A warm Neptune's methane reveals core mass and vigorous atmospheric mixing

Abstract

Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane, which has only recently been identified atmospherically. The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior. However, the interiors are largely unconstrained along with the vertical mixing strength and only upper limits on the CH${}_4$ depletion have been available. The warm Neptune WASP-107 b stands out among exoplanets with an unusually low density, reported low core mass, and temperatures amenable to CH${}_4$ though previous observations have yet to find the molecule. Here we present a JWST NIRSpec transmission spectrum of WASP-107 b which shows features from both SO${}_2$ and CH${}_4$ along with H${}_2$O, CO${}_2$, and CO. We detect methane with 4.2$\ddot{I}ƒ$ significance at an abundance of 1.0$\pm$0.5 ppm, which is depleted by 3 orders of magnitude relative to equilibrium expectations. Our results are highly constraining for the atmosphere and interior, which indicate the envelope has a super-solar metallicity of 43$\pm$8$\times$ solar, a hot interior with an intrinsic temperature of T${}_{\mathrm{int}}$=460$\pm$40 K, and vigorous vertical mixing which depletes CH4 with a diffusion coefficient of Kzz = 10${}^{11.6\pm 0.1}$ cm${}^2$/s. Photochemistry has a negligible effect on the CH${}_4$ abundance, but is needed to account for the SO${}_2$. We infer a core mass of 11.5${}_{-3.6}^{+3.0}$ M${}_{\odot }$, which is much higher than previous upper limits, releasing a tension with core-accretion models.

Published in Nature at this URL, https://www.nature.com/articles/s41586-024-07395-z . This is the authors version of the manuscript, 20 pages including Methods. Data from Figs. 1, 2, and 3 are available at this URL, https://doi.org/10.5281/zenodo.10891400