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During the brief visit of Comet 3I/ATLAS last year, astronomers were given an extremely rare glimpse of the cosmos beyond our solar system. As only the third known interstellar object to pass by Earth, this icy, dusty rock has already provided researchers with unexpected new data about deep space. But in addition to recently learning about the comet’s surprisingly alcohol-laden, ice-volcano-covered composition, astronomers now say they have a better idea of its age. According to chemical traces detailed in a study published today in the journal Nature Astronomy3I/ATLAS is very, very old.
“3I/ATLAS is a really exciting opportunity to investigate the composition of another planetary system, one that formed long before our Sun and solar system existed,” Rosemary Dorsey, an astrophysicist at the University of Helsinki and co-author of the study, said in a statement.
3I/ATLAS was extremely bright when it approached Earth in July 2025, unlike the previously documented interstellar objects 1I/ʻOumuamua and 2I/Borisov. This allowed Dorsey and his colleagues a never-before-seen opportunity to examine details such as their isotopic ratios: varying amounts of different forms of the same element. Using the European Southern Observatory’s Very Large Telescope, they paid special attention to the carbon and nitrogen isotopes within the cyanide molecules spinning within the gas cloud of 3I/ATLAS. These trails are very susceptible to the conditions present during a comet’s formation period, but then do not alter much as it spends eons traveling through space.
Unlike local comets, the interstellar tourist contains unusually high isotopic ratios of nitrogen and carbon. Based on this and other examinations, the team believes that the comet was born in an outer region surrounding an ancient, low-metallicity star. These stellar objects contain few elements heavier than helium, implying that they originated during a much younger era of the universe before it became more chemically diverse. Combined with evidence from other recent studies, it now appears that 3I/ATLAS began its travels long before the sun existed. The comet may even be more than twice as old as our yellow star, making it more than 9 billion years old.
“[Interstellar comets] They are a kind of fossils from a planetary formation process that occurred very far away, but that we have the opportunity to study from much closer,” added Cyrielle Opitom, astronomer at the University of Edinburgh and co-author of the study.
3I/ATLAS is becoming increasingly difficult to observe as it continues its journey out of the solar system, but the vast amounts of data astronomers collected while they could will keep them busy for years. At least, until our next interstellar visitor arrives.
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