NASA's Swift Observatory has detected water vapor from the interstellar comet 3I/ATLAS, marking the first such observation for an object from outside our solar system. The comet, the third confirmed interstellar visitor, was producing water at a rate of 40 kilograms per second when it was nearly three times farther from the sun than Earth. This discovery provides insights into the chemistry of distant planetary systems.
The comet 3I/ATLAS entered our solar system this summer, becoming only the third known interstellar comet observed by astronomers. Researchers from Auburn University used NASA's Neil Gehrels Swift Observatory to study it, detecting a faint ultraviolet glow from hydroxyl gas, a byproduct of water. This observation, made when the comet was well beyond the typical activation zone for solar system comets, revealed unexpected activity.
At a distance nearly three times Earth's from the sun, 3I/ATLAS was releasing water at about 40 kilograms per second. This rate is comparable to that of a fire hose and suggests processes beyond simple surface sublimation, such as the heating of detached icy particles. Ground-based telescopes could not detect this ultraviolet signal due to Earth's atmosphere, but Swift's orbital position allowed for clear observation.
"When we detect water -- or even its faint ultraviolet echo, OH -- from an interstellar comet, we're reading a note from another planetary system," said Dennis Bodewits, professor of physics at Auburn. "It tells us that the ingredients for life's chemistry are not unique to our own."
Zexi Xing, a postdoctoral researcher and lead author of the study, noted, "Every interstellar comet so far has been a surprise. 'Oumuamua was dry, Borisov was rich in carbon monoxide, and now ATLAS is giving up water at a distance where we didn't expect it. Each one is rewriting what we thought we knew about how planets and comets form around stars."
The findings, published in The Astrophysical Journal Letters (2025; 991 (2): L50), enable comparisons between 3I/ATLAS and native solar system comets. Previous interstellar objects like 'Oumuamua and Borisov showed varied compositions, highlighting differences in volatile ices across star systems. The comet has since dimmed and is currently unobservable, but it is expected to reappear after mid-November for further study as it approaches the sun closer.