Astronomers have used the European Southern Observatory's Very Large Telescope (ESO's VLT) to study the composition of 3I/ATLAS, the brightest interstellar object ever seen, in detail. By measuring specific chemical fingerprints — the first observations of this kind for a comet that formed outside the Solar System — they found that 3I/ATLAS likely originated in the outskirts of an old star system. The findings shine new light on the formation history of this comet, indicating that it may be much older than the Sun.
Interstellar comets are icy objects formed around a star other than the Sun that occasionally wander into our Solar System. "They are sort of fossils from a planetary formation process that happened very far away, but that we get the chance to study from much closer," says astronomer Cyrielle Opitom, a researcher at the University of Edinburgh, United Kingdom. Together with Jean Manfroid and Damien Hutsemékers of the University of Liège, Belgium, Opitom led a study of 3I/ATLAS published today in Nature Astronomy.
3I/ATLAS is the third interstellar object ever discovered, after 1I/ʻOumuamua and 2I/Borisov. It was found as it was approaching the Sun, spending enough time in our Solar System for astronomers to study it in detail. While it was difficult to measure the composition of the first two interstellar objects — in the first astronomers didn't detect gas and the second was too faint — this was not the case for 3I/ATLAS. Thanks to the object's unprecedented brightness, Opitom, Manfroid, Hutsemékers and their team were able to measure the comet's isotopic ratios: the relative amounts of different forms of the same element.
Using the UVES instrument on ESO's VLT , the team measured ratios of carbon and nitrogen isotopes in cyanide molecules present in the gas around the comet. These ratios are known to be a good indicator of a comet's origin, as they are very sensitive to the physical conditions in the formation environment and are not expected to change much as the comet travels on through space.
"Unlike comets from our Solar System, this interstellar visitor carries unusually high carbon and nitrogen isotopic ratios," explains Aravind Krishnakumar, a researcher at the University of Liège and co-author on the new study. A similar study led by Martin Cordiner at the NASA Goddard Space Flight Center, US, that was published late last month in Nature, found a similar isotopic ratio of carbon, as well as elevated levels of deuterium, also called heavy hydrogen [1] . The study used data from the James Webb Space Telescope, a joint project of the US, European and Canadian space agencies.
Overall, the findings by Opitom's team indicate that the comet likely formed in the outer regions around an old, 'low-metallicity' star. A low-metallicity star is one with few elements heavier than helium in its composition, that is thought to have formed when the Universe was much younger — and less chemically rich — than it is now. The team suspects that 3I/ATLAS therefore originated around a star much older than the Sun. "3I/ATLAS is a really exciting opportunity to probe the composition of another planetary system, one that formed long before our Sun and Solar System even existed," says co-author Rosemary Dorsey, a researcher at the University of Helsinki, Finland. Evidence from the studies by the different teams points to 3I/ATLAS being more than twice as old as the Sun .
As 3I/ATLAS moves away from the Sun and gets progressively fainter, its observations at the VLT are also nearing their end. ESO's upcoming Extremely Large Telescope ( ELT ) will allow similar measurements for future interstellar objects, including those less bright than 3I/ATLAS. "The field of interstellar objects is still very new, and we do not really know what to expect. Every time a new one is discovered, we have new surprises," Opitom concludes.
Notes
[1] A team lead by Salazar-Manzano and Paneque-Carreño used the Atacama Large Millimeter/submillimeter Array (ALMA) , in which ESO is a partner, to measure deuterated (or semi-heavy) water in 3I/ATLAS. They also found elevated levels of this type of water compared to those found in Solar System comets.
More information
This research was presented in a paper to appear in Nature Astronomy ( doi:10.1038/s41550-026-02921-7 ).
The team is composed of C. Opitom (Institute for Astronomy, University of Edinburgh, Royal Observatory, UK [Edinburgh]), J. Manfroid (STAR Institute, University of Liège, Belgium [STAR]), D. Hutsemékers (STAR), E. Jehin (STAR), M. M. Knight (Volgenau Department of Physics, United States Naval Academy, Annapolis, MD, USA), K. Aravind (STAR), L. Ferellec (Faculty of Science and Engineering, Northumbria University, Newcastle, UK), D. Bodewits (Physics Department, Edmund C. Leach Science Center, Auburn University, AL, USA), V. V. Guzmán (Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Santiago, Chile), M. Cordiner (Department of Physics, Catholic University of America, Washington, DC, USA and Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA), R. C. Dorsey (Department of Physics, University of Helsinki, Finland), F. La Forgia (Department of Physics and Astronomy, University of Padova, Italy), M. Lippi (INAF - Osservatorio Astrofisico di Arcetri, Firenze, Italy), B. P. Murphy (Edinburgh), C. Snodgrass (Edinburgh).
The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO's headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal, ESO will host and operate the south array of the Cherenkov Telescope Array Observatory, the world's largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building "the world's biggest eye on the sky" — ESO's Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.
Links
- Research paper
- Chasing Starlight video of 3I/ATLAS, featuring Cyrielle Opitom (from December 2025)
- Photos of the VLT
