Interior redox state effects on the stability of secondary atmospheres and observational manifestations: LP 791-18 d as a case study for outgassing rocky exoplanets

¹ Ludwig Maximilian University, Faculty of Physics, University Observatory, Scheinerstrasse 1, Munich 81679, Germany
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ ETH Zürich, Center for Origin and Prevalence of Life, Department of Earth and Planetary Sciences, Institute for Geochemistry and Petrology, 8092 Zurich, Switzerland
⁴ Department of Astronomy, University of California, Berkeley, Berkeley, CA 94720-3411, USA
⁵ Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720-4767, USA

^★ Corresponding author: leo.gkouvelis@physik.lmu.de

Received: 19 February 2025
Accepted: 2 June 2025

Abstract

Recent advances in space-based and ground-based facilities now allow the atmospheric characterization of a selected sample of rocky exoplanets. These atmospheres offer key insights into planetary formation and evolution, but their interpretation requires models that couple atmospheric processes with both the planetary interior and the surrounding space environment. This work focuses on the Earth-size planet LP 791-18 d, which is estimated to receive continuous tidal heating due to the orbital configuration of the system; thus, it is expected to exhibit volcanic activity. Using a 1D radiative-convective model coupled with chemical kinetics and an outgassing scheme at the lower boundary, we simulated the planet’s atmospheric composition across a range of oxygen fugacities, surface pressures, and graphite activities. We estimated the mantle temperature of ≈1680–1880 K, balancing the competing contributions of interior tidal heating and convective cooling. Our results show that the atmospheric mean molecular weight gradient is controlled by oxygen fugacity rather than bulk metallicity. Furthermore, we used the atmospheric steady-state solutions produced from the interior redox state versus surface pressure parameter space, and explored their atmospheric stability. We find that stability is achieved only in highly oxidized scenarios, fO₂ − IW ≳ 2, while reduced interior states fall into the hydrodynamic escape regime with mass loss rates on the order of ≈10⁵−10⁸ kg/s. We argue that scenarios with reduced interior states are likely to have exhausted their volatile budget during the planet’s lifetime. Furthermore, we predict the atmospheric footprint of the planet’s interior based on its oxidation state and assess its detectability using current or forthcoming tools to constrain the internal and atmospheric composition. We show that the degeneracy between bare rock surfaces and thick atmospheres can be resolved by using three photometric bands to construct a color-color diagram that accounts for potential effects from photochemical hazes and clouds. For JWST/MIRI, this discrimination is possible only in the case of highly oxidized atmospheres. The case of LP 791-18 d enables the investigation of secondary atmosphere formation through outgassing, with implications for similar rocky exoplanets. Our modeling approach connects interior and atmospheric processes, providing a basis for exploring volatile evolution and potential habitability.