How the chemical composition of solids influences the formation of planetesimals

¹ MAUCA – Master track in Astrophysics, Université Côte d’Azur & Observatoire de la Côte d’Azur, Parc Valrose, 06100 Nice, France
² Department of Physics, University College Cork, Cork, Ireland
³ Center for Star and Planet Formation, Globe Institute, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen, Denmark
⁴ Department of Physics, Muni University, PO Box 725, Arua, Uganda

^★ Corresponding author: bbitsch@ucc.ie

Received: 20 May 2025
Accepted: 11 July 2025

Abstract

The formation of planetesimals is a necessary step for the formation of planets. While several methods exist that can explain the formation of planetesimals, an increase in the local dust-to-gas ratio above unity is a strong requirement to trigger the collapse of the pebble cloud to form planetesimals. One prime location for this to happen is at the water-ice line, where large water-rich pebbles evaporate and release their smaller silicate cores, resulting in an increase in the local dust-to-gas ratio originating from the different inward velocities of the large and small pebbles. While previous work indicated that planetesimal formation becomes very challenging at overall dust-to-gas ratios below 0.6%, in line with the occurrence of close-in super-Earths, it is unclear how the overall disc composition influences the formation of planetesimals. Observations of stellar abundances not only indicate a decrease in the overall C/O ratio for low metallicity stars, they also show a large spread in the C/O ratios. However, the C/O ratio sets the abundance of water ice within the disc. Using the 1D numerical disc evolution code chemcomp, we simulated protoplanetary discs with varying C/O ratios and dust-to-gas ratios over a 3 Myr timescale. Planetesimal formation is modelled by implementing conditions based on dust-gas dynamics and pebble fragmentation. Our results confirm that planetesimal formation is highly dependent on disc metallicity with lower metallicity discs forming significantly fewer planetesimals. We find that a decreased carbon fraction generally enhances planetesimal formation, while a higher carbon fraction suppresses it due to a reduced water abundance at the same dust-to-gas ratio. The opposite is the case with the oxygen ratio, where larger oxygen fractions allow a more efficient formation of planetesimals at the same overall dust-to-gas ratio. Consequently we make the prediction that planets around low metallicity stars should be more common if the stars have low C/O ratios, especially when their oxygen abundance is increased compared to other elements, testable through observations. Our simulations thus open a pathway to understanding whether the composition of the planet-forming material influences the growth of planets.