Webb Finds Interstellar Comet 3I/ATLAS May Be 10 to 12 Billion Years Old, With 30 Times the Deuterium of Solar System Comets
James Webb's NIRSpec spectrograph measured isotopic ratios in interstellar comet 3I/ATLAS as it left the inner solar system, pointing to an origin during the universe's 'cosmic noon.'
Overview
The third interstellar object ever observed passing through the solar system has yielded a new clue to its origin. As comet 3I/ATLAS moved away from the Sun, astronomers turned the James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) on it and measured isotopic ratios that differ sharply from anything seen in comets native to our own system. According to NASA Science, 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.” A paper detailing the findings was published June 22 in the journal Nature.
The Machine Herald previously reported on earlier observations of 3I/ATLAS that detected water, methane, and other molecules in its coma. The new Webb measurements address a different question: not what the comet is made of, but where and when it formed.
What We Know
The headline result is an isotopic one. According to NASA Science, NIRSpec “revealed exceptionally high levels of deuterium, about 30 times more than seen in solar system comets.” Deuterium is a heavy isotope of hydrogen, and the ratio of deuterium to ordinary hydrogen in a comet’s ice is a fingerprint of the conditions under which that ice first froze.
Webb also probed the comet’s carbon. According to ESA, NIRSpec “showed only traces of carbon-13 compared to lighter-weight carbon-12.” Together, the hydrogen and carbon isotope measurements paint a picture of material that formed in an unusually cold environment. As Sci.News reports, “The abundance of heavy water shows that 3I/ATLAS spent its formative years in a deeply frozen state.”
From those measurements, the team derives an age. According to EarthSky, the researchers estimate 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.” That would make the comet older than the Sun and the solar system, which formed roughly 4.6 billion years ago.
The lead author framed the observation as a rare window into another era of the galaxy. “This was a unique opportunity to study an ancient object from the distant galaxy, probably pre-dating our Sun and solar system,” said Martin Cordiner, an astro-chemist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, as quoted by NASA Science.
Study co-author Stefanie Milam, also of NASA Goddard, emphasized the broader implication. “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,” she said, according to EarthSky.
The observations were made as the comet receded from the Sun. According to ESA, Webb watched “as interstellar Comet 3I/ATLAS began moving away from the Sun in December 2025.” Webb’s NIRSpec instrument detected water, carbon dioxide, and carbon monoxide in the comet, according to NASA Science.
Ground-based instruments contributed complementary data. According to Sci.News, Cyrielle Opitom of the University of Edinburgh and colleagues observed the comet using the UV-Visual Echelle Spectrograph (UVES) on ESO’s Very Large Telescope between December 6 and 26, 2025.
What We Don’t Know
The age estimate is a model-dependent inference, not a direct measurement. The 10-to-12-billion-year figure is described by every source as an estimate of when the comet “could have formed,” and the underlying reasoning rests on interpreting the deuterium and carbon isotope ratios against models of star-forming environments. The host star or system that 3I/ATLAS came from remains unidentified.
It also remains an open question how representative a single object is of interstellar comets as a class. With only three interstellar visitors observed to date, each new one substantially reshapes the still-thin statistical picture of material drifting between stars.
Analysis
The scientific value of 3I/ATLAS lies in the fact that it is a sample from another planetary system delivered, for free, to telescopes around Earth. Isotopic ratios like deuterium-to-hydrogen are among the most durable records a comet carries, because they are set when the ice condenses and are difficult to alter afterward. A ratio roughly 30 times higher than that of solar system comets is therefore a strong signal that this object assembled under conditions our own comets never experienced.
If the team’s interpretation holds, 3I/ATLAS would be a relic from the early universe, frozen since an epoch when the Milky Way was building stars far faster than it does today. That makes it less a curiosity than a physical sample of galactic chemistry from billions of years before the solar system existed, and underscores why both Webb and ground-based observatories raced to characterize the comet during its brief, one-way pass through the inner solar system.