The James Webb Space Telescope (JWST) has produced the most detailed information ever on an exoplanet, making it the world we know most about after the eight major planets of our Solar System. Observations of the planet, called WASP-39b, reveal patchy clouds, an intriguing chemical reaction in its atmosphere, and provide hints about its formation.“We’ve studied lots of planets before,” says Laura Kreidberg, director of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and part of the observation team, which posted five papers on their observations on the arXiv preprint server on 22 November. “But we’ve never seen a data set like this.”...Using three of its instruments, JWST was able to observe light from the planet’s star as it filtered through WASP-39b’s atmosphere, a process known as transmission spectroscopy. This allowed a team of more than 300 astronomers to detect water, carbon monoxide, sodium, potassium and more in the planet’s atmosphere, in addition to the carbon dioxide. The gives the planet a similar composition to Saturn, although it has no detectable rings.The team were also surprised to detect sulfur dioxide, which had appeared as a mysterious bump in early observation data. Its presence suggests a photochemical reaction is taking place in the atmosphere as light from the star hits it, similar to how our Sun produces ozone in Earth’s atmosphere. In WASP-39b’s case, light from its star, slightly smaller than the Sun, splits water in its atmosphere into hydrogen and hydroxide, which reacts with hydrogen sulfide to produce sulfur dioxide.“Photochemistry, because it is such an important process here on Earth, is probably an important process on other potentially habitable planets,” says Jacob Bean, an astronomer at the University of Chicago in Illinois and the observation team’s co-leader. Until now, “we’ve only been able to test our understanding of photochemistry in our Solar System. But planets around other stars give us access to completely different physical conditions”.