The water ice locked inside interstellar comet 3I/ATLAS condensed at temperatures below minus 400 degrees Fahrenheit, according to a peer-reviewed study published in Nature Astronomy in June 2026. That is colder than any formation environment linked to a comet in our own solar system, where even the most distant Kuiper Belt objects are thought to have assembled at roughly 30 to 50 Kelvin (about minus 405 to minus 370 degrees Fahrenheit). The finding represents the first measurement of a water deuterium-to-hydrogen ratio from an object born around another star, turning a single icy wanderer into a thermometer for a faraway planetary nursery.
A comet from somewhere else
3I/ATLAS, formally designated C/2025 N1, was first flagged on July 1, 2025, after the ATLAS survey picked it up. Pre-discovery images were later pulled from ATLAS archives and the Zwicky Transient Facility, according to NASA’s overview of the object. Its trajectory was the giveaway: the comet follows a hyperbolic orbit, meaning it is not gravitationally bound to the Sun and will never return. Both NASA and the European Space Agency independently confirmed that classification, earning the object the “3I” interstellar tag, only the third ever assigned.
The comet reached its closest point to the Sun on October 30, 2025, and swung nearest to Earth on December 19, 2025, at roughly 1.8 astronomical units (about 167 million miles). A Hubble Space Telescope observation from August 20, 2025, placed the nucleus at a few kilometers across. By the standards of solar system comets, that is modest. By the standards of interstellar visitors, it was large enough to study in detail, a stroke of luck that astronomers seized.
Reading the ice with ALMA
The key measurement came from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, one of the most sensitive radio telescope networks on Earth. The research team, whose results appear in the Nature Astronomy paper, targeted emissions from HDO, a heavier variant of ordinary water in which one hydrogen atom is replaced by deuterium, a hydrogen isotope carrying an extra neutron. By comparing the HDO signal to models of the comet’s total water output, the team derived a lower-bound constraint on the deuterium-to-hydrogen ratio, or D/H.
That ratio came back far higher than anything measured in Earth’s oceans or in the dozens of solar system comets sampled over the past three decades. The chemistry behind the interpretation is well established: when water ice forms at extremely low temperatures, deuterium preferentially swaps into the molecule. The colder the environment, the more deuterium ends up in the ice. Working backward from the D/H lower bound, the researchers concluded that 3I/ATLAS’s water must have frozen out at temperatures below roughly 20 Kelvin, or about minus 424 degrees Fahrenheit.
The corresponding preprint on arXiv lays out the enrichment factors relative to Earth and solar system comets and connects the signal to those ultra-cold formation conditions, while acknowledging modeling assumptions that future observations could sharpen.
What scientists still do not know
The result is a lower bound, not a pinpoint value. That means the actual deuterium enrichment could be even more extreme, pushing the implied formation temperature still colder. How much colder remains unconstrained.
Nobody knows which star system ejected 3I/ATLAS. Interstellar comets drift for millions of years, and gravitational nudges along the way scramble the trail. Current orbital reconstructions narrow the inbound direction to a broad swath of the Milky Way, but no published study has claimed a specific parent star.
There is also a question of interpretation. The leading explanation ties the extreme D/H to formation in the frigid outer reaches of a protoplanetary disk, the rotating belt of gas and dust where planets and comets coalesce around a young star. But some fraction of the deuterium enrichment could have accumulated during the comet’s long cruise through interstellar space, where cosmic rays and ultraviolet radiation can alter ice chemistry over millions of years. The Nature Astronomy paper addresses this possibility but cannot fully rule it out with a single object.
Even within our solar system, comets show a spread in D/H values depending on where and when they formed. One interstellar comet with an unusually high ratio might trace a particularly cold pocket in its home disk rather than the average conditions there. Until astronomers measure water isotopes in additional interstellar visitors, it is impossible to say whether 3I/ATLAS is typical of its birthplace or an outlier.
Why it matters beyond one comet
Before 3I/ATLAS, scientists had detected only two interstellar objects passing through the solar system: 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. ‘Oumuamua was spotted too late and was too small for detailed compositional work. Borisov showed signs of water outgassing and carried an unusual carbon monoxide abundance, but no D/H ratio was extracted from its water. That makes 3I/ATLAS the first interstellar body to yield a direct isotopic measurement of water, a data point planetary scientists have wanted for decades.
If the comet’s water truly formed below 20 Kelvin, its parent system may have hosted a more extended or colder protoplanetary disk than the one that built our solar system. That hypothesis could be tested by performing the same ALMA-style spectroscopy on future interstellar comets. No formal observing program dedicated to that goal has been announced, but the Vera C. Rubin Observatory, expected to begin its main survey in the coming years, is projected to detect interstellar objects at a higher rate than current surveys, potentially giving researchers more targets.
How 3I/ATLAS compares to every other water measurement from beyond the solar system
The broader question hovering over the discovery is whether water this deuterium-rich could have played a role in seeding rocky planets with volatile materials in other star systems. In our own solar system, comets are one proposed delivery mechanism for Earth’s water. If interstellar comets routinely carry water forged at extreme cold, the chemistry of oceans on distant worlds could look quite different from our own. That idea remains speculative with a sample size of one, but 3I/ATLAS has cracked open a door that was previously sealed shut: for the first time, scientists can compare the water recipe of another star’s building blocks with the one that made our own. There is, as of June 2026, no second data point to set beside it.
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*This article was researched with the help of AI, with human editors creating the final content.
