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

David K. Sing; Zafar Rustamkulov; Daniel P. Thorngren; Joanna K. Barstow; Pascal Tremblin; Catarina Alves de Oliveira; Tracy L. Beck; Stephan M. Birkmann; Ryan C. Challener; Nicolas Crouzet; Néstor Espinoza; Pierre Ferruit; Giovanna Giardino; Amélie Gressier; Elspeth K. H. Lee; Nikole K. Lewis; Roberto Maiolino; Elena Manjavacas; Bernard J. Rauscher; Marco Sirianni; Jeff A. Valenti

doi:10.1038/s41586-024-07395-z

paper

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

arXiv:2405.11027 · doi:10.1038/s41586-024-07395-z

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 $σ$ 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 $_{int}$ =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 $_{⊙}$ , 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

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

Abstract

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