SO 2 , silicate clouds, but no CH 4 detected in a warm Neptune.

WASP-107b is a warm ( ~ 740 K) transiting planet with a Neptune-like mass of ~ 30.5 M ⊕ and Jupiter-like radius of ~ 0.94 R J 1,2 whose extended atmosphere is eroding 3 . Previous observations showed evidence for water vapour and a thick high-altitude condensate layer in WASP-107b's atmosphere 4,5 . Recently, photochemically produced sulphur dioxide (SO 2 ) was detected in the atmosphere of a hot ( ~ 1,200 K) Saturn-mass planet from transmission spectroscopy near 4.05 μm 6,7 , but for temperatures below ~ 1,000 K sulphur is predicted to preferably form sulphur allotropes instead of SO 2 8,9,10 . Here we report the 9σ-detection of two fundamental vibration bands of SO 2 , at 7.35 μm and 8.69 μm, in the transmission spectrum of WASP-107b using the Mid-Infrared Instrument (MIRI) of the JWST. This discovery establishes WASP-107b as the second irradiated exoplanet with confirmed photochemistry, extending the temperature range of exoplanets exhibiting detected photochemistry from ~ 1,200 K down to ~ 740 K. Additionally, our spectral analysis reveals the presence of silicate clouds, which are strongly favoured ( ~ 7σ) over simpler cloud setups. Furthermore, water is detected ( ~ 12σ), but methane is not. These findings provide evidence of disequilibrium chemistry and indicate a dynamically active atmosphere with a super-solar metallicity.