Webb Telescope Discovers Interstellar Comet 3I/ATLAS Has Unexpected Chemistry, May Have Originated 12 Billion Years Ago

The third confirmed interstellar comet in human history, 3I/ATLAS, has a surprisingly unusual chemical composition. New observations from the James Webb Space Telescope suggest this comet may have formed 10 to 12 billion years ago, far older than our Solar System. The study was published June 22 in the journal Nature.

Webb Seizes the Observation Window

In December 2025, as interstellar comet 3I/ATLAS began moving away from the Sun, astronomers seized the opportunity to turn the powerful Webb telescope toward this rare visitor. The comet had just been freshly warmed from its closest pass by the Sun, and its ancient ice had been converted to a bright coma of gas ideal for observation.

The research team received approval to interrupt Webb's planned schedule of observations to use its NIRSpec (Near-Infrared Spectrograph) instrument for detailed analysis. NIRSpec can map specific chemical and molecular signatures of cosmic objects, making it crucial for a rare object like 3I/ATLAS—only the third comet from outside the Solar System ever studied, and the first observed by an instrument capable of capturing as much detail as NIRSpec.

Surprising Chemical Findings

The observations yielded unexpected results. NIRSpec data showed exceptionally high levels of deuterium (heavy hydrogen) in 3I/ATLAS, about 30 times more than seen in Solar System comets. This implies the comet may have originated in a very cold system much earlier in the history of our galaxy. During its formation, the material that became incorporated into 3I/ATLAS was likely exposed to plenty of radiation, but not any long-term warmth that would have reprocessed its heavy water ice.

Additionally, NIRSpec showed only traces of carbon-13 compared to lighter-weight carbon-12. This also points to a very old origin, as stellar systems become enriched with carbon-13 over time as generations of stars are born and die in the galaxy. This is why there are higher levels of carbon-13 in our system, around our Sun, which formed relatively recently at 4.5 billion years ago.

Tracing the Comet's Origins

Based on these data, the research team estimates that 3I/ATLAS could have formed as long as 10 to 12 billion years ago, during the Universe's "cosmic noon" when star formation was at its height. Its young origin system was likely ensconced in a relatively cold, dense cloud. The abundance of heavy water shows that 3I/ATLAS spent its formative years in a deeply frozen state.

Martin Cordiner, an astro-chemist at NASA's Goddard Space Flight Center and lead author of the study, said: "This was a unique opportunity to study an ancient object from the distant Galaxy, probably pre-dating our Sun and Solar System. On the one hand, we get direct insight into that distant time and place, and on the other, we learn something about how unusual our own Solar System may be."

Cross-Telescope Observations

The European Southern Observatory's Very Large Telescope (VLT) also participated in observing 3I/ATLAS. A separate study led by astronomer Cyrielle Opitom of the University of Edinburgh analyzed the comet's carbon and nitrogen varieties in the form of cyanide, complementing Webb's findings.

Stefanie Milam of NASA Goddard and co-author of the study said: "For us as scientists, finding these rare isotopes is fascinating, but the bigger picture here is looking at the possibilities of prebiotic chemistry elsewhere in the galaxy. So far, we know of only one place in the vast cosmos where chemical ingredients led to life—our Solar System, our Earth. Analysis of these interstellar objects is a major step towards learning how common, or uncommon, the conditions for the evolution of life are in the Universe."

Further Reading

Original article: Webb finds clues to ancient, distant origin of Comet 3I/ATLAS

Research paper: Nature