, Ireland

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 August 2025
Accepted: 3 December 2025

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 since the level of heavy element enrichment a planet can accrete, as well as its atmospheric composition, is strongly dependent on where in the disc it is forming. We used a planet formation model that simulates the evolution of the protoplanetary disc, accounting for pebble growth, drift and evaporation, and the formation of planets from pebble and gas accretion. We simulated the growth and migration of planetary embryos in ten different protoplanetary discs whose chemical compositions are matched to the host stars of the planets that we aim to reproduce; this provided a more realistic model of their growth than previous studies. The heavy element content of giant exoplanets was used to infer their formation location and thus to estimate their atmospheric abundances. We focused 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 nine out of the ten simulated planets 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, which is 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 placed on the planet’s formation.