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WASP-107b is a warm (approximately 740 K) transiting planet with a Neptune-like mass of roughly 30.5 M ⊕ and Jupiter-like radius of about 0.94 R J (refs.  1,2 ), whose extended atmosphere is eroding 3 . Previous observations showed evidence for water vapour and a thick, high-altitude condensate layer in the atmosphere of WASP-107b (refs.  4,5 ). Recently, photochemically produced sulfur dioxide (SO 2 ) was detected in the atmosphere of a hot (about 1,200 K) Saturn-mass planet from transmission spectroscopy near 4.05 μm (refs.  6,7 ), but for temperatures below about 1,000 K, sulfur is predicted to preferably form sulfur allotropes instead of SO 2 (refs.  8-10 ). 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 JWST. This discovery establishes WASP-107b as the second irradiated exoplanet with confirmed photochemistry, extending the temperature range of exoplanets exhibiting detected photochemistry from about 1,200 K down to about 740 K. Furthermore, our spectral analysis reveals the presence of silicate clouds, which are strongly favoured (around 7σ) over simpler cloud set-ups. Furthermore, water is detected (around 12σ) but methane is not. These findings provide evidence of disequilibrium chemistry and indicate a dynamically active atmosphere with a super-solar metallicity., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Brown dwarfs serve as ideal laboratories for studying the atmospheres of giant exoplanets on wide orbits, as the governing physical and chemical processes within them are nearly identical 1,2 . Understanding the formation of gas-giant planets is challenging, often involving the endeavour to link atmospheric abundance ratios, such as the carbon-to-oxygen (C/O) ratio, to formation scenarios 3 . However, the complexity of planet formation requires further tracers, as the unambiguous interpretation of the measured C/O ratio is fraught with complexity 4 . Isotope ratios, such as deuterium to hydrogen and 14 N/ 15 N, offer a promising avenue to gain further insight into this formation process, mirroring their use within the Solar System 5-7 . For exoplanets, only a handful of constraints on 12 C/ 13 C exist, pointing to the accretion of 13 C-rich ice from beyond the CO iceline of the disks 8,9 . Here we report on the mid-infrared detection of the 14 NH 3 and 15 NH 3 isotopologues in the atmosphere of a cool brown dwarf with an effective temperature of 380 K in a spectrum taken with the Mid-Infrared Instrument (MIRI) of JWST. As expected, our results reveal a 14 N/ 15 N value consistent with star-like formation by gravitational collapse, demonstrating that this ratio can be accurately constrained. Because young stars and their planets should be more strongly enriched in the 15 N isotope 10 , we expect that 15 NH 3 will be detectable in several cold, wide-separation exoplanets., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)