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In December 2025, as the comet was moving away from our star, JWST pointed its instruments at it. Solar heat had made the comet brighter, facilitating observations. Researchers were able to measure the proportions of different molecules, particularly carbon and heavy hydrogen. The ratios obtained correspond to nothing known in our Solar System, unambiguously confirming that 3I/ATLAS comes from another stellar system.
Researchers using JWST find clues to the origins of comet 3I/ATLAS. Credit: NASA, ESA, CSA, STScI, Martin Cordiner (CUA, NASA-GSFC); Image processing: Alyssa Pagan (STScI)
These results were published in the journal Nature on June 22. Astronomers analyzed the comet's chemical composition while it was still active, which revealed signatures characteristic of interstellar molecular clouds. This discovery reinforces the idea that 3I/ATLAS formed in a region very different from our own protoplanetary disk.
The excitement around this object was such that rumors quickly circulated on social media. Some suggested the possibility of an extraterrestrial spacecraft, as had been the case for 'Oumuamua a few years earlier. But JWST's chemical data clearly show it is a classic comet, albeit of exotic origin.
This graph shows the differences between 3I/ATLAS and comets in our Solar System. Credit: NASA, ESA, CSA, Martin Cordiner (CUA, NASA-GSFC), Leah Hustak (STScI)
The comet is now continuing its journey toward the outskirts of the Solar System, never to return. Astronomers continue to analyze JWST data to learn more about its history. Each new interstellar object is an opportunity to discover the diversity of planetary systems in our galaxy.
These observations show how effective recent telescopes can be for studying such fleeting visitors. Thanks to JWST, scientists were able not only to confirm the comet's interstellar origin, but also to obtain clues about the physical and chemical conditions prevailing in its home system.