The interstellar comet 3I/ATLAS is a frozen fragment of an ancient planetary system that formed 10 to 12 billion years ago, at temperatures below 30 Kelvin, in a region of the Galaxy that was still chemically young. That is the conclusion of a study published in Nature by a team led by Martin Cordiner at NASA Goddard Space Flight Center, who used the James Webb Space Telescope to measure isotopic ratios in the comet’s gas and dust.
The findings represent the first direct isotopic evidence of an exoplanetary icy planetesimal, and they push the known age of preserved planetary material back into the first few billion years after the Big Bang.
What 3I/ATLAS is
3I/ATLAS (also designated C/2025 N1) is the third confirmed interstellar object to pass through the Solar System, after 1I/’Oumuamua and 2I/Borisov. It was discovered on July 1, 2025, by the ATLAS survey telescope in Hawaii. Like Borisov before it, 3I/ATLAS is an active comet with a solid icy nucleus less than one kilometer in diameter, surrounded by a coma of gas and dust.
It reached perihelion on October 29, 2025, at 1.36 astronomical units from the Sun, between the orbits of Earth and Mars, and is now exiting the Solar System permanently at approximately 58 kilometers per second.
What JWST saw
Cordiner’s team trained JWST’s NIRSpec instrument on the comet during its outbound leg after perihelion, capturing infrared spectra of the gas in its coma. They measured three isotopic systems: deuterium to hydrogen (D/H) in water, and carbon-13 to carbon-12 in carbon dioxide and carbon monoxide.
The results were unlike anything seen in Solar System comets.
The D/H ratio in 3I/ATLAS’s water was 0.95 percent, or approximately 9,500 parts per million. Solar System comets have D/H ratios of 0.02 to 0.03 percent, or 200 to 300 parts per million. The interstellar comet’s water is more than 10 times richer in deuterium.
The carbon isotope ratios were similarly extreme. 12C/13C in carbon dioxide ranged from 141 to 191, and in carbon monoxide from 123 to 172. Solar System values cluster between 68 and 89.
What the isotopes reveal
Deuterium enrichment of this magnitude can only be produced by low-temperature ion-molecule chemistry in dense molecular clouds. At cryogenic temperatures below approximately 30 Kelvin, deuterium fractionation is strongly enhanced because the zero-point energy differences between HDO and H2O favor deuterium incorporation in ion-neutral reactions. The comet’s ices, the team concluded, formed at temperatures no higher than 30 Kelvin, colder than any region where Solar System comets are thought to have formed.
The carbon isotope ratios tell a different story. Extreme 12C/13C enrichment indicates formation in a metal-poor environment, where fewer generations of stars have enriched the interstellar medium with the heavier carbon isotope. Using Galactic chemical evolution models, the team calculated that 3I/ATLAS’s parent molecular cloud was enriched by only one to two generations of massive stars. This places the accretion epoch at approximately 10 to 12 billion years ago, just two to four billion years after the Big Bang.
The comet is thus a preserved fragment of an ancient planetary system, frozen in a molecular cloud during the early star-forming epoch of the Universe and ejected into interstellar space by gravitational interactions with its parent system.
Caveats and context
The age estimate depends on Galactic chemical evolution models, which are themselves dependent on assumptions about how carbon isotopes are produced and distributed by asymptotic giant branch stars. Different yield tables or star-formation histories could shift the age estimate.
The D/H measurement is derived from gas-phase line ratios in the coma, not from direct ice sampling. Fractionation during sublimation or photodissociation in the coma could affect the measured ratios, though the authors accounted for these effects as best possible.
This is also a single object. Only three interstellar objects have ever been observed: ‘Oumuamua (a dry, elongated rock with no outgassing), Borisov (a relatively normal comet with Solar System-like composition), and 3I/ATLAS (an extreme isotopic outlier). We cannot yet know how representative any of them are.
The paper was published as an unedited manuscript subject to final copyediting, with minor numerical differences between preprint and published versions, the D/H value appears as both 0.95 and 0.98 percent in different versions of the text.
What it means
The discovery confirms that interstellar comets can preserve a chemical record of environments far more extreme than anything preserved in our own Solar System. 3I/ATLAS is not just another comet; it is a messenger from a time when the Galaxy was still assembling its heavy elements, carrying ices that formed in a molecular cloud before the Sun even existed. Its passage through our neighborhood, brief as it is, offers a direct glimpse into the chemistry of planet formation in the early Universe.
Source: Cordiner, M., Roth, N.X., Micheli, M. et al. Isotopic evidence for a cold and distant origin of 3I/ATLAS. Nature (2026). DOI: 10.1038/s41586-026-10771-6