Older than the Sun: new clues reveal the ancient origin of interstellar comet 3I/ATLAS

A team of astronomers has used ESO’s Very Large Telescope to carry out the most detailed chemical analysis ever performed on an interstellar object, revealing that comet 3I/ATLAS likely formed in the outer reaches of a stellar system far older than our own Sun. The findings, published in Nature Astronomy, mark the first time isotopic ratios have been measured in a comet born outside the Solar System, opening a direct window into the chemistry of an alien planetary formation environment.

A fossil from another world

Interstellar comets are icy bodies that formed around a star other than the Sun and were subsequently ejected into the galaxy, occasionally drifting through our Solar System. They are, in essence, fossils of planetary formation processes that occurred in entirely different stellar environments. «They are a kind of fossil of a planetary formation process that took place far away, but that we have the chance to study from much closer,» said Cyrielle Opitom, astronomer at the University of Edinburgh and lead author of the study, carried out together with Jean Manfroid and Damien Hutsemékers from the University of Liège in Belgium.

3I/ATLAS is only the third interstellar object ever identified, after 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. Unlike its predecessors, however, 3I/ATLAS was bright enough and remained visible long enough for astronomers to perform high-resolution spectroscopy, a feat that was impossible with the earlier visitors. 1I/’Oumuamua showed no detectable outgassing at all, and 2I/Borisov was simply too faint for isotopic measurements.

Isotopic fingerprints point to an ancient origin

Using the UVES spectrograph on ESO’s VLT, the team measured the isotopic ratios of carbon and nitrogen in cyanide molecules present in the gas surrounding the comet’s nucleus. Isotopic ratios are particularly powerful tracers of origin because they are set by the physical and chemical conditions of the environment where the comet formed and are not expected to change significantly during the object’s journey through interstellar space.

The results were striking. 3I/ATLAS carries unusually high carbon and nitrogen isotopic ratios compared to comets from our Solar System. A complementary study led by Martin Cordiner at NASA’s Goddard Space Flight Center, published in Nature, found a similarly elevated carbon isotopic ratio using data from the James Webb Space Telescope, along with enhanced levels of deuterium (heavy hydrogen). An additional measurement by a team led by Salazar-Manzano and Paneque-Carreño using ALMA detected elevated levels of deuterated (semi-heavy) water in 3I/ATLAS, further reinforcing the picture of an origin chemically distinct from our own neighbourhood.

Born around a metal-poor star

Taken together, the isotopic signatures suggest that 3I/ATLAS formed in the outer regions around an ancient, low-metallicity star. In astrophysics, metallicity refers to the abundance of elements heavier than helium. Stars with low metallicity are understood to have formed when the universe was much younger and chemically less enriched than it is today. The converging evidence from multiple independent teams and instruments points to 3I/ATLAS being more than twice as old as the Sun, which itself formed approximately 4.6 billion years ago.

«3I/ATLAS is a really exciting opportunity to investigate the composition of another planetary system, one that formed long before our Sun and our Solar System existed,» said co-author Rosemary Dorsey from the University of Helsinki.

What comes next

As 3I/ATLAS recedes from the Sun and fades from view, VLT observations are drawing to a close. But the upcoming Extremely Large Telescope (ELT), currently under construction by ESO in Chile, will be capable of performing similar measurements on future interstellar visitors, including those significantly fainter than 3I/ATLAS. The field of interstellar object science remains in its infancy, with only three confirmed detections to date, and each new discovery has brought surprises that challenge existing assumptions about planetary formation and the chemical diversity of the galaxy. «The field of interstellar objects is still very young, and we really don’t know what to expect. Every time a new one is discovered, we get new surprises,» concluded Opitom.

The study was published in Nature Astronomy. The complementary JWST study by Cordiner et al. appeared in Nature. The ALMA detection of deuterated water was carried out using ESO’s Atacama Large Millimeter/submillimeter Array.

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Total or partial reproduction prohibited without express authorization. Original source: European Southern Observatory (ESO) / Nature Astronomy.