Interstellar comet 3I/ATLAS is spraying water across the solar system

This summer, that traveler entered our solar system and received the name 3I/ATLAS, becoming only the third confirmed interstellar comet ever observed. When researchers at Auburn University aimed NASA's Neil Gehrels Swift Observatory at the object, they uncovered something extraordinary: the first detection of hydroxyl (OH) gas coming from it, a clear chemical sign of water. Swift was able to detect a faint ultraviolet glow that ground based telescopes cannot see because it operates above Earth's atmosphere, where this type of light is not blocked before reaching the surface.

First Detection of Water on Interstellar Comet 3I/ATLAS

Identifying water through its ultraviolet byproduct, hydroxyl, marks an important step in understanding how interstellar comets behave and change over time. In comets that formed within our own solar system, water serves as the primary measure of activity. Scientists use it to determine how sunlight triggers the release of other gases and to compare the mix of frozen materials inside a comet's nucleus. Detecting the same water signature in 3I/ATLAS means astronomers can now evaluate it using the same standards applied to familiar solar system comets. That comparison opens the door to studying how planetary systems across the galaxy may differ or resemble our own.

Unexpected Water Activity Far From the Sun

What makes 3I/ATLAS especially intriguing is the distance at which this water activity was observed. Swift detected hydroxyl when the comet was nearly three times farther from the Sun than Earth is, well beyond the region where surface ice would normally turn directly into vapor. Even at that distance, the comet was losing water at a rate of about 40 kilograms per second, comparable to water blasting from a fully opened fire hose. Most comets native to our solar system remain relatively inactive that far out.

The strong ultraviolet signal suggests that additional processes may be involved. One possibility is that sunlight is warming tiny icy particles that have broken away from the nucleus. As those grains heat up, they could release vapor and supply the surrounding cloud of gas. Only a small number of distant comets have shown this kind of extended water source, and it points to layered ices that may preserve information about how and where the object originally formed.

Clues to Planet Formation Beyond Our Solar System

Each interstellar comet discovered so far has revealed something different about chemistry in other planetary systems. Together, these visitors show that the ingredients that build comets, especially volatile ices, can vary widely from one star system to another. Those differences provide insight into how temperature, radiation, and chemical makeup shape the materials that eventually form planets and possibly create conditions suitable for life.

How NASA's Swift Observatory Made the Discovery

Detecting that faint ultraviolet signal was also a technical achievement. NASA's Neil Gehrels Swift Observatory carries a relatively small 30 centimeter telescope, yet from its position in orbit it can observe ultraviolet wavelengths that are mostly absorbed by Earth's atmosphere. Without interference from air and sky brightness, Swift's Ultraviolet/Optical Telescope can reach a sensitivity comparable to a 4 meter class ground telescope at those wavelengths. Its ability to respond quickly allowed the Auburn team to observe 3I/ATLAS within weeks of its discovery, before it became too faint or moved too close to the Sun for safe observation from space.

"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."

"Every interstellar comet so far has been a surprise," added Zexi Xing, postdoctoral researcher and lead author of the study. "'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."

3I/ATLAS has since dimmed and is currently out of view, but it is expected to become observable again after mid November. That return will give scientists another opportunity to monitor how its activity changes as it moves closer to the Sun. The detection of hydroxyl, detailed in The Astrophysical Journal Letters, offers the first solid proof that this interstellar comet is releasing water far from the Sun. It also highlights how even a modest space based telescope, operating above Earth's atmosphere, can capture faint ultraviolet signals that connect this rare visitor to the broader family of comets and to the distant planetary systems where such objects are born.