A chemical compound (SO2) produced by photochemical reactions in exoplanet atmospheres.Sulfur dioxide (SO2) is a colorless gas with a pungent odor, naturally produced by volcanic eruptions and industrial processes. In the context of exoplanet atmospheres, SO2 is a key tracer of photochemical reactions—it forms when ultraviolet radiation from a host star breaks down other sulfur-bearing molecules like hydrogen sulfide. Detecting SO2 in an exoplanet's atmosphere provides evidence of active chemistry and can reveal the planet's composition and temperature structure.
The first unambiguous detection of SO2 in an exoplanet atmosphere was reported in 2023 by a team using the James Webb Space Telescope, observing the hot Saturn-mass exoplanet WASP-39b. This marked a milestone in atmospheric chemistry studies of exoplanets.
In exoplanet atmospheres, SO2 is typically produced when UV light photolyzes hydrogen sulfide (H2S) or sulfur dioxide itself, leading to a chain of reactions that create and destroy SO2. Its presence is detected via absorption features in the planet's transmission spectrum—when the planet passes in front of its star, gases in its atmosphere absorb specific wavelengths of starlight, leaving a fingerprint. The abundance of SO2 depends on the balance between photochemical production and destruction, which is influenced by temperature and the intensity of stellar radiation.
SO2 detection is used to characterize exoplanet atmospheres, helping scientists understand planetary formation, atmospheric dynamics, and the potential habitability of worlds beyond our solar system.
On Earth, SO2 is a major air pollutant that causes acid rain, but in exoplanet atmospheres, it can be a sign of a planet's active volcanic activity or photochemical processes.






















