Isotopes in comet 3I/ATLAS point to an old, metal poor star, with very wide error bars

The European Southern Observatory announced on 6 July 2026 that astronomers had measured carbon and nitrogen isotope ratios in the interstellar comet 3I/ATLAS, the first isotopic fingerprint taken of a comet that formed outside the Solar System. The measurements were made with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope in Chile, on 18 January and 18 February 2026, and the results were published in Nature Astronomy under the digital object identifier 10.1038/s41550-026-02921-7.

The object itself is now well characterised. NASA records that 3I/ATLAS was discovered on 1 July 2025 by the NASA funded ATLAS survey telescope at Rio Hurtado in Chile, that it passed perihelion on 30 October 2025 at about 1.4 astronomical units, and that it made its closest approach to Earth on 19 December 2025 at about 1.8 astronomical units, or 270 million kilometres. Hubble observations on 20 August 2025 bounded the nucleus at not less than 440 metres and not more than 5.6 kilometres across.

The measurement was made on the cyanide radical in the coma, and the numbers are worth stating rather than summarising. In the preprint of the study, posted on 7 March 2026 by Cyrielle Opitom of the University of Edinburgh with Jean Manfroid, Damien Hutsemekers and colleagues, the reported ratios are a carbon 12 to carbon 13 ratio of 147, with an upper uncertainty of 87 and a lower uncertainty of 40, and a nitrogen 14 to nitrogen 15 ratio of 343, with an upper uncertainty of 454 and a lower uncertainty of 124. Both are high compared with Solar System comets. The nitrogen ratio is consistent with values measured in the interstellar medium, and the authors read the pair as indicating an origin in the outer disc around an older, low metallicity star.

The error bars are not a footnote. A nitrogen ratio quoted as 343 with an upper bound near 800 and a lower bound near 219 is a measurement that excludes typical cometary values while leaving a wide range of possible true values above them. It supports the direction of the claim and constrains the magnitude only loosely. Coverage that reported the ratios as high generally did not report how high the uncertainty allows them to be.

A second, independent line of evidence points the same way. On 23 April 2026 the Atacama Large Millimeter/submillimeter Array reported the detection of semi heavy water in the comet, with the paper appearing in Nature Astronomy the following day. Luis E. Salazar Manzano and Teresa Paneque-Carreno of the University of Michigan report at least thirty times the proportion of HDO found in Solar System comets, and more than forty times the proportion in Earth's oceans. Because deuterium enrichment in water is strongly temperature sensitive, the team infers formation below about 30 kelvin. Ordinary water was not detected directly; the water production rate was inferred from methanol line excitation, which is a modelled step rather than a measured one. A parallel preprint led by Martin Cordiner of NASA Goddard, posted on 6 March 2026, reaches a compatible conclusion, describing formation at temperatures below roughly 30 kelvin in a relatively metal poor environment early in the history of the Galaxy, with accretion perhaps ten to twelve billion years ago.

It is worth being precise about what an isotope ratio can do. It records the temperature and chemistry of the gas and ice from which a molecule formed, and it is largely preserved through the freezing of a cometary nucleus. It does not identify a star, it does not date a star, and it does not measure a star's metallicity directly. The inference runs from the ratios, through models of how carbon and nitrogen fractionate in cold molecular material, to a class of formation environment. Every step in that chain is calibrated against Solar System comets and against the interstellar medium of our own Galaxy, and 3I/ATLAS is the first object of its kind for which any of it has been attempted.

The researchers say as much. Opitom described interstellar comets as "fossils from a planetary formation process that happened very far away", and told Sky at Night Magazine that each new discovery brings new surprises, which is a plain statement that expectations in this field are weakly constrained. Rosemary Dorsey of the University of Helsinki framed the object as an opportunity to probe another planetary system rather than a settled result about one.

What is not known is whether 3I/ATLAS is representative of anything. Three interstellar objects have been observed, of which one has now been measured isotopically. Whether the high nitrogen ratio is a property of old metal poor systems generally, or a property of this one comet, cannot be established from a sample of one.