WASP-107b is a warm (~740 K) transiting planet with a Neptune-like mass of ~30.5 Earth masses and Jupiter-like radius of ~0.94 Jupiter radius, whose extended atmosphere is eroding. Previous observations showed evidence for water vapour and a thick high-altitude condensate layer in WASP-107b's atmosphere. Recently, photochemically produced sulphur dioxide (SO2) was detected in the atmosphere of a hot (~1200 K) Saturn-mass planet from transmission spectroscopy near 4.05 microns, but for temperatures below 1000 K sulphur is predicted to preferably form sulphur allotropes instead of SO2. Here we report the 9-sigma detection of two fundamental vibration bands of SO2, at 7.35 microns and 8.69 microns, 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 ~1200 K down to ~740 K. Additionally, our spectral analysis reveals the presence of silicate clouds, which are strongly favoured (~7-sigma) over simpler cloud setups. Furthermore, water is detected (~12-sigma), but methane is not. These findings provide evidence of disequilibrium chemistry and indicate a dynamically active atmosphere with a super-solar metallicity.
@article{arxiv.2311.12515,
title = {SO2, silicate clouds, but no CH4 detected in a warm Neptune},
author = {Achrene Dyrek and Michiel Min and Leen Decin and Jeroen Bouwman and Nicolas Crouzet and Paul Mollière and Pierre-Olivier Lagage and Thomas Konings and Pascal Tremblin and Manuel Güdel and John Pye and Rens Waters and Thomas Henning and Bart Vandenbussche and Francisco Ardevol Martinez and Ioannis Argyriou and Elsa Ducrot and Linus Heinke and Gwenael Van Looveren and Olivier Absil and David Barrado and Pierre Baudoz and Anthony Boccaletti and Christophe Cossou and Alain Coulais and Billy Edwards and René Gastaud and Alistair Glasse and Adrian Glauser and Thomas P. Greene and Sarah Kendrew and Oliver Krause and Fred Lahuis and Michael Mueller and Goran Olofsson and Polychronis Patapis and Daniel Rouan and Pierre Royer and Silvia Scheithauer and Ingo Waldmann and Niall Whiteford and Luis Colina and Ewine F. van Dishoeck and Thomas Greve and Göran Ostlin and Tom P. Ray and Gillian Wright},
journal= {arXiv preprint arXiv:2311.12515},
year = {2025}
}