Direct Evidence of Photochemistry in an Exoplanet Atmosphere by Shang-Min Tsai et al. on Monday 21 November
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 (SO$_2$) 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 SO$_2$ is produced by successive oxidation of sulphur radicals
freed when hydrogen sulphide (H$_2$S) is destroyed. The SO$_2$ distribution
computed by the photochemical models robustly explains the 4.05 $\mu$m 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 SO$_2$ 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
$\sim$10$\times$ solar for WASP-39b. Through providing improved constraints on
bulk metallicity and sulphur abundance, the detection of SO$_2$ 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 $\gtrsim$750 K. The
confirmation of photochemistry through the agreement between theoretical
predictions and observational data is pivotal for further atmospheric
characterisation studies.
arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.10490v1
