An Oxidation Gradient Straddling the Small Planet Radius Valley

Creators: Cherubim, Collin^{1, 2}; Wordsworth, Robin¹; Bower, Dan J.³; Sossi, Paolo A.³; Adams, Danica¹; Hu, Renyu^{4, 5}

1. Harvard University
2. Harvard-Smithsonian Center for Astrophysics
3. ETH Zurich
4. Jet Propulsion Lab
5. California Institute of Technology

Abstract

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.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

This research has made use of the NASA Exoplanet Archive (NASA Exoplanet Archive 2024)⁸ This data set or service is made available by the NASA Exoplanet Science Institute at IPAC, which is operated by the California Institute of Technology under contract with the National Aeronautics and Space Administration.

R.W. acknowledges funding from the Leverhulme Center for Life in the Universe, Joint Collaborations Research Project grant G119167, LBAG/312.

D.J.B. and P.A.S. were supported by the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract No. MB22.00033, a SERI-funded ERC Starting grant "2ATMO." PAS also thanks the Swiss National Science Foundation (SNSF) through an Eccellenza Professorship (203668).

D.J.A. is funded by NASA through the NASA Hubble Fellowship Program grant HST-HF2-51523.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.

Software References

SciPy (P. Virtanen et al. 2020), Atmodeller (D. J. Bower et al. 2025, in preparation), IsoFATE (C. Cherubim et al. 2024).

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