JWST reveals abundant methane and depleted carbon dioxide on the temperate sub-Neptune LP791-18c

Pierre-Alexis Roy ( Université de Montréal )

While the last decade of HST science led to a handful of atmosphere detections on sub-Neptune exoplanets, the JWST's wider wavelength coverage and high precision now provides us with the first opportunity to pinpoint the chemical inventories for multiple species in sub-Neptune envelopes, and to infer their interior structure. With three transiting planets spanning both ends of the radius valley, including an outer sub-Neptune whose density indicates the presence of a substantial atmosphere, the small M dwarf system LP791-18 is the ideal laboratory to pursue this endeavor. Here, we present the detection of methane in the atmosphere of the temperate sub-Neptune LP791-18c via a JWST NIRSpec/PRISM transit observation. The transmission spectrum of LP791-18c is dominated by strong methane absorption and by the presence of opaque hazes at short wavelengths. While our retrieved methane abundance is consistent with that recently measured on K2-18b, which is of similar size and temperature, LP791-18c’s spectrum, in stark contrast, shows no sign of carbon dioxide absorption, allowing us to derive stringent constraints on its depletion. Instead, our super-solar methane measurement is accompanied by a tentative detection of hydrogen sulfide, which could be indicative of strong vertical mixing in the planet’s atmosphere. These findings point towards LP791-18c hosting a deep reducing atmosphere, with no traces of oxidized species, and no evidence of a hard surface beneath the H2-rich envelope. This discovery represents a powerful example of the diversity of sub-Neptune envelopes, as even analog planets in density and temperature are observed with vastly different chemical regimes. Our work further showcases the new era for sub-Neptune science, with JWST enabling us to infer the presence or absence of multiple chemical species in small exoplanet's envelopes, and gain insight into their interior structure.