[ESO] 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.
This image shows part of the spectrum of interstellar comet 3I/ATLAS, captured between 6 and 26 December 2025 with the UVES instrument on ESO’s Very Large Telescope (VLT). Using UVES, astronomers studied the spectral signatures of cyanide, a molecule comprised of a carbon atom and a nitrogen one. More precisely, they looked at their isotopic ratios: the relative amounts of different forms of the same atoms. These ratios are sensitive to the conditions under which 3I/ATLAS formed, and are not expected to change much as the comet travels through space. The spectrum shown here contains spectral features produced by 12C, an isotope of carbon with 6 protons and 6 neutrons, and 13C, which has 7 neutrons instead. These features are very faint, but astronomers know the exact wavelength regions where to look for them in the spectrum of the comet. Adding together several of these wavelength regions averages out the noise, making the real features pop out. The team performed similar measurements with two isotopes of nitrogen, 14N and 15N. By comparing the 12C/13C and 14N/15N ratios with those measured in Solar System comets and in the discs of material around young stars, the team concluded that 3I/ATLAS likely formed in the outskirts of the disc around a star older than the Sun. Credit: ESO/C. Opitom, J. Manfroid et al. Comet image: O. Hainaut
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.
IMAGE
This animation shows the orbit of interstellar comet 3I/ATLAS as it crossed our Solar System over the course of 2025 and 2026. Credit: ESO/spaceengine.org
“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).
High nitrogen and carbon isotopic ratios in the interstellar comet 3I/ATLAS, Nature Astronomy (open access)
Astrobiology, Astrochemistry,
