New isotope readings sharpen the origin story of interstellar comet 3I/ATLAS

A second telescope confirms an unusually old visitor

New spectroscopic observations of the interstellar comet 3I/ATLAS, taken with the European Southern Observatory’s Very Large Telescope in Chile, independently support an earlier finding that the object is far older than the Sun and was forged in a chemically alien environment. The VLT data, published on 6 July 2026 in Nature Astronomy, give astronomers the first isotopic ratios ever measured from an interstellar comet and tighten the case that 3I/ATLAS is a relic of planet formation around an early, metal-poor star.

The work was led by J. Manfroid and D. Hutsemékers of the University of Liège, together with Cyrielle Opitom of the University of Edinburgh, according to both the journal article and Space.com’s reporting on the team. Other co-investigators include E. Jehin, M. M. Knight, K. Aravind, L. Ferellec, D. Bodewits, V. V. Guzmán, M. Cordiner, R. C. Dorsey, F. La Forgia, M. Lippi, B. P. Murphy, C. Snodgrass and M. Bannister, drawn from institutions in Belgium, the United Kingdom, the United States, Italy, Finland and New Zealand.

What the new numbers say

Using the Ultraviolet and Echelle Spectrograph (UVES) on the VLT, the team targeted the CN molecule released in the comet’s coma and tail as 3I/ATLAS warmed on its approach to the Sun. They report a carbon-12 to carbon-13 ratio of 151, with an uncertainty range that runs from 107 to 261, and a nitrogen-14 to nitrogen-15 ratio of 363, with an uncertainty range that runs from 210 to 996. Both ratios are significantly higher than the values typically measured in comets native to our solar system, where 14N/15N sits near 150.

Space.com notes that carbon-13 is produced in greater abundance than carbon-12 over cosmic time, mainly in red giant stars, so an unusually carbon-12-rich object such as 3I/ATLAS must have formed before much of that enrichment had built up in the galaxy. The 12C/13C reading matches the higher-than-solar value previously returned by NASA’s James Webb Space Telescope, giving the age estimate an independent cross-check.

An origin at the cold edge of another solar system

The nitrogen reading is the more striking of the two. According to the Nature Astronomy paper, the 14N/15N ratio in 3I/ATLAS is close to values measured in the interstellar medium, in prestellar cloud cores and in the outer reaches of protoplanetary disks around young stars — but not to the ratios typically found in the inner, warmer parts of such disks where most solar system comets are believed to have formed. The authors conclude that the measurements are compatible with an origin for 3I/ATLAS in the outer disk of an older, low-metallicity star.

Space.com quotes Aravind Krishnakumar, a team member at the University of Liège, summarising the finding in plain terms: “Unlike comets from our solar system, this interstellar visitor carries unusually high carbon and nitrogen isotopic ratios.” Rosemary Dorsey, an astronomer at the University of Helsinki, called 3I/ATLAS “a really exciting opportunity to probe the composition of another planetary system, one that formed long before our Sun and solar system even existed.” Opitom separately described interstellar objects as “sort of fossils from a planetary formation process that happened very far away, but we get the chance to study from much closer.”

A timeline of 3I/ATLAS and its place among interstellar visitors

3I/ATLAS was discovered in July 2025 by the ATLAS survey at about 5 astronomical units from the Sun and was already active at that distance, according to the Nature Astronomy paper. Its perihelion, the point in its orbit closest to the Sun, occurred on 29 October 2025. JWST observations taken at 3.3 au before perihelion found a coma that is extremely rich in carbon dioxide and, to a lesser extent, carbon monoxide, relative to water. Post-perihelion measurements at 2.4 au recorded a CO/H2O ratio of 2.33 ± 0.07, which the paper describes as high compared with the average comet at that distance.

Ground-based optical spectra also revealed a very high nickel abundance and, as the comet approached perihelion, the presence of iron in the coma, with an initial Ni I/Fe I ratio exceeding that seen in solar system comets. A separate study found an exceptionally high CH3OH/HCN ratio, higher than in all but one solar system comet. Taken together, the Nature Astronomy authors write, “these compositional signatures suggest that 3I formed under conditions markedly different from those that prevailed in the Solar System.”

Why the first two interstellar objects could not deliver this answer

Astronomers have now catalogued three confirmed interstellar visitors. The first, 1I/’Oumuamua, was discovered in 2017 and never showed detectable gas, eliminating the possibility of measuring its isotopes. The second, 2I/Borisov, was discovered in 2019 but was too faint for the high-resolution spectroscopy needed to extract reliable isotopic ratios from its coma. 3I/ATLAS was therefore the first interstellar object bright and active enough for the UVES technique to succeed, a point made in both the journal paper and Space.com’s coverage.

How 3I/ATLAS may have been ejected into interstellar space

The Nature Astronomy paper stops short of identifying a specific ejection mechanism, focusing instead on the disk location implied by the isotopes. Space.com lays out one possible pathway, citing models in which migrating giant planets can fling small bodies into interstellar space, but notes that because 3I/ATLAS appears to have formed far from such planetary activity — in a Kuiper-belt-like region of its parent system — it is also plausible that a passing star gravitationally stripped the comet and sent it on a long, solitary trajectory through the galaxy.

Why it matters for planetary science

Direct samples of material from other planetary systems are rare. Telescopes can study protoplanetary disks around young stars, but at distances of hundreds of light-years the level of detail is limited. An interstellar comet that swings through the inner solar system can be observed with instruments such as JWST, UVES and other high-resolution spectrographs at far closer range, allowing astronomers to measure isotopic ratios, molecular abundances and dust properties that would otherwise be inaccessible. The Nature Astronomy authors describe interstellar objects as enabling “a unique window into the conditions prevailing in these disks where planetesimals and planets are forming.”

The findings also have implications for how common planet formation is around metal-poor stars, which were much more common in the early Milky Way than the Sun-like, metal-rich stars that dominate the solar neighbourhood today. If 3I/ATLAS is representative, planetesimal formation in the cold outer regions of such disks may have been efficient enough to seed many of them with comet-like bodies.

Caveats and open questions

The Nature Astronomy paper is open access and was published on 6 July 2026, with Manfroid and Hutsemékers listed as joint lead authors. The reported 14N/15N ratio carries very large uncertainties, spanning roughly 210 to 996, and the authors urge caution in over-interpreting the absolute value. They argue, however, that the lower bound alone is sufficient to place the ratio firmly above the solar system comet range and into the regime of outer-disk and prestellar material.

The 12C/13C ratio is consistent with the JWST result published earlier in 2026 by a team led by Martin Cordiner of NASA’s Goddard Space Flight Center, which estimated 3I/ATLAS’s age at 10 to 12 billion years on the basis of carbon and deuterium isotopes. No source consulted reports a numerical age update from the VLT team; Space.com simply describes the new carbon reading as supporting the JWST observations.

What to watch next

Astronomers will be looking to apply the same UVES technique to the next bright interstellar visitor, since the first two known objects did not yield isotopic measurements. Continued monitoring of 3I/ATLAS as it recedes from the Sun may allow JWST and other facilities to extend the inventory of measured molecules beyond CN, tightening constraints on where and when the comet formed. As Opitom’s team argues, each new interstellar object is a chance to test whether outer-disk formation around older, metal-poor stars is the rule or the exception among planetary systems in the Milky Way.