Webb detects unusual isotopes in interstellar comet 3I/ATLAS, suggesting formation in an ancient cold environment

NASA said Monday that new James Webb Space Telescope observations of the interstellar comet 3I/ATLAS have revealed unusual isotope ratios in its water and carbon-bearing gases, offering the clearest evidence yet that the object formed in an extremely cold environment early in the Milky Way’s history, likely before the sun existed.

The finding matters because 3I/ATLAS — also designated C/2025 N1 — is only the third confirmed interstellar object ever detected. Unlike 1I/ʻOumuamua, which did not show a clearly measurable gaseous coma, 3I/ATLAS is active enough for astronomers to analyze the cloud of gas around its nucleus directly. That makes it one of the few known visitors from beyond the solar system whose chemistry can be studied in detail.

The comet was first reported by the NASA-funded ATLAS survey on July 1, 2025. It reached perihelion, its closest point to the sun, on Oct. 29, 2025. The James Webb Space Telescope observed it on Dec. 22 and 23, 2025, after it had begun heading back outward. Using NIRSpec integral-field spectroscopy — a technique that splits light into its component wavelengths across the coma — researchers measured isotopologues, or molecular variants containing different isotopes, in water and carbon-bearing gases.

The central result is that 3I/ATLAS appears to be highly enriched in deuterium, a heavy form of hydrogen, relative to ordinary hydrogen in its water. NASA’s summary said the comet’s deuterium-to-hydrogen ratio was on the order of 1%, far above the roughly 1 to 3 × 10^-4 typically seen in solar system comet water. The comet also showed unusually high carbon-12 to carbon-13 ratios in carbon-bearing gases compared with typical solar system comets. Together, those isotope ratios act as chemical fingerprints, pointing to formation conditions unlike those that produced comets around the sun.

An independent study using the Atacama Large Millimeter/submillimeter Array, or ALMA, published in Nature Astronomy on April 23, 2026, found a lower bound for the comet’s water deuterium ratio of greater than 6.6 × 10^-3, broadly supporting the Webb result. NASA’s announcement summarized the new Webb work as the underlying study has circulated in preprint and posted versions, while the ALMA paper has been peer-reviewed.

Researchers led by Martin A. Cordiner of NASA’s Goddard Space Flight Center and the Catholic University of America interpret the isotope measurements as evidence that 3I/ATLAS formed in a very cold environment, about 30 kelvin or below, and in a relatively metal-poor setting. Using galactic chemical evolution models, they estimate the material may have accreted roughly 10 billion to 12 billion years ago — a model-based inference rather than a directly measured age. If that interpretation holds, the comet would preserve matter from an earlier era of the galaxy than anything yet sampled in the solar system.

“This was a unique opportunity to study an ancient object from the distant galaxy, probably pre-dating our Sun and solar system,” Cordiner said in NASA’s summary.