A NASA space telescope has made a surprising discovery at comet 3I/ATLAS that's got a team of astronomers rather excited.
The discovery is said to be a major breakthrough in understanding how comets evolve.
More about comet 3I/ATLAS
It's also a scientific breakthrough because of exactly where the comet was when the team made the discovery.
And it has major implications for our understanding of how planets form across the Galaxy, and whether they could ever host life.
The fingerprints of comet chemistry
Comet 3I/ATLAS was discovered on 1 July 2025, and since then, astronomers have been doing everything they can to learn as much about it as they can.
That's because, unlike other comets, 3I/ATLAS doesn't orbit the Sun. It's an interstellar comet, and one of only three we've ever seen.
This means it entered our Solar System from elsewhere in the Galaxy, and will eventually exit it. When it does leave our Solar System, it will be gone forever.
3I/ATLAS is thought to be at least 7 billion years old, making it likely twice as old as Earth, and the oldest comet we've ever seen.
It's a primordial relic from a distant star system so, understandably, astronomers are keen to study it in detail while it's here.
Scientists have even been able to get Mars rovers and orbiters to look at 3I/ATLAS while it's hidden from Earth, as it travels close to the Sun.
A team of astronomers at Auburn University in Alabama, USA, managed to point NASA’s Neil Gehrels Swift Observatory at the comet, and a newly-released study using that data shows it detected hydroxyl (OH) gas, a chemical fingerprint of water.
The space-based telescope was able to spot a faint ultraviolet glow that ground observatories couldn't see, because it was able to capture light that never reaches Earth's surface.
Why this is a big deal
The team behind the detection say it's a major breakthrough in understanding how interstellar comets evolve.
When looking at comets that originated in our Solar System, scientists analyse water to measure how active that comet is.
They study how heat from the Sun causes the release of frozen gases as the comet gets closer to the inner Solar System.
Finding the same signal in an interstellar object means astronomers can begin to study 3I/ATLAS with the same criteria they use to study Solar System comets.
This, by extension, is a chance to begin studying the chemistry of planetary systems beyond our Sun.
A strange location
The team behind the detection are particularly interested in where the water activity is occurring.
Swift detected OH when 3I/ATLAS was nearly three times farther from the Sun than Earth.
That should have been far beyond the distance from the Sun where water ice on a comet’s surface would normally turn into a gas.
In fact, the team measured a water-loss rate of about 40 kg per second. At that sort of distance from the Sun, most Solar System comets are relatively quiet.
This strong ultraviolet signal from 3I/ATLAS suggests something else is going on.
One explanation is that sunlight is heating small icy grains released from the comet's nucleus, allowing them to vaporise and feed the surrounding cloud of gas.
Interstellar comets can tell scientists a lot about the chemistry involved in planet formation beyond our Solar System.
These deep-space interlopers also reveal how the building blocks of comets vary dramatically from one star system to another.
That hints at the potential diversity in planet-forming regions across the Galaxy, and the likelihood that distance planets could host life.
How they made the detection
NASA’s Neil Gehrels Swift Observatory is equipped with a 30cm telescope, which most amateur astronomers will likely tell you isn't particularly big.
But Swift's location above Earth's atmosphere means it can see ultraviolet wavelengths that are almost completely absorbed before reaching the ground.
This enabled the team to observe comet 3I/ATLAS within weeks of discovery, before it grew too faint or too close to the Sun to study.
"When we detect water – or even its faint ultraviolet echo, OH – from an interstellar comet, we’re reading a note from another planetary system," says 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," says Zexi Xing, postdoctoral researcher and lead author of the study.
"[Previous interstellar comets] 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 faded from view but will become observable again after mid-November 2025, offering another chance to track how its activity evolves as it approaches the Sun.
Read the full paper via the Astrophysical Journal Letters
