2025ApJ...990L..35L

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 a recent theory of carbon-rich planet formation outside 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 times solar metallicity atmospheres. We further show that carbon-rich interiors lead to atmospheric compositions that match Hubble Space Telescope 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 observations but require 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 the interior C/O.