Dottorato in ASTRONOMIA

Titolo della tesi: The multifaceted connection between the atmospheric composition of giant planets and their native disc

During their formation and migration to their final orbit, giant planets can interact with physically and chemically diverse environments in their native protoplanetary disc. This interaction influences the accretion of gas and solids by the planets and leaves chemical fingerprints in the composition of their atmospheres. Our understanding of these fingerprints is mainly limited to two chemical elements, namely carbon and oxygen, and a few scenarios for the structure of the protoplanetary disc. The aim of this thesis is to identify fingerprints of planet formation in a wider range of chemical elements and to investigate how sensitive they are to the physical and chemical conditions in the planet formation environment and to the planet formation process itself. The developed framework simulates the composition of primordial planetary atmospheres by combining numerical simulations of the chemical and dynamical evolution of gas and solids in protoplanetary discs with detailed N-body simulations of giant planet formation and migration in the core accretion paradigm. We use the simulated atmospheric abundances of chemical elements characterised by different volatility, such as carbon, oxygen, nitrogen, and sulphur, to study the combined effects of different initial chemical conditions in the native disc, the viscous evolution of the disc gas, the size-dependent dynamical evolution of the disc dust, planetary migration, and the accretion of gas and planetesimals. We find that disc evolution strongly influences the radial distribution of carbon and oxygen in the disc and their partitioning between gas and solids. Since planets derive their composition from the accreted disc material, this chemical diversity in the disc is transported into planetary atmospheres. In particular, we find that the planetary C/O ratio does not allow a clear interpretation. However, when the C/O ratio is compared to other elemental ratios, including the refractory-to-volatile ratio as probed by the S/N ratio, and the composition of the host star, we can constrain various aspects of planet formation, such as the source of planetary metallicity, the extent of planetary migration, and the properties of the formation environment. For example, we can identify signatures of accretion histories dominated by planetesimals, gas, or gas enriched in volatiles due to the sublimation of drifting icy grains. The thesis provides a robust interpretative framework that has already been applied to ground-based spectroscopic characterisations of exoplanets and will support the analysis of current and future observations with the next generation of telescopes, such as JWST, Ariel, and ELT.