k2-18b

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.