Kavli Affiliate: Sara Seager
| First 5 Authors: , , , ,
| Summary:
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 $leq100times$ 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.
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