2023Natur.623..709B

The abundances of main carbon- and oxygen-bearing gases in the atmospheres of giant exoplanets provide insights into atmospheric chemistry and planet formation processes. Thermochemistry suggests that methane (CH₄) should be the dominant carbon-bearing species below about 1,000 K over a range of plausible atmospheric compositions; this is the case for the solar system planets and has been confirmed in the atmospheres of brown dwarfs and self-luminous, directly imaged exoplanets. However, CH₄ has not yet been definitively detected with space-based spectroscopy in the atmosphere of a transiting exoplanet, but a few detections have been made with ground-based, high-resolution transit spectroscopy including a tentative detection for WASP-80b (ref. ). 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.