Carbon-rich Sub-Neptune Interiors Are Compatible with JWST Observations
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arXiv
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| Auteurs principaux: | , |
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| Format: | Preprint |
| Publié: |
2025
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| _version_ | 1866909746607423488 |
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| author | Lin, Zifan Seager, Sara |
| author_facet | Lin, Zifan Seager, Sara |
| contents | Many possible interior compositions exist for sub-Neptunes: ice-poor, ice-rich, and water-dominated interiors can all match the measured masses and radii. Motivated by recent theory of carbon-rich planet formation outside of the refractory organic carbon "soot line" and observations of carbon-rich protoplanetary disks around late M dwarfs, we propose another possible sub-Neptune composition: a carbon-rich composition consisting of an iron-silicate core, a carbon layer, and a hydrogen/helium-dominated envelope. We show that the interiors of three prototypical sub-Neptunes with high-quality spectral observations - TOI-270 d, GJ 1214 b, and K2-18 b - are consistent with carbon-rich compositions if they have $\leq100\times$ solar metallicity atmospheres. We further show that carbon-rich interiors lead to atmospheric compositions that match HST and JWST observations. Simulated carbon-rich TOI-270 d transmission spectra pass the $χ^2$ test under a wide range of C/O, haze, and cloud scenarios. K2-18 b spectral models are broadly consistent with observation, but requires additional sources for carbon species to be fully compatible. GJ 1214 b models, however, are incompatible with observations, ruling out a carbon-rich interior composition, if the atmosphere of the planet is primordial and reflects interior C/O. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2508_15117 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Carbon-rich Sub-Neptune Interiors Are Compatible with JWST Observations Lin, Zifan Seager, Sara Earth and Planetary Astrophysics Many possible interior compositions exist for sub-Neptunes: ice-poor, ice-rich, and water-dominated interiors can all match the measured masses and radii. Motivated by recent theory of carbon-rich planet formation outside of the refractory organic carbon "soot line" and observations of carbon-rich protoplanetary disks around late M dwarfs, we propose another possible sub-Neptune composition: a carbon-rich composition consisting of an iron-silicate core, a carbon layer, and a hydrogen/helium-dominated envelope. We show that the interiors of three prototypical sub-Neptunes with high-quality spectral observations - TOI-270 d, GJ 1214 b, and K2-18 b - are consistent with carbon-rich compositions if they have $\leq100\times$ solar metallicity atmospheres. We further show that carbon-rich interiors lead to atmospheric compositions that match HST and JWST observations. Simulated carbon-rich TOI-270 d transmission spectra pass the $χ^2$ test under a wide range of C/O, haze, and cloud scenarios. K2-18 b spectral models are broadly consistent with observation, but requires additional sources for carbon species to be fully compatible. GJ 1214 b models, however, are incompatible with observations, ruling out a carbon-rich interior composition, if the atmosphere of the planet is primordial and reflects interior C/O. |
| title | Carbon-rich Sub-Neptune Interiors Are Compatible with JWST Observations |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2508.15117 |