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

Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane 1,2,3,4 , which has only recently been identified atmospherically 5,6 . The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior 7,8,9 . 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 10 , and temperatures amenable to CH 4 though previous observations have yet to find the molecule 2,4 . 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 4.2σ significance at an abundance of 1.0±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±8× solar, a hot interior with an intrinsic temperature of T int =460±40 K, and vigorous vertical mixing which depletes CH 4 with a diffusion coefficient of K zz = 10 11.6±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 10 , releasing a tension with core-accretion models 11 .