transmission-spectroscopy

In July 2022, the James Webb Space Telescope detected carbon dioxide in the atmosphere of WASP-39b — a hot Jupiter 700 light-years away — with a clarity no previous telescope could match. It was the sharpest single spectrum ever taken of an exoplanet atmosphere and the opening demonstration of what JWST was built to do. But the CO₂ detection was only the beginning: the same data revealed sulfur dioxide formed by photochemistry, a chemical map of how the planet formed, and a template for reading the atmospheres of smaller, cooler, potentially habitable worlds.

For most of the history of exoplanet science, astronomers could detect planets but not smell them. They could measure orbital periods, estimate masses, and infer temperatures — but the chemical composition of another world's atmosphere was almost entirely out of reach. JWST changed that. In its first two years, it detected carbon dioxide on a gas giant 700 light-years away, found evidence of methane and CO₂ on a possible ocean world 120 light-years out, and ruled out thick atmospheres on the rocky TRAPPIST-1 planets — all using a technique called transmission spectroscopy that is as elegant as it is precise.