JWST Identifies Methane Emission in a Cold Extrasolar World

Beyond our solar system, aurorae have been inferred from radio observations of isolated brown dwarfs. Within our solar system, Jupiter and Saturn have auroral emission at their poles, driven by the interaction of the planetary magnetic fields and bombarding electrons, thought to be sourced from the solar wind with spots contributed by active moons. Solar system giants have auroral signatures across the electromagnetic spectrum from X-ray to radio wavelengths, and in the infrared are associated with both strong H3+ and methane emission. Isolated brown dwarfs with auroral signatures in the radio on the other hand, have been searched for corresponding infrared features but have only had null detections. CWISEP J193518.59-154620.3. (W1935 for short) is a brown dwarf with a temperature of ~482 K (about 350 K higher than Jupiter), with no host star. Here we report serendipitous JWST observations of strong methane emission from W1935 at 3.326 microns. Atmospheric modeling leads us to conclude that a temperature inversion of about 300 K centered in the 1-10 millibar pressure range is required to replicate the feature. This represents an atmospheric temperature inversion seen in a Jupiter-like atmosphere without irradiation from a host star. A plausible explanation for the strong inversion is heating by auroral processes, although other internal and/or external dynamical processes cannot be ruled out. The best fit model rules out the contribution of H3+ emission which is prominent in solar system gas giants however this is consistent with rapid destruction of H3+ at the higher pressure where the W1935 emission originates.