An Oxidation Gradient Straddling the Small Planet Radius Valley
We present a population-level view of volatile gas species (H _2 , He, H _2 O, O _2 , CO, CO _2 , CH _4 ) 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 mo...
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2025-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/adbca9 |
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| author | Collin Cherubim Robin Wordsworth Dan J. Bower Paolo A. Sossi Danica Adams Renyu Hu |
| author_facet | Collin Cherubim Robin Wordsworth Dan J. Bower Paolo A. Sossi Danica Adams Renyu Hu |
| author_sort | Collin Cherubim |
| collection | DOAJ |
| description | We present a population-level view of volatile gas species (H _2 , He, H _2 O, O _2 , CO, CO _2 , CH _4 ) 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 O _2 -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 O _2 detection. We motivate future atmospheric characterization surveys by providing a target list of planet candidates predicted to have O _2 -, He-, and deuterium-rich atmospheres. |
| format | Article |
| id | doaj-art-3487e71dba3b46869f1819adf098a154 |
| institution | DOAJ |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
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| series | The Astrophysical Journal |
| spelling | doaj-art-3487e71dba3b46869f1819adf098a1542025-08-20T03:05:25ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-0198329710.3847/1538-4357/adbca9An Oxidation Gradient Straddling the Small Planet Radius ValleyCollin Cherubim0https://orcid.org/0000-0002-8466-5469Robin Wordsworth1https://orcid.org/0000-0003-1127-8334Dan J. Bower2https://orcid.org/0000-0002-0673-4860Paolo A. Sossi3https://orcid.org/0000-0002-1462-1882Danica Adams4https://orcid.org/0000-0001-9897-9680Renyu Hu5https://orcid.org/0000-0003-2215-8485Department of Earth and Planetary Sciences, Harvard University , 20 Oxford St., Cambridge, MA 02138, USA ; ccherubim@g.harvard.edu; Center for Astrophysics ∣ Harvard & Smithsonian , 60 Garden St., Cambridge, MA 02138, USADepartment of Earth and Planetary Sciences, Harvard University , 20 Oxford St., Cambridge, MA 02138, USA ; ccherubim@g.harvard.edu; School of Engineering and Applied Sciences, Harvard University , 20 Oxford St., Cambridge, MA 02138, USAInstitute of Geochemistry and Petrology , Department of Earth and Planetary Sciences, ETH Zurich, Clausiusstrasse 25, Zurich CH-8092, SwitzerlandInstitute of Geochemistry and Petrology , Department of Earth and Planetary Sciences, ETH Zurich, Clausiusstrasse 25, Zurich CH-8092, SwitzerlandDepartment of Earth and Planetary Sciences, Harvard University , 20 Oxford St., Cambridge, MA 02138, USA ; ccherubim@g.harvard.eduJet Propulsion Laboratory, California Institute of Technology , Pasadena, CA 91109, USA; Division of Geological and Planetary Sciences, California Institute of Technology , Pasadena, CA 91125, USAWe present a population-level view of volatile gas species (H _2 , He, H _2 O, O _2 , CO, CO _2 , CH _4 ) 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 O _2 -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 O _2 detection. We motivate future atmospheric characterization surveys by providing a target list of planet candidates predicted to have O _2 -, He-, and deuterium-rich atmospheres.https://doi.org/10.3847/1538-4357/adbca9ExoplanetsExoplanet atmospheresExoplanet atmospheric evolutionExoplanet atmospheric compositionPlanetary climatesPlanetary atmospheres |
| spellingShingle | Collin Cherubim Robin Wordsworth Dan J. Bower Paolo A. Sossi Danica Adams Renyu Hu An Oxidation Gradient Straddling the Small Planet Radius Valley The Astrophysical Journal Exoplanets Exoplanet atmospheres Exoplanet atmospheric evolution Exoplanet atmospheric composition Planetary climates Planetary atmospheres |
| title | An Oxidation Gradient Straddling the Small Planet Radius Valley |
| title_full | An Oxidation Gradient Straddling the Small Planet Radius Valley |
| title_fullStr | An Oxidation Gradient Straddling the Small Planet Radius Valley |
| title_full_unstemmed | An Oxidation Gradient Straddling the Small Planet Radius Valley |
| title_short | An Oxidation Gradient Straddling the Small Planet Radius Valley |
| title_sort | oxidation gradient straddling the small planet radius valley |
| topic | Exoplanets Exoplanet atmospheres Exoplanet atmospheric evolution Exoplanet atmospheric composition Planetary climates Planetary atmospheres |
| url | https://doi.org/10.3847/1538-4357/adbca9 |
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