Proto-planetary disk composition-dependent element volatility in the context of rocky planet formation

¹ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
² Institute of Geophysics, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland
³ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁴ Center for Star and Planet Formation, Globe Institute, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark
⁵ Bavarian Geoinstitute for Experimental Geochemistry and Geophysics, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany
⁶ HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, 15-17 Konkoly Thege Miklos ut, Budapest 1121, Hungary
⁷ Department of Geological Sciences, University of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, CO 80309, USA
⁸ Institute of Geochemistry and Petrology, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland

^★ Corresponding authors: spaargaren@astro.rug.nl; oliver.herbort@univie.ac.at; haiyang.wang@sund.ku.dk

Received: 18 June 2025
Accepted: 5 September 2025

Abstract

Aims. The inferred compositions of the Solar System’s terrestrial (rocky) bodies are fractionated from that of the Sun, where elemental depletions in the bulk rocky bodies correlate with element volatility, expressed in its 50% condensation temperature. However, because element volatility depends on disk gas composition, it is not mandated that elemental fractionation trends derived from the solar-terrestrial scenario apply to other planetary systems. Here, we expand upon previous efforts to quantify elemental volatility during disk condensation, and how this affects rocky planet compositional diversity.

Methods. We simulated condensation sequences for a sample of 1000 initial disk compositions based on observed stellar abundances. Based on these simulations, we present parametrisations of how element 50% condensation temperatures depend on disk composition and apply element fractionation trends with appropriate element volatility to stellar abundances to simulate compositions of rocky exoplanets with the same volatile depletion pattern as the Earth, providing a robust and conservative lower limit to the compositional diversity of rocky exoplanets.

Results. Here we show that Earth-like planets emerge from low-C/O disks (C/O ≤ 0.75) and graphite-bearing planets from medium-to-high-C/O disks (C/O > 0.75). Furthermore, we identify an intermediate-C/O (0.84–1.04) class of planets characterised by Mg and Si depletion, leading to relatively high abundances of Fe, Ca, and A1. We show that devolatilisation patterns could be adapted potentially with disk composition-dependent condensation temperatures to make predictions of rocky planet bulk compositions within individual systems, although such patterns could be further modified by the dynamics of planetary accretion, which remains under-constrained for most exoplanetary systems. The outcomes of our analysis suggest that accounting for disk composition-dependent condensation temperatures means that we can expect an even broader range of possible rocky planet compositions than has previously been considered.