Using the ultraviolet/optical telescope on the NASA Neil Gehrels Swift Observatory, astronomers have detected hydroxyl gas (OH) – a chemical water footprint – from the interstellar 3i/Atlas object.
Stacked images of the interstellar comet 3i/Atlas acquired with the Ultraviolet/Optical Telescope on board the NASA Neil Gehrels Swift Observatory: the first were obtained on July 31 and August 1, 2025 (visit 1, upper half of the figure) and the second on August 19, 2025 (visit 2, lower half of the figure). Image credit: Xing and others., Two: 10.3847/2041-8213/AE08AB.
The discovery of the third interstellar object, 3i/Atlas, on July 1, 2025, launched a large campaign of characterization around the world.
Following the lessons learned from the previous interstellar objects 1i/’Oumuamua and 2i/Borisov, observation campaigns were launched to quickly capture its initial light, its morphology, its light curves, its color and its optical and close infrared spectrum.
Given the apparent brightness and the early extension of coma, gas production has been supposed and sought after, but has not been found.
Characterizing the early activity of interstellar objects is essential to understand their chemical and physical evolution during the solar approach, because this perhaps represents the first time that they have been heated significantly during their very long dynamic lifespan.
“Water detection is a major advance to understand how the interstellar comets evolve,” said Dennis Bodewits, astronomer at the University of Auburn, and his colleagues.
“In the comets of the solar system, water is the stallion by which scientists measure their global activity and follow the way in which sunlight leads to the release of other gases.”
“This is the chemical reference on which any comparison of volatile ice is based in the core of a comet.”
“Finding the same signal in an interstellar object means that, for the first time, we can start to place 3i/Atlas on the same scale as that used to study the native comets of the solar system – a step towards comparison of the chemistry of planetary systems through our Milky Way. »»
“What makes 3i/Atlas remarkable is the place where this aquatic activity occurs.”
Swift detected the hydroxyl while the comet was almost three times more distant from the sun than the earth-far beyond the region where water ice on the surface of a comet can easily be enhanced-and has measured a water loss rate of around 40 kg per second. At these distances, most of the solar system comets remain silent.
The strong ultraviolet signal of 3i/Atlas suggests that something else is at work: perhaps sunlight heats small frozen grains released by the nucleus, allowing them to vaporize and feed the surrounding gas cloud.
Such extended sources of water were only observed in a handful of distant comets and indicate complex layers of ice which preserve clues on the way in which these objects have formed.
Each interstellar comet discovered so far has revealed a different side of planetary chemistry beyond our sun.
Together, they demonstrate that the constituent elements of the comets – and the volatile ice that shapes them – can vary considerably from one stellar system to another.
These differences suggest to what extent the planet training environments can be diversified and how processes such as temperature, radiation and composition shape the materials that in the end generate planets and, potentially, life.
Capture this whisper of ultraviolet light from 3i/Atlas was a technical triumph in itself.
Swift transports a modest 30 cm telescope, but orbit above the earth’s atmosphere, it can see the ultraviolet wavelengths which are almost completely absorbed before reaching the ground.
Released from the glare of the sky and air interference, the Ultraviolet/Optical Telescope of SWIFT reaches the sensitivity of a 4 m class ground telescope for these wavelengths.
Its rapid targeting capacity allowed astronomers to observe the comet a few weeks after its discovery, long before it became too weak or too close to the sun to be studied from space.
“When we detect water – or even its low ultraviolet echo, oh – from an interstellar comet, we read a note from another planetary system,” said Professor Bodewits.
“This tells us that the ingredients of life chemistry are not unique in ours.”
“So far, each interstellar comet has been a surprise,” added Dr. Zexi Xing, a postdoctoral researcher at the University of Auburn.
“‘Oumaumua was dry, Borisov was rich in carbon monoxide, and now Atlas abandons water at a distance where we did not expect it.”
“Everyone rewrites what we thought about how planets and comets are formed around the stars.”
An article describing the results was published on September 30 in the Astrophysical newspapers.
_____
Zexi Xing and others. 2025. Water production rate of the interstellar object 3i/Atlas. Apjl 991, L50; Two: 10.3847/2041-8213/AE08AB
