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 …