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

Sing, David K; Rustamkulov, Zafar; Thorngren, Daniel P; Barstow, Joanna K; Tremblin, Pascal; de Oliveira, Catarina Alves; Beck, Tracy L; Birkmann, Stephan M; Challener, Ryan C; Crouzet, Nicolas; Espinoza, Néstor; Ferruit, Pierre; Giardino, Giovanna; Gressier, Amélie; Lee, Elspeth K H; Lewis, Nikole K; Maiolino, Roberto; Manjavacas, Elena; Rauscher, Bernard J; Sirianni, Marco; ... (2024). A warm Neptune's methane reveals core mass and vigorous atmospheric mixing. Nature, 630(8018), pp. 831-835. Springer Nature 10.1038/s41586-024-07395-z

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Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane1,2,3,4, which has only recently been identified atmospherically5,6. The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior7,8,9. 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-107 b 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 molecule2,4. Here we present a JWST NIRSpec transmission spectrum of WASP-107 b which 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 Tint=460±40 K, and vigorous vertical mixing which depletes CH4 with a diffusion coefficient of Kzz = 1011.6±0.1 cm2/s. Photochemistry has a negligible effect on the CH4 abundance, but is needed to account for the SO2. We infer a core mass of M⊕, which is much higher than previous upper limits10, releasing a tension with core-accretion models11.

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