Astronomers report findings on the interstellar comet 3I/ATLAS, the third known object from outside the solar system to pass through it. Discovered after it was detected in 2025 by the ATLAS survey, the comet closely approaches the Sun and has provided rare observational access to material formed elsewhere in the Milky Way. Using data including observations from the James Webb Space Telescope and other facilities such as ESO’s Very Large Telescope, researchers analyze the comet’s chemical and isotopic “fingerprints,” including ratios of hydrogen and carbon isotopes. Results suggest 3I/ATLAS is likely extremely old, potentially forming about 10 to 12 billion years ago—roughly two to three times older than the solar system—and originating from a colder, less metal-rich environment than the one that produced Earth and the other planets. The measured deuterium-to-hydrogen ratio in its water is reported to be far higher than in solar-system comets, and its carbon isotope ratios differ from those seen in local objects and some other astrophysical environments. The comet’s detailed composition also appears rich in organic molecules. Its exact birthplace in the Milky Way and whether it came from a specific stellar system remain uncertain, though researchers say a planetary system origin is most likely. 3I/ATLAS is expected to travel through the outer solar system in coming years and depart beyond Pluto’s orbit around 2029.