This discovery provides crucial data for comparing 3I/ATLAS to both solar system comets and other interstellar objects like 'Oumuamua (dry) and Borisov (carbon monoxide-rich), thereby enhancing understanding of exoplanetary system chemistry.
Since its arrival this summer, scientists have been racing to understand as much as possible about 3I/ATLAS, the third recorded visitor from outside our solar system. A breakthrough study published on Sept. 30 has scientists exclaiming, "OH!": the first detection of hydroxyl gas (OH) from an interstellar object. The finding provides a crucial touchstone for understanding how this alien visitor compares to the comets born in our solar system.
Unlike the two interstellar objects that preceded it, 3I/ATLAS is offering astronomers a unique glimpse into the chemistry of another star system. A team of Auburn University scientists, led by Zexi Xing, a postdoctoral researcher, published their findings in The Astrophysical Journal Letters.
They found telltale signs of water in the form of hydroxyl, a byproduct created when water vapor is broken apart by sunlight. Water is the measuring stick by which scientists study comet activity. The discovery provides a useful point of reference, allowing scientists to compare this alien comet to comets born in our own solar system. 3I/ATLAS, with its surprising water activity, proves that each interstellar guest has a new story to tell.
"Every interstellar comet so far has been a surprise," said Xing in a press release. "'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."
Comets are essentially giant orbs of frozen gases, rock, and dust. As they approach a star, the heat causes their solid nucleus to spew gas and dust, creating a glowing head, or coma, and often a tail. First spotted on July 1, 2025, by the ATLAS survey in Chile, comet 3I/ATLAS was confirmed to be interstellar (not from our solar system) because of its hyperbolic trajectory -- an open-ended path proving it is not gravitationally bound to our Sun.
The detection of water from 3I/ATLAS was a technical triumph. Scientists couldn't see the water directly. Instead, they looked for its chemical fingerprint: a molecule called hydroxyl, which is what's left after sunlight breaks apart a water molecule.
Hydroxyl gives off a faint glow in ultraviolet (UV) light, a wavelength that is almost entirely blocked by Earth's atmosphere. This is where NASA's Neil Gehrels Swift Observatory comes in. As a space-based telescope orbiting high above the atmosphere, Swift has a clear, unobstructed view of the cosmos in UV light, allowing it to see what ground-based telescopes cannot.
Its specialized telescope captured the faint UV signal from hydroxyl, providing the first concrete proof that 3I/ATLAS was "outgassing" -- releasing water vapor into space. Finding this water signature is a breakthrough, as it allows astronomers to measure the comet's activity and compare it directly to comets from our own solar system.
Even more surprising than the presence of water was its location. The comet was actively shedding water vapor while nearly three times as far from the Sun as Earth. At this distance, sunlight is usually not strong enough to turn a comet's surface ice directly into gas, a process known as sublimation.
However, 3I/ATLAS was spewing out water at an astonishing rate of about 88 pounds (40 kilograms) per second -- around the same rate as a firehose at full blast, according to the press release. This has led scientists to theorize that a different mechanism is at play in the comet's outgassing process, one distinct from sublimation on the main nucleus. Instead, they believe the nucleus is ejecting a spray of tiny, ice-coated dust grains.
These smaller particles have a much larger surface area compared to their volume. This allows the faint sunlight to heat them efficiently, turning their icy coatings into gas and creating the vast cloud of water vapor observed around the comet. Such an extended source of water has been seen in only a handful of distant comets, suggesting a complex structure for this interstellar visitor.
"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, a professor of physics at Auburn. "It tells us that the ingredients for life's chemistry are not unique to our own."
Adding to the excitement, two Mars orbiters caught a glimpse of the comet as it passed by the Red Planet in early October. ESA's ExoMars Trace Gas Orbiter (TGO) captured images of a fuzzy white dot -- the comet's icy nucleus shrouded in its thousand-kilometer-wide coma. While the nucleus itself was too distant to resolve, the observation confirmed the comet's activity. The task was incredibly challenging because the comet was some 100,000 times fainter than the orbiter's usual subject, the martian surface.
The images provide another valuable data point in the ongoing study of this rare visitor, a true outsider carrying clues about the formation of worlds far beyond our own.