Astronomers utilizing ESO’s Very Large Telescope (VLT) have measured the carbon and nitrogen isotope ratios of 3I/ATLAS, the third known interstellar visitor to pass through our solar system. Their findings indicate that interstellar comets likely formed in the frigid outer regions of protoplanetary disks surrounding significantly older and less metal-rich stars than our Sun.

3I/ATLAS is just the third interstellar object ever documented, following 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. Its brightness allowed for isotopic measurements that were not feasible with its predecessors.

“Interstellar objects originating from planetary systems beyond our own present a unique chance to explore materials formed in various protoplanetary disks, each likely experiencing distinct physical and chemical conditions,” stated Dr. Ciriel Opitom, an astronomer at the University of Edinburgh.

“When such an object becomes active and sublimates, the resulting gas can be analyzed spectroscopically, enabling direct determination of its volatile composition and isotopic ratios.”

“Isotope ratios serve as vital tools for tracing the origins and evolutionary pathways of different species.”

“Given that the fractionation process is influenced by temperature and radiation environments, isotopic ratios can map the chemical evolution of materials from the prestellar phase to fully formed planets and planetesimals.”

Dr. Opitom and his collaborators observed 3I/ATLAS from December 6 to 26, 2025, following its closest approach to the Sun.

Using the VLT’s Ultraviolet-Visible Echelle Spectrometer (UVES), they examined emissions from cyanide (CN), a molecule frequently found in comet atmospheres, and measured the carbon to nitrogen isotope ratios.

The carbon isotope ratio (carbon-12/carbon-13) was approximately 151, while the nitrogen isotope ratio (nitrogen-14/nitrogen-15) was around 363.

In contrast, most solar system comets possess carbon isotope ratios near 90 and nitrogen isotope ratios close to 150.

This image of interstellar comet 3I/ATLAS was captured on January 18, 2026, using the FORS2 instrument aboard ESO’s Very Large Telescope. Image credit: ESO/O. Hainaut.

“3I/ATLAS presents an exciting opportunity to investigate the composition of other planetary systems that formed long before our Sun and Solar System came into existence,” noted Dr. Rosemary Dorsey, an astronomer at the University of Helsinki.

The elevated nitrogen ratio in 3I/ATLAS aligns with the hypothesis that it formed far from its parent star, where the effectiveness of isotope-selective chemistry diminishes.

Chemical evolution models of galaxies suggest that older, metal-poor stars yield planetary materials rich in carbon-12 compared to carbon-13.

The astronomers’ high carbon ratio measurement aligns with these predictions, corroborating previous findings indicating that comets originated around ancient stars relatively deficient in heavy elements.

“The nitrogen-14/nitrogen-15 ratio exceeds the typical value of 150 found in solar system comets, approaching values observed in the interstellar medium, prestellar phases, or outside protoplanetary disks,” the researchers stated.

“The carbon-12/carbon-13 ratio also surpasses commonly measured figures in solar system comets and the local interstellar medium.”

“These measurements support the theory that 3I/ATLAS originated in the outer disk surrounding older, less metallic stars, offering potential insights into planetesimal formation efficiency around such stars.”

For further details, the team’s study is published in today’s issue of Nature Astronomy.

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C. Opitom others. High isotope ratios of nitrogen and carbon in interstellar comet 3I/ATLAS. Nat Astron published online on July 6, 2026. doi: 10.1038/s41550-026-02921-7