The interstellar comet 3I/ATLAS is older than our Sun
An international team involving the University of Edinburgh, the University of Liège, the European Southern Observatory (ESO) and several partner institutions has, for the first time, measured the isotopic composition of the interstellar comet 3I/ATLAS. The scientific team, notably researchers from the STAR Institute at ULiège, drew on expertise developed over more than twenty years in Liège in this type of measurement. The results have been published in the journal Nature Astronomy.
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nterstellar comets are icy objects formed around stars others than the Sun, which can sometimes pass through our solar system. “They are, in a sense, the ‘fossils’ of a planetary formation process that took place very far away from us, but which we are fortunate enough to be able to study much more closely,” explains astronomer Cyrielle Opitom, a researcher at the University of Edinburgh. 3I/ATLAS is only the third interstellar object ever discovered, following 1I/‘Oumuamua and 2I/Borisov. It was spotted as it approached the Sun, spending enough time in our solar system for astronomers to study it in detail. Whilst it had been difficult to determine the composition of the first two interstellar objects (astronomers had not detected any gas in the first, and the second was too faint), this was not the case for 3I/ATLAS. Thanks to the object’s unprecedented brightness, the team was able to measure the comet’s isotopic ratios – the relative amounts of different forms of the same element.
Using the UVES instrument installed on ESO’s VLT, the team led by Cyrielle Opitom, in collaboration with astronomers Jean Manfroid, Damien Hutsemékers and Emmanuel Jehin from ULiège, measured the isotopic ratios of carbon and nitrogen in the cyanide molecules present in the gas surrounding the comet. “We know that these ratios are a good indicator of a comet’s origin, as they are highly sensitive to the physical conditions of the formation environment and should not change significantly as the comet travels through space,” explains Emmanuel Jehin, FNRS Research Director and head of the COMETA group at the University of Liège. “Astronomers in Liège have been dedicated for several decades to this very challenging quest to measure rare isotopes in comets within the Solar System, and this time, it was a very special comet!”
“Unlike comets in our solar system, this interstellar visitor exhibits unusually high carbon and nitrogen isotope ratios,” explains Krishnakumar Aravind, a researcher at the University of Liège and co-author of the new study. A similar study led by Martin Cordiner at NASA’s Goddard Space Flight Centre in the United States, and published at the end of June in the journal *Nature*, revealed a similar carbon isotope ratio, as well as high levels of deuterium, also known as heavy hydrogen [1]. This study drew on data from the James Webb Space Telescope, a joint project of the US, European and Canadian space agencies.
Overall, the team’s findings suggest that the comet probably formed in the outer regions around an old, ‘low-metallicity’ star. A low-metallicity star is one whose composition contains few elements heavier than helium; it is thought to have formed at a time when the Universe was much younger - and chemically poorer - than it is today. The team therefore suspects that 3I/ATLAS originated around a star much older than the Sun. “3I/ATLAS offers a truly exciting opportunity to study the composition of another planetary system, which formed long before our Sun and our Solar System even existed,” explains co-author Rosemary Dorsey, a researcher at the University of Helsinki in Finland. According to the results of studies carried out by various teams, 3I/ATLAS could therefore be twice as old as the Sun.
ESO’s future Extremely Large Telescope (ELT) will enable similar measurements to be carried out on future interstellar objects, including those less luminous than 3I/ATLAS. “The field of interstellar objects is still very new, and we don’t really know what to expect. Every time a new object is discovered, we are in for new surprises,” concludes Cyrielle Opitom.
Scientific reference
C. Opitom, J. Manfroid, D. Hutsemékers, E. Jehin, M. M. Knight, K. Aravind, L. Ferellec, D. Bodewits, V. V. Guzmán, M. Cordiner, R. C. Dorsey, F. La Forgia, M. Lippi, B. P. Murphy, C. Snodgrass & M. Bannister, High nitrogen and carbon isotopic ratios in the interstellar comet 3I/ATLAS, Nature Astronomy, 6 july 2026. doi.org/10.1038/s41550-026-02921-7
