WASP-39b’s atmosphere gave JWST its first clear detection of carbon dioxide in an exoplanet atmosphere.
The James Webb Space Telescope observed WASP-39b in 2022 with its Near-Infrared Spectrograph, or NIRSpec. As the planet passed in front of its star, a small fraction of starlight filtered through the atmosphere. Molecules absorbed particular wavelengths, leaving a distinctive transmission spectrum with a strong carbon-dioxide feature near 4.3 micrometres.
WASP-39b is a hot, inflated gas giant about 700 light-years away. It has a mass roughly comparable to Saturn but a diameter larger than Jupiter, and it circles its star at only about 0.05 astronomical units. Earlier observations had suggested other atmospheric ingredients, including water vapour, sodium, and potassium, but JWST provided the first unambiguous exoplanet CO₂ detection.
The result does not mean the planet is habitable: WASP-39b is intensely hot. Instead, carbon dioxide acts as a valuable tracer of atmospheric chemistry and planetary formation, demonstrating JWST’s ability to analyse distant worlds.