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

TL;DR

This paper explores the possibility that sub-Neptune planets have carbon-rich interiors, demonstrating that such compositions can explain observed spectra of certain planets and are compatible with JWST data, expanding understanding of planetary diversity.

Contribution

It introduces a new model of sub-Neptune interiors with carbon-rich compositions and shows their consistency with spectral observations, including JWST data, for specific planets.

Findings

Carbon-rich interiors can match observed spectra of TOI-270 d and K2-18 b.

GJ 1214 b's spectrum rules out a carbon-rich interior if its atmosphere is primordial.

Simulated spectra of TOI-270 d fit observations across various atmospheric scenarios.

Abstract

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 $\chi^2$ 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.