A chemical perspective on planet formation in reduced systems

¹ Institute of Geochemistry and Petrology, ETH Zurich, Switzerland
² Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark

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Received: 14 November 2025
Accepted: 17 March 2026

Abstract

Context. The relative abundances of refractory elements in planets are widely assumed to reflect those of their host stars. However, because elements are classified according to their behaviour in the solar nebula, this implicitly assumes that condensation is independent of nebular chemistry, despite contradictory evidence in chemically reduced systems with high molar carbon-to-oxygen (C/O) ratios.

Aims. We investigated how variations in stellar C/O ratio and disk pressure modify condensation chemistry, and assessed the reliability of mapping stellar compositions to planetary building blocks in reduced environments.

Methods. For a sample of FGK stars with C/O ratios spanning 0.65–0.95 (solar = 0.59±0.08), we computed the equilibrium phase stability using FactSage over 1900–400 K at total pressures of 10⁻², 10⁻⁴, and 10⁻⁶ bar. We tracked the phase evolution and key chemical transitions across C/O, temperature, and pressure. Bulk planet(esimal) compositions were derived using a stochastic accretion framework that aggregates condensates from temperature-dependent feeding zones.

Results. We identified three distinct condensation regimes: (i) solar-like (C/O ≲ 0.7), (ii) transitional (C/O ~ 0.7–0.91), and (iii) reduced (C/O ≳ 0.92). Relative to solar-like sequences, oxygen-bearing silicates condense at lower temperatures in transitional and reduced regimes, while carbides, silicides, and sulfides appear. Bulk planetesimal Fe/Mg, Fe/Si, and Fe/O ratios deviate substantially from their host stellar values in transitional and reduced sequences, thus producing more diverse rocky building blocks within the same disk, ranging from metal-rich C- and S-bearing bodies to more Earth-like compositions.

Conclusions. Condensation sequences are not universal across stellar compositions. In reduced disks, elemental ratios commonly treated as refractory based on the solar system condensation temperatures may not reliably trace planetary bulk composition. The distinct building blocks produced in high C/O systems thus provide potential formation pathways for metal-enriched super-Mercury analogues and distinct C- and S-rich rocky planets, expanding the diversity of terrestrial compositions beyond solar system analogues.