Methane throughout the atmosphere of the warm exoplanet WASP-80b

The abundances of main carbon- and oxygen-bearing gases in the atmospheres of giant exoplanets provide insights into atmospheric chemistry and planet formation processes ^1,2. Thermochemistry suggests that methane (CH₄) should be the dominant carbon-bearing species below about 1,000 K over a range of plausible atmospheric compositions ³; this is the case for the solar system planets ⁴ and has been confirmed in the atmospheres of brown dwarfs and self-luminous, directly imaged exoplanets ⁵. However, CH₄ has not yet been definitively detected with space-based spectroscopy in the atmosphere of a transiting exoplanet ^6–11, but a few detections have been made with ground-based, high-resolution transit spectroscopy ^12,13 including a tentative detection for WASP-80b (ref. ¹⁴). Here we report transmission and emission spectra spanning 2.4–4.0 μm of the 825 K warm Jupiter WASP-80b taken with the NIRCam instrument of the JWST, both of which show strong evidence of CH₄ at greater than 6σ significance. The derived CH₄ abundances from both viewing geometries are consistent with each other and with solar to sub-solar C/O and around five times solar metallicity, which is consistent with theoretical predictions ^15–17.