JWST Indicates an Auroral Signature in an Extrasolar World

Methane emission is a well observed phenomenon in the atmospheres of solar system giant planets with both Jupiter and Saturn showing strong spectral emission in the infrared [1]. For solar system gas giants, the emission is driven by a combination of solar pumping and aurorae largely brought on from interactions with plasma emit- ted by active moons such as Io (around Jupiter; [2]) and Enceladus (around Saturn; [3]). Studies of extrasolar aurorae in the planetary temperature regime have so far lacked the detection of strong spectral signatures of aurorae seen in solar system planets such as emission from H+3 or CH4, despite searches for the former in the spectra of radio emitting brown dwarfs (e.g. [4]). Here we present JWST data of the extrasolar object WISEP J193518.59-154620.3 (W1935 for short) that demonstrates – for the first time – methane in emission at 3.326 μm, making this a strong atmospheric signature of an aurora detected beyond our solar system. Modeling the spectra using an atmospheric retrieval code we conclude a temperature inversion of some ∼300K centered at 1-10 milibar is required to replicate the feature. Given that W1935 is isolated, with no host star, we surmise it is an excellent candidate for a moon hosting system akin to Jupiter and Io.