The compositional diversity of rocky exoplanets around K-dwarf stars

¹ Centre for Planetary Habitability (PHAB), Department of Geosciences, University of Oslo, PO Box 1028 Blindern, 0315 Oslo, Norway
² Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom
³ Konkoly Observatory, HUN-REN CSFK, MTA Centre of Excellence ; Konkoly Thege Miklos St. 15–17, 1121 Budapest, Hungary

^★ Corresponding author; petra.hatalova@geo.uio.no

Received: 11 June 2024
Accepted: 16 December 2024

Abstract

Context. The mass-radius distribution of currently known exoplanets suggests a fascinating variety in terms of their chemical compositions. Still, the interior structures and compositions of rocky exoplanets are essentially inaccessible to observations. Here, we present a model that combines planet formation simulations with equilibrium condensation models to estimate the interior composition and structure of exoplanets around K-dwarf stars.

Aims. In a previous study, we reproduced the observed population of close-in super-Earths around K-dwarf stars with suitable initial conditions. However, the masses and radii of these simulated planets are not self-consistent, since the radius information is postprocessed based on an assumed constant average density (3 g cm⁻³). In this work, we have incorporated compositions into the N-body simulations using the chemistry of the protoplanetary disk from equilibrium condensation sequences, together with thermodynamics of the various mineral phases. This has allowed us to develop models of likely interior structures, bulk densities, and, thus, radii for rocky exoplanets.

Methods. We selected the outcomes of ten N-body simulations around a 0.8 M_⊙ star that best reproduced the observed population. We employed stellar abundances from six K-dwarf stars with a range of metallicities. Based on the condensation sequence and the temperature-pressure profile of a disk, we assigned bulk elemental compositions to planetesimals and then tracked materials accreted onto planets during their formation.

Results. We have obtained a set of planets with more realistic radii determinations than those purely based on one preset density for all bodies. We formed various types of planets: i) Ca- and Al-rich(er), ii) Mg-depleted, iii) fully oxidized core-less planets, iv) planets similar to Earth or Mars in composition and core size, and v) planets with different core mass fractions (from significantly smaller than the Martian core to significantly larger than the Earth’s core); however, we do not have Mercury-like planet with a huge core.