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, 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 depletion have been available. The warm Neptune WASP-107b stands out among exoplanets with an unusually low density, reported low core mass, and temperatures amenable to CH, though previous observations have yet to find the molecule. Here we present a JWST-NIRSpec transmission spectrum of WASP-107b that shows features from both SO and CH along with HO, CO, 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 = 460 ± 40 K, and vigorous vertical mixing which depletes CH with a diffusion coefficient of K = 10 cm s. Photochemistry has a negligible effect on the CH abundance but is needed to account for the SO. We infer a core mass of , which is much higher than previous upper limits, releasing a tension with core-accretion models.