Direct evidence of photochemistry in an exoplanet atmosphere

Photochemistry is a fundamental process of planetary atmospheres that is integral to habitability, atmospheric composition and stability, and aerosol formation. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Here we show that photochemically produced sulphur dioxide (SO₂) is present in the atmosphere of the hot, giant exoplanet WASP-39b, as constrained by data from the JWST Transiting Exoplanet Early Release Science Program and informed by a suite of photochemical models. We find that SO₂ is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H₂S) is destroyed. The SO₂ distribution computed by the photochemical models robustly explains the 4.05 μm spectral feature seen in JWST transmission spectra [Rustamkulov et al.(submitted), Alderson et al.(submitted)] and leads to observable features at ultraviolet and thermal infrared wavelengths not available from the current observations. The sensitivity of the SO₂ feature to the enrichment of heavy elements in the atmosphere ("metallicity") suggests that it can be used as a powerful tracer of atmospheric properties, with our results implying a metallicity of ∼10× solar for WASP-39b. Through providing improved constraints on bulk metallicity and sulphur abundance, the detection of SO₂ opens a new avenue for the investigation of giant-planet formation. Our work demonstrates that sulphur photochemistry may be readily observable for exoplanets with super-solar metallicity and equilibrium temperatures ≳750 K. The confirmation of photochemistry through the agreement between theoretical predictions and observational data is pivotal for further atmospheric characterisation studies.