Interstellar comet 3I/ATLAS carried water with an isotope signature that may date its formation to 11 or 12 billion years ago, according to a peer-reviewed study published in Nature in June 2026. If that model-based age is broadly correct, the comet’s ice formed roughly six to seven billion years before the Sun and planets.

Two qualifications belong near the top. July 2025 was when astronomers discovered 3I/ATLAS, not necessarily when it “arrived” in the solar system. And the water was not dated directly. The age comes from comparing measured isotope ratios with models of how the Milky Way’s chemistry changed over time.

This is one study, not settled consensus. The phrase “oldest water ever observed passing through our neighborhood” is a reasonable shorthand for the paper’s implication, but it is not a formal record category maintained by astronomers.

July 2025 was the discovery date

The NASA-funded ATLAS survey telescope at Rio Hurtado, Chile, reported 3I/ATLAS to the Minor Planet Center on July 1, 2025. NASA’s observation history says later searches found pre-discovery images dating to June 14. The comet had entered the solar system earlier and was already moving inward when it was noticed.

Its orbit supplied the clearest fact about its origin. The trajectory was strongly hyperbolic, meaning the object was moving too fast to be gravitationally bound to the Sun. Tracing that path backward placed its origin outside our planetary system. The “3I” designation identifies it as the third confirmed interstellar object, after 1I/‘Oumuamua and 2I/Borisov.

As the comet passed through sunlight, its frozen material sublimated into gas and dust surrounding the nucleus. That coma allowed telescopes to read its chemistry from afar.

Webb measured unusually heavy water

Martin Cordiner of NASA’s Goddard Space Flight Center led the Nature study of Webb observations made on December 22 and 23, 2025. Webb’s Near-Infrared Spectrograph measured water, carbon monoxide and carbon dioxide in the coma and separated common isotopes from rarer, heavier versions.

The water’s deuterium-to-hydrogen ratio was 0.98 percent, with an uncertainty of 0.06 percentage points. Deuterium is hydrogen with a neutron added to its nucleus. The measured proportion was more than an order of magnitude above the values found in known solar system comets.

The authors argue that most of this water ice formed below about 30 kelvin, or minus 243 degrees Celsius, in a cold and irradiated environment. It then avoided enough later warming to preserve the heavy-water signal. Substantial high-temperature processing would have altered it.

An independent clue had arrived from the Atacama Large Millimeter/submillimeter Array. A Nature Astronomy paper led by Luis Salazar Manzano reported a lower limit on the comet’s heavy-water enrichment using ALMA observations from November 2025. We previously covered what that ALMA measurement did and did not establish.

The carbon ratios produced the ancient age

The Webb team also measured unusually high ratios of carbon-12 to carbon-13 in carbon monoxide and carbon dioxide. Carbon-13 accumulates as successive generations of stars manufacture elements and return them to interstellar space. Material formed early in the Milky Way’s history can therefore carry less carbon-13 than material produced later.

When Cordiner and colleagues compared the comet’s ratios with models of Galactic chemical evolution, they found that an origin roughly 11 to 12 billion years ago could explain the measurements. The estimate places formation during a period of intense early star formation, long before the solar system formed about 4.6 billion years ago.

NASA’s June 22 account of the result described 3I/ATLAS as a fragment of an ancient planetary system. That description fits the paper. It should not be read as a direct age stamped into individual water molecules.

The model cannot identify a birthday or home star

The Nature paper spells out important uncertainties. Galactic isotope models depend on incomplete knowledge of star formation rates, gas moving into and out of the Galaxy, the mix of stellar masses and the quantities of elements produced by different stars. Carbon-isotope ratios also vary with distance from the Milky Way’s center.

Those limitations leave room around the age estimate. They do not erase the measured chemistry: 3I/ATLAS has water and carbon ratios unlike those of known solar system comets, and the combination points toward a cold, old and relatively metal-poor formation environment.

The observations cannot identify the parent star. Nor do they show that the comet spent 11 or 12 billion years moving freely through interstellar space. A planetary system first had to form the object, and that system may have kept it for some unknown period before a gravitational encounter ejected it.

What the “oldest water” claim means

Astronomers observe stars, galaxies and ancient gas from far earlier in cosmic history, so 3I/ATLAS is not the oldest matter or oldest object humanity has detected. The narrower statement concerns water preserved in a comet from another planetary system that physically passed through our solar neighborhood.

Even within that narrower frame, “oldest” remains conditional on the chemical-evolution interpretation. Researchers did not collect the ice, count radioactive decay products or trace the comet back to a known star. They measured isotopes in gas released from the ice and inferred the environment and era that best fit them.

The next useful comparison will require another interstellar comet observed with similar sensitivity. With only three confirmed interstellar objects and detailed water-isotope measurements for one, astronomers do not yet know whether 3I/ATLAS is an extreme outlier or part of a population of ancient icy bodies moving between stars.