Astronomers have uncovered important new clues about the origin ofย 3I/ATLAS, the brightest interstellar object ever observed. Using the European Southern Observatoryโs Very Large Telescope (ESOโs VLT), scientists studied the cometโs chemical composition in unusual detail and found evidence suggesting that it may have formed in the distant outer region of an ancient star system.
The study provides fresh insight into the history of 3I/ATLAS and indicates that the comet could be far older than the Sun.
Interstellar comets are icy bodies that were born around stars outside our solar system before being thrown into space. From time to time, some of these objects pass through our solar system, giving astronomers a rare opportunity to examine material from other planetary systems.
According to astronomer Cyrielle Opitom of the University of Edinburgh, interstellar comets act like preserved remains from the process of planet formation that occurred far beyond our solar neighbourhood. Although they formed around other stars, their journey brings them close enough for scientists on Earth to study.
Opitom led the new research alongside Jean Manfroid and Damien Hutsemรฉkers of the University of Liรจge in Belgium. Their findings were published in Nature Astronomy.
A Rare Chance to Study an Interstellar Comet Closely
3I/ATLAS is only the third interstellar object ever detected, following 1I/สปOumuamua and 2I/Borisov. Unlike the earlier two objects, 3I/ATLAS was bright enough and remained visible long enough for researchers to investigate its chemical makeup in detail.
Studying the first two interstellar visitors was difficult. In the case of 1I/สปOumuamua, astronomers did not detect gas around it, while 2I/Borisov was too faint for certain types of detailed measurements. However, 3I/ATLAS offered a much better opportunity because of its exceptional brightness.
Using the UVES instrument on ESOโs VLT, researchers measured the cometโs isotopic ratios, which are the relative amounts of different versions of the same chemical element. These measurements were taken from cyanide molecules found in the gas surrounding the comet.
The team focused on the isotopes of carbon and nitrogen. These chemical fingerprints are valuable because they can reveal the conditions in which a comet formed. They are also believed to remain mostly unchanged as a comet travels through space, making them useful indicators of its birthplace.
The results showed that 3I/ATLAS has unusually high carbon and nitrogen isotopic ratios compared with comets from our own solar system. According to Aravind Krishnakumar, a researcher at the University of Liรจge and co-author of the study, this makes the interstellar visitor chemically distinct from familiar solar system comets.
Evidence of an Ancient Star System
A separate study led by Martin Cordiner of NASAโs Goddard Space Flight Center also found a similar carbon isotopic ratio in 3I/ATLAS. That study, published in Nature, used observations from the James Webb Space Telescope, a joint mission of the United States, Europe, and Canada. It also detected elevated levels of deuterium, also known as heavy hydrogen.
Taken together, these findings suggest that 3I/ATLAS may have formed in the outer region of an old star system. More specifically, the comet may have originated around a low-metallicity star.
Low-metallicity stars contain relatively small amounts of elements heavier than helium. Such stars are believed to have formed when the universe was younger and less enriched with heavy elements than it is today. This points to the possibility that 3I/ATLAS came from a planetary system that existed long before the Sun and our solar system formed.
Co-author Rosemary Dorsey of the University of Helsinki described 3I/ATLAS as an exciting opportunity to study the chemical composition of another planetary system, especially one that appears to have formed much earlier than our own. Evidence from the new studies suggests that the comet may be more than twice as old as the Sun.
Preparing for Future Interstellar Visitors
As 3I/ATLAS continues moving away from the Sun, it is becoming dimmer, and observations using the VLT are gradually coming to an end. However, future instruments will make it possible to study other interstellar objects in even greater detail.
ESOโs upcoming Extremely Large Telescope is expected to give astronomers the ability to perform similar measurements on future interstellar visitors, including those that are much fainter than 3I/ATLAS.
For researchers, each new interstellar object offers a fresh chance to learn about worlds beyond our solar system. As Opitom noted, the study of interstellar objects is still a young field, and every discovery brings new surprises.
Journal Reference:
Opitom, C., Manfroid, J., Hutsemรฉkers, D., et al. (2026). High nitrogen and carbon isotopic ratios in the interstellar comet 3I/ATLAS. Nature Astronomy. https://doi.org/10.1038/s41550-026-02921-7