Planet formation in chemically diverse and evolving discs

I. Composition of planetary building blocks

¹ INAF Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
² INAF Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
³ Institut für Theortische Astrophysik, Universität Heidelberg, Zentrum für Astronomie, Albert-Ueberle-Str 2, 69120 Heidelberg, Germany
⁴ School of Physics and Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
⁵ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁶ Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁸ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
⁹ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁰ ICSC – National Research Centre for High Performance Computing, Big Data and Quantum Computing, Via Magnanelli 2, 40033 Casalecchio di Reno, Italy
¹¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Viale Berti Pichat 6/2, Bologna, Italy
¹² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

^★ Corresponding author: elenia.pacetti@inaf.it

Received: 3 February 2025
Accepted: 20 June 2025

Abstract

Protoplanetary discs are dynamic environments where the interplay between chemical processes and mass transport shapes the composition of gas and dust available for planet formation. We investigate the combined effects of volatile chemistry (including both gas-phase and surface reactions), viscous gas evolution, and radial dust drift on the composition of planetary building blocks. We explore scenarios of chemical inheritance and reset under varying ionisation conditions and dust grain sizes in the submillimetre regime. We simulated the disc evolution using a semi-analytical 1D model that integrates chemical kinetics with gas and dust transport, accounting for viscous heating, turbulent mixing, and refractory organic carbon erosion. We find that mass transport plays a role in the chemical evolution of even sub-μm grains, especially in discs that have experienced strong heating or are exposed to relatively high levels of ionising radiation. The radial drift of relatively small (~100 μm) icy grains can yield significant volatile enrichment in the gas phase within the snowlines, increasing the abundances of species like H₂O, CO₂, and NH₃ by up to an order of magnitude. Early planetesimal formation can lead to volatile depletion in the inner disc on timescales shorter than 0.5 Myr, while the erosion of refractory organic carbon can lead to markedly superstellar gas-phase C/O and C/N ratios. Notably, none of the analysed scenarios were able to reproduce the classical monotonic radial trend of the gas-phase C/O ratio predicted by early models. Our results also show that a pairwise comparison of elemental ratios, in the context of the host star’s composition, is key to isolating signatures of different scenarios in specific regions of the disc. We conclude that accurate models of planet formation must concurrently account for the chemical and dynamical evolution of discs, as well as the possible diversity of their initial chemical and physical conditions.