Astrogeology

An Oxidation Gradient Straddling the Small Planet Radius Valley

By Keith Cowing

Status Report

astro-ph.EP

March 12, 2025

Filed under astro-ph.EP, astrochemistry, astrogeology, atmosphere, exoplanet, radius valley, rocky exoplanet, Spectroscopy, sub-Neptune

An Oxidation Gradient Straddling the Small Planet Radius Valley — Atmospheric composition trends for planets around M stars after 5 Gyr. Colors are chosen arbitrarily and color intensity indicates species molar concentrations (panels a, b) and simulated planet density (panels c, d) as a function of orbital period vs. planet radius/atmospheric scale height. For panels c and d, each group corresponds to atmospheres with xi ≥ 50%, except H2O, for which the cutoff is 5%. Contour lines in panels c and d trace the various planet families. Planets trend toward greater oxidation with smaller radii and shorter orbital periods as a result of atmospheric escape-driven fractionation and magma ocean volatile exchange. H2O-rich planets are exceptional in that they are not well confined in P − Rp space and typically do not reach atmospheric molar concentrations above ≈ 10% as a result of our dry start assumption. The dashed line in panel a shows the radius valley calculated from the present simulations and the dotted line shows that from Cherubim et al. (2024). An animated version of this plot is available at: https://github.com/cjcollin37/IsoFATE/blob/main/animated figures.md. — astro-ph.EP

We present a population-level view of volatile gas species (H₂, He, H₂O, O₂, CO, CO₂, CH₄) distribution during the sub-Neptune to rocky planet transition, revealing in detail the dynamic nature of small planet atmospheric compositions.

Our novel model couples the atmospheric escape model 𝙸𝚜𝚘𝙵𝙰𝚃𝙴 with the magma ocean-atmosphere equilibrium chemistry model 𝙰𝚝𝚖𝚘𝚍𝚎𝚕𝚕𝚎𝚛 to simulate interior-atmosphere evolution over time for sub-Neptunes around G, K and M stars.

Chiefly, our simulations reveal that atmospheric mass fractionation driven by escape and interior-atmosphere exchange conspire to create a distinct oxidation gradient straddling the small-planet radius valley.

We discover a key mechanism in shaping the oxidation landscape is the dissolution of water into the molten mantle, which shields oxygen from early escape, buffers the escape rate, and leads to oxidized secondary atmospheres following mantle outgassing.

Our simulations reproduce a prominent population of He-rich worlds along the upper edge of the radius valley, revealing that they are stable on shorter timescales than previously predicted.

Our simulations also robustly predict a broad population of O₂-dominated atmospheres on close-in planets around low mass stars, posing a potential source of false positive biosignature detection and marking a high-priority opportunity for the first-ever atmospheric O₂ detection. We motivate future atmospheric characterization surveys by providing a target list of planet candidates predicted to have O₂-, He-, and deuterium-rich atmospheres.

Collin Cherubim, Robin Wordsworth, Dan Bower, Paolo Sossi, Danica Adams, Renyu Hu

Comments: Accepted in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2503.05055 [astro-ph.EP] (or arXiv:2503.05055v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2503.05055
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Submission history
From: Collin Cherubim
[v1] Fri, 7 Mar 2025 00:22:48 UTC (10,980 KB)
https://arxiv.org/abs/2503.05055
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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