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SO2, silicate clouds, but no CH4 detected in a warm Neptune

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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 RJ 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 (SO2) 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 SO2 8,9,10. Here we report the 9σ-detection of two fundamental vibration bands of SO2, 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.

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Correspondence to Achrène Dyrek.

Supplementary information

Supplementary Information

This file contains Supplementary Methods: a complete methodology of data reduction on both JWST and HST data, retrieval techniques and models with a peculiar emphasis on a specific non-linearity correction derived to reduce the JWST data. Included are Supplementary Figs. 1–15 and Supplementary Tables 1–3.

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Supplementary Data for Supplementary Fig. 13

Dataset describes the input stellar spectral energy distributions and photo-absorption cross sections of SO2, H2O, H2S, and CH4 used for the photochemical model.

Supplementary Data for Supplementary Fig. 14

Dataset that provides the model-predicted SO2 molar fractions when all photo-absorption cross sections are taken into account and when they are taken into account separately. Also provides the values of predicted SO2 molar fractions without vertical mixing, without photodissociation of H2O and without including the thermochemical reaction H2O + HH2 + OH.

Supplementary Data for Supplementary Fig. 15

Dataset that provides the [OH]/[H] ratio for equilibrium and disequilibrium predictions through the [OH]/[H] proxy and the molar fraction of [OH]/[H] obtained with the model.

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Dyrek, A., Min, M., Decin, L. et al. SO2, silicate clouds, but no CH4 detected in a warm Neptune. Nature (2023).

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