Astrogeology

Carbon-rich Sub-Neptune Interiors Are Compatible With JWST Observations

By Keith Cowing

Press Release

August 22, 2025

Filed under astrochemistry, exoplanet, GJ 1214 b, K2-18 b, M dwarf, Protoplanetary Disk, soot line, Spectroscopy, sub-Neptune, TOI-270 d

Carbon-rich Sub-Neptune Interiors Are Compatible With JWST Observations — Possible interior compositions of 55 Cnc e. Compositions consistent within the 1σ, 2σ, and 3σ M-R uncertainties are shown in orange, dark teal, and light teal, respectively. The red dashed and solid lines show xcarbon = 12 wt% and 24 wt%, respectively, the upper bounds of xcarbon assuming no volatile loss (E. A. Bergin et al. 2023). The blue curve shows interior solution from N. Madhusudhan et al. (2012), to the right of which are allowed values (1σ uncertainty). Ternary diagrams are shown for two radius assumptions: (a) visible radius, 2.04 ± 0.15 R⊕, and (b) gray radius, 2.20 ± 0.12 R⊕, following N. Madhusudhan et al. (2012). Our interior models with updated carbon and SiC EOSs are consistent with earlier studies. — astro-ph.EP

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,

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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