How initial disc conditions sculpt the atmospheric composition of giant planets

¹ Department of Physics, University College Cork, College Rd, Cork T12 K8AF, Ireland
² Max Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany

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

Received: 28 July 2025
Accepted: 7 January 2026

Abstract

Past studies have revealed the dependency of disc parameters (e.g. disc mass, radius, viscosity, grain fragmentation velocity, dust-togas ratio) on the formation of giant planets, where more massive discs seem beneficial to the formation of giant planets. However, it is unclear how different disc properties influence the composition of forming giant planets. In particular, the idea that atmospheric abundances can directly trace the formation location of planets is put into question by the fact that the chemical evolution of a disc can be caused by inward drifting, evaporating pebbles and chemical reactions. This notion complicates the idea of a simple relation between atmospheric abundances and planet formation locations. Here, we use planet formation simulations that include the effects of pebble drift and evaporation, and we investigate how different disc parameters influence the atmospheric composition of growing giant planets. We focus on the atmospheric C/O, C/H, O/H, and S/H ratios, which allow us to probe tracers of volatiles and refractories and thus, different accretion pathways of giant planets. We find that most of the disc parameters only have a limited influence on the atmospheric abundances of gas giants, except for the dust-to-gas ratio, where a larger dust-to-gas ratio results in higher atmospheric abundances. However, the atmospheric abundances are determined by the planetary formation location, even in the pebble drift and evaporation scenario. Our study suggests that volatile-rich giant exoplanets predominantly form in the inner disc regions, where they can accrete large fractions of vapour-enhanced gas. Our study shows that simulations that try to trace the origin of giant planets via their atmospheric abundances do not have to probe all disc parameters as long as the disc parameters allow for the formation of giant planets (e.g. a sufficiently high disc mass). Consequently, our study suggests that the diversity of observed planetary compositions is a direct consequence of their formation location and migration history.