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 ≤100× 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 χ2 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.
Zifan Lin, Sara Seager
Comments: 15 pages, 10 figures, accepted for publication in the Astrophysical Journal Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.15117 [astro-ph.EP] (or arXiv:2508.15117v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.15117
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Related DOI:
https://doi.org/10.3847/2041-8213/adfcc8
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Submission history
From: Zifan Lin
[v1] Wed, 20 Aug 2025 23:07:02 UTC (1,655 KB)
https://arxiv.org/abs/2508.15117
Astrobiology, astrochemistry,
