Since it began science operations, the James Webb Space Telescope has observed the thin skies of distant worlds by watching starlight filter through them during transits. When a planet crosses its star, a tiny fraction of that light passes through the atmosphere, and molecules in the gas leave fingerprints in the spectrum. Webb's infrared instruments are sensitive enough to read those fingerprints for planets that earlier telescopes could only guess about, and the results have already rewritten parts of the exoplanet story.

The most important thing to understand is what a detection actually means. Webb does not photograph an atmosphere or sample its air. It measures which wavelengths are absorbed more strongly, and scientists match those patterns against laboratory data for gases like water vapour, carbon dioxide, methane, and sulphur dioxide. A strong match is evidence, not proof of habitability, and every claim sits on top of careful modelling of the star, the planet, and the instrument itself.

Webb's earliest exoplanet results delivered the clearest atmospheric spectra ever recorded for planets beyond our solar system. For hot gas giants such as WASP-39 b, the telescope identified water vapour, carbon dioxide, sodium, potassium, and even sulphur dioxide, a molecule produced when starlight drives chemical reactions in the upper atmosphere. That photochemistry detection mattered because it showed Webb can trace active processes, not just list ingredients.

No Webb exoplanet result has attracted more attention than the observations of K2-18 b, a sub-Neptune orbiting in its star's habitable zone. Early analysis reported methane and carbon dioxide, plus a tentative hint of dimethyl sulphide, a gas that on Earth is produced by marine life. Later reanalyses showed the dimethyl sulphide signal was far weaker than the headlines suggested, and other teams argued the data could also fit a magma-ocean world or a gas-rich planet with no surface ocean at all.

Webb's exoplanet program continues with repeated observations of the most promising small planets, stacking transits to beat down the noise from stellar activity. Targets include the TRAPPIST-1 system, where several Earth-sized worlds orbit an ultra-cool dwarf star, and a growing list of sub-Neptunes whose atmospheres may reveal whether they are water worlds, gas dwarfs, or something in between. Each additional transit sharpens the spectrum a little more.