Planet Formation in Chemically Diverse and Evolving Discs

Giant planets form in protoplanetary discs, where the coupled dynamical and chemical evolution of gas and solids determines the composition of the material they accrete. We investigate how planet formation and migration in evolving discs shape the primordial elemental makeup of giant planet atmospheres. Our aim is to establish a multi-element framework linking atmospheric compositions to the formation pathways of planets and the time-dependent chemical properties of their natal discs. We coupled one-dimensional models of viscously evolving discs – incorporating radial dust drift and a comprehensive treatment of volatile chemistry – with N -body simulations of planetesimals interacting with a growing and migrating giant planet. Four chemical scenarios (inheritance and reset under low and high ionisation) and three representative grain sizes (0.1, 20, and 100 μm) were explored. The coupled outputs were post-processed to track the accretion of carbon, oxygen, nitrogen, and sulphur, and to derive atmospheric elemental ratios normalised to stellar values (* denotes stellar normalisation). We identified three atmospheric classes corresponding to distinct accretion regimes: gas-dominated (GD), characterised by N/O* > C/O* > C/N* and unconstrained or substellar S/N* (near-stellar C/S*); planetesimal-dominated (PD), showing N/O* < C/O* < C/N*, S/N* ≥ C/N*, and C/S* ≤ C/O*; and drift-enhanced (DE), exhibiting N/O* < C/O* < C/N* and markedly super-stellar volatile-to-refractory ratios. We see that N/O*, C/N*, and S/N* vary systematically with migration extent, although degeneracies arise for planets forming beyond the CO and N 2 snow lines; in addition, C/O* remains largely insensitive. Metallicity alone does not uniquely trace the balance between solid and gas accretion in drift-dominated regimes. Variations in the disc’s chemical state and dust size imprint distinctive volatile-ratio patterns across the atmospheric classes, providing complementary constraints on disc properties. This multi-element framework establishes predictive trends to guide the interpretation of atmospheric spectra from current and forthcoming facilities, such as JWST and Ariel, in the context of giant planet formation.