Every once in a while, our solar system receives a visitor from the vast, dark reaches of interstellar space. These travelers are rare—so rare that in all of human history we’ve only confirmed three of them. The newest one, known as 3I/ATLAS, was discovered in July, and it is already rewriting what we thought we knew about comets.
Unlike the familiar comets born in the frozen outer realms of our own solar system, interstellar comets come from elsewhere—alien star systems far beyond our own. Each one is a time capsule, carrying with it the chemistry of a distant birthplace. And thanks to the James Webb Space Telescope (JWST), we’ve just gotten an intimate look at what 3I/ATLAS is made of. What astronomers found was not just surprising—it was unlike anything seen before.
The James Webb Space Telescope Turns Its Eyes Outward
On August 6th, when 3I/ATLAS was about 3.32 astronomical units (AU) from the Sun—roughly the distance between the Sun and the asteroid belt—JWST pointed its NIRSpec infrared camera toward the comet’s glowing atmosphere, or coma. This coma forms as sunlight heats up the comet’s nucleus, releasing gas and dust in a shimmering halo.
What JWST found stunned astronomers. The coma of 3I/ATLAS was dominated by carbon dioxide (CO₂). While water, carbon monoxide (CO), and carbonyl sulfide were also present—as expected in comets—the ratio of CO₂ to water was extraordinary: 8 to 1, the highest ever observed. That ratio wasn’t just unusual—it was a full six standard deviations above the norm, meaning it wasn’t a fluke of data but a genuine anomaly.
Strangely, the ratio of carbon monoxide to water—at 1.4—was much more in line with other comets. Why CO₂ should dominate so overwhelmingly while CO remains ordinary is a puzzle scientists are still working to solve.
What Makes This Comet So Unusual?
There are several possible explanations for this lopsided chemistry. One theory suggests that 3I/ATLAS was born in a star system bathed in unusually strong ultraviolet radiation. Such radiation could have altered the chemistry of its ices, making carbon dioxide far more abundant relative to water.
Another possibility is that the comet formed beyond the “CO₂ ice line” in its original system. Just as our solar system has “ice lines” that dictate where different volatile materials can freeze solid, 3I/ATLAS may have originated in a region where CO₂ ice was plentiful while water ice was scarce.
A more subtle explanation involves how the comet responds to heat from the Sun. Water ice requires more energy to sublimate than CO₂ ice. If 3I/ATLAS has a dense or insulated nucleus, water trapped beneath its surface might not yet be escaping, while CO₂, with its lower melting point, would boil off first. That could temporarily skew the observed ratio, with water releasing later as the comet draws nearer to the Sun.
A Familiar Signature in an Alien Object
One finding stood out as oddly reassuring. JWST measured the ratio of two stable carbon isotopes—carbon-12 and carbon-13—in the comet’s coma. Remarkably, this ratio closely matches what we see on Earth. That suggests that at least some aspects of chemical evolution in distant star systems mirror our own. In other words, while 3I/ATLAS carries exotic fingerprints, it also carries echoes of home.
Putting 3I/ATLAS Into Context
To appreciate how important these observations are, it helps to remember how few interstellar visitors we’ve had the chance to study. The first, ʻOumuamua, passed by in 2017, but it was faint and unusual—showing no coma at all, only a strange elongated shape and mysterious acceleration. The second, 2I/Borisov, discovered in 2019, did have a coma, and its chemistry revealed high levels of carbon monoxide relative to water.
Now comes 3I/ATLAS—the third known interstellar object, and only the second with a measurable coma. And already, its chemistry is unlike Borisov’s, raising the possibility that interstellar comets are astonishingly diverse. If so, every one we study could reveal a new piece of the puzzle about how planetary systems form across the galaxy.
Racing Against Time
The challenge with interstellar comets is that they don’t linger. Their paths carry them on hyperbolic trajectories—once they swing past the Sun, they’re gone forever, back into the interstellar dark.
For 3I/ATLAS, the clock is ticking. Its perihelion, or closest approach to the Sun, will happen in October, when it will be hidden from Earth’s view by the Sun’s glare. Some hope that spacecraft near Mars might catch a glimpse, but for Earth-based and space-based telescopes like JWST, the window is closing fast.
By December, the comet will reappear, but by then it will already be on its way out, shedding the last of its ices into space. Astronomers have only a brief chance to study it before it disappears forever.
Why This Matters
At first glance, studying the chemistry of a fleeting comet might seem like an esoteric pursuit. But in truth, these interstellar visitors are gifts—messengers from other worlds, carrying information no probe could bring us. They are physical fragments of alien star systems, offering direct evidence of how planets, ices, and atmospheres might form elsewhere in the galaxy.
Each comet gives us a unique data point in the story of cosmic evolution. ʻOumuamua hinted at the strangeness of extrasolar objects. Borisov suggested a composition rich in carbon monoxide. Now 3I/ATLAS, with its astonishing abundance of carbon dioxide, adds a third chapter—one that will force scientists to rethink their models of cometary chemistry.
A Cosmic Goodbye
Soon, 3I/ATLAS will vanish from our skies, becoming just another icy wanderer in the endless night between stars. But before it leaves, it will have whispered secrets to us—about its birthplace, about the diversity of other solar systems, about the endless creativity of nature in shaping worlds.
We will not see it again. But the data we collect now will ripple forward for decades, helping future scientists refine their theories about the origins of planetary systems and perhaps even the ingredients for life itself.
Science often advances not by what is expected, but by what surprises us. And 3I/ATLAS has surprised us in the best possible way—reminding us that the universe is stranger, richer, and more full of wonder than we can imagine.
More information: Martin A. Cordiner et al, JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS, arXiv (2025). DOI: 10.48550/arxiv.2508.18209
