Interstellar Comet 3I/ATLAS Formed Up To 12 Billion Years Ago, New Observations Suggest

The interstellar comet 3I/ATLAS carries a chemical fingerprint unlike anything in our Solar System, and it may have formed 10 to 12 billion years ago, before our Sun even existed, according to two papers published in the journal Nature.

Discovered in July 2025, 3I/ATLAS is the third confirmed interstellar object to pass through the inner Solar System.

Despite detailed observations of this comet, the precise age, origin, and path of 3I/ATLAS have been uncertain.

Estimates of its age have ranged from 3-10 billion years old based on its velocity.

Measurements of the ratio of isotopes can offer clues about the physical and chemical conditions where 3I/ATLAS formed.

“This was a unique opportunity to study an ancient object, probably pre-dating our Sun and Solar System,” said Dr. Martin Cordiner, an astronomer at NASA’s Goddard Space Flight Center.

“On the one hand, we get direct insight into that distant time and place, and on the other, we learn something about how unusual our own Solar System may be.”

As 3I/ATLAS began moving away from the Sun in December 2025, Dr. Cordiner and colleagues took the opportunity to turn the NASA/ESA/CSA James Webb Space Telescope in its direction and capture detailed measurements of its chemical components.

Webb’s NIRSpec (Near-Infrared Spectrograph) instrument revealed exceptionally high levels of deuterium, about 30 times more than seen in solar system comets.

This implies that 3I/ATLAS may have originated in a very cold system much earlier in the history of our Milky Way Galaxy.

During its formation, the material that became incorporated into the comet was likely exposed to plenty of radiation, but not any long-term warmth that would have reprocessed its heavy water ice into the type of water ice we are familiar with on Earth.

Additionally, NIRSpec showed only traces of carbon-13 compared to lighter-weight carbon-12. This also points to a very old origin for 3I/ATLAS.

That is why there are higher levels of carbon-13 in our planetary system, around our Sun, which formed relatively recently, 4.5 billion years ago.

The authors estimate that 3I/ATLAS could have formed as long as 10 to 12 billion years ago, during the Universe’s ‘cosmic noon,’ when star formation was at its height.

Its young origin system was likely ensconced in a relatively cold, dense cloud.

The abundance of heavy water shows that 3I/ATLAS spent its formative years in a deeply frozen state.

In a separate study, University of Edinburgh astronomer Cyrielle Opitom and colleagues performed observations of the interstellar visitor with the UV-Visual Echelle Spectrograph (UVES) on ESO’s Very Large Telescope between December 6 and 26, 2025.

Their results complement Webb’s findings with an analysis of 3I/ATLAS’s carbon and nitrogen varieties in the form of the chemical .

“For us as scientists, finding these rare isotopes is fascinating, but the bigger picture here is looking at the possibilities of prebiotic chemistry elsewhere in the Galaxy,” said Dr. Stefanie Milam, also from NASA’s Goddard Space Flight Center.

“So far, we know of only one place in the vast cosmos where chemical ingredients led to life — our Solar System, our Earth.”

“Analysis of these interstellar objects is a major step towards learning how common, or uncommon, the conditions for the evolution of life are in the Universe.”

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M. Cordiner et al. Isotopic evidence for a cold and distant origin of 3I/ATLAS. Nature, published online June 22, 2026; doi: 10.1038/s41586-026-10771-6

C. Opitom et al. 2026. High nitrogen and carbon isotopic ratios in the interstellar comet 3I/ATLAS. Nature, in press; arXiv: 2603.07187