Recent observations by astronomers utilizing ESO’s Very Large Telescope (VLT) revealed significant insights into the carbon and nitrogen isotope ratios of 3I/ATLAS, the third known interstellar visitor to pass through our solar system. This analysis indicates that interstellar comets like 3I/ATLAS originated in the frigid outer regions of protoplanetary disks surrounding ancient stars that are older and less metal-rich compared to our Sun.
This image illustrates part of the spectrum of interstellar comet 3I/ATLAS obtained by ESO’s Very Large Telescope’s UVES instrument in December 2025. Image credit: ESO / C. Opitom / Manfroid et al. / O. Hainaut.
3I/ATLAS marks the third interstellar object identified, following 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. Its brightness facilitated isotopic analysis that was not feasible for earlier interstellar visitors.
According to Dr. Ciriel Opitom, an astronomer at the University of Edinburgh, “Interstellar objects from beyond our solar system present a unique opportunity to explore material that formed in other protoplanetary disks under distinct physical and chemical environments.”
“When these objects become active and sublimate, the resulting gas can be analyzed spectroscopically, enabling us to ascertain their volatile composition and isotopic ratios directly.”
“Isotope ratios serve as crucial indicators of the origin and evolution of various species,” Dr. Opitom added.
“Given that the fractionation process is sensitive to temperature and radiation conditions, isotopic ratios can trace the chemical evolution from the prestellar phase through protostellar and protoplanetary disk stages until fully realized planets and planetesimals emerge.”
Dr. Opitom and his collaborators monitored 3I/ATLAS from December 6 to December 26, 2025, following its closest solar approach.
Employing the VLT’s Ultraviolet-Visible Echelle Spectrometer (UVES), they analyzed the emissions from cyanide (CN)—a molecule prevalent in cometary atmospheres—and measured carbon to nitrogen isotope ratios.
They discerned that the carbon isotope ratio (carbon-12/carbon-13) was approximately 151, while the nitrogen isotope ratio (nitrogen-14/nitrogen-15) stood at about 363.
In contrast, most known solar system comets exhibit carbon isotope ratios near 90 and nitrogen isotope ratios around 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.
Dr. Rosemary Dorsey, an astronomer at the University of Helsinki, expressed, “3I/ATLAS offers an extraordinary opportunity to examine the composition of planetary systems formed long before the existence of the Sun and our Solar System.”
The elevated nitrogen ratio recorded by 3I/ATLAS aligns with its formation taking place far from the parent star, where the efficiency of isotope-selective chemistry declines substantially.
Chemical evolution models of galaxies predict that older, metal-poor stars result in planetary material rich in carbon-12 relative to carbon-13.
The notably high carbon ratio observed by astronomers reaffirm these predictions and correlate with previous findings suggesting that comets formed around ancient stars that possessed fewer heavy elements.
Researchers noted, “The nitrogen-14/nitrogen-15 ratio we found is significantly higher than the typical value of 150 observed in solar system comets, approaching measurements seen in the interstellar medium, prestellar phase, or outside the protoplanetary disk.”
Additionally, they stated, “The carbon-12/carbon-13 ratio exceeds those typically recorded in solar system comets and the local interstellar medium.”
“These findings support the premise that 3I/ATLAS originated in the outer disks surrounding older, metal-poor stars, offering potential insights into the feasibility of planetesimal formation around such stars.”
The research team’s findings are detailed in a recent publication in the journal Nature Astronomy.
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C. Opitom et al.. 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
Source: www.sci.news
