3I/ATLAS, a comet from the Galactic cold

3I/ATLAS, the third confirmed interstellar object to pass through our Solar System, carries a chemical signature unlike that of any known comet. Observations with the James Webb Space Telescope, complemented by ALMA data, reveal water highly enriched in deuterium and carbon isotopic ratios very different from those measured in Solar System bodies. These clues suggest that 3I/ATLAS formed in a very cold environment, probably below 30 K, within an ancient and relatively metal-poor planetary system. With contributions from LIRA researchers Nicolas Biver, Dominique Bockelée-Morvan and Jacques Crovisier, this result makes 3I/ATLAS a preserved fragment of another planetary system — and possibly a witness to the early chemical history of the Milky Way.