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^1-4, 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^7-9. However, the interiors are largely unconstrained along with the vertical mixing strength and only upper limits on the CH₄ depletion have been available. The warm Neptune WASP-107b stands out among exoplanets with an unusually low density, reported low core mass¹⁰, and temperatures amenable to CH₄, though previous observations have yet to find the molecule^2,4. Here we present a JWST-NIRSpec transmission spectrum of WASP-107b that shows features from both SO₂ and CH₄ along with H₂O, CO₂, 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 CH₄ with a diffusion coefficient of K_zz = 10^11.6±0.1 cm² s^-1. Photochemistry has a negligible effect on the CH₄ abundance but is needed to account for the SO₂. We infer a core mass of $$ , which is much higher than previous upper limits¹⁰, releasing a tension with core-accretion models¹¹.