Interstellar comet 3I/ATLAS is twice as old as the Sun, study finds

Astronomers using the European Southern Observatory's Very Large Telescope in Chile have measured the chemical fingerprints of interstellar comet 3I/ATLAS, finding that it likely formed in the outer regions around an ancient, chemically poor star — making it potentially more than twice as old as our Sun.

A Fossil From Another Star System

The findings, published Monday in Nature Astronomy, represent the first isotopic ratio measurements ever made of a comet that formed outside our solar system. Using the UVES instrument on the VLT, a team led by Cyrielle Opitom of the University of Edinburgh measured ratios of carbon and nitrogen isotopes in cyanide molecules present in the gas surrounding the comet.nature+1

"They are sort of fossils from a planetary formation process that happened very far away, but that we get the chance to study from much closer," Opitom said in a statement released by ESO.eso

The isotopic ratios proved unlike anything seen in comets from our own solar system. "Unlike comets from our Solar System, this interstellar visitor carries unusually high carbon and nitrogen isotopic ratios," said co-author Aravind Krishnakumar of the University of Liège.eso

Corroborating Evidence From Webb

The VLT results align with a separate study published in June in Nature, led by Martin Cordiner at NASA's Goddard Space Flight Center, which used the James Webb Space Telescope and found a similar carbon isotopic ratio along with elevated levels of deuterium — about 30 times more than seen in solar system comets. Additional observations from the Atacama Large Millimeter/submillimeter Array also detected elevated levels of heavy water.smithsonianmag+3

Together, the evidence points to 3I/ATLAS forming around a "low-metallicity" star — one with few elements heavier than helium, indicating it formed when the universe was much younger and less chemically enriched. Multiple teams now estimate the comet could be more than 12 billion years old.eso+2

A New Window Into Planetary Formation

3I/ATLAS, discovered on July 1, 2025, by the ATLAS survey telescope in Chile, is the third interstellar object ever found after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Its brightness gave scientists an unprecedented opportunity to study an object born around another star.science.nasa+1

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

As the comet fades from view, Opitom noted that ESO's upcoming Extremely Large Telescope will enable similar studies of future, potentially fainter interstellar visitors. "The field of interstellar objects is still very new, and we do not really know what to expect," she said. "Every time a new one is discovered, we have new surprises."eso