A Comet From Another Star System Just Passed Through Ours
A comet possibly older than our Sun drifted through our solar system in 2025. It was made around a different star, under different conditions, and its chemistry proves it. Every major telescope turned to look. It will never come back.
Somewhere between 7.6 and 14 billion years ago, in a star system we will probably never identify, a comet was born. It was knocked loose from its home, drifted into interstellar space, and spent longer than our Sun has existed travelling through the dark between stars. In July 2025, it rolled into our solar system. We pointed every telescope we have at it. And then it left. It will never come back.
The object was named 3I/ATLAS. The “3I” means it’s the third confirmed interstellar object ever detected, after 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019. Unlike those earlier visitors, 3I/ATLAS was big enough and close enough for scientists to study in serious detail.
A snowball from someone else’s backyard
Comets are essentially dirty snowballs: lumps of ice, dust, and rock left over from the formation of a solar system. Ours were all made from the same cloud of gas and dust that formed our Sun 4.6 billion years ago. Their chemistry reflects our neighbourhood. 3I/ATLAS was made somewhere else entirely, around a different star, under different conditions. And the chemistry proves it.
The James Webb Space Telescope found that 3I/ATLAS is unusually rich in carbon dioxide: a ratio of roughly 8 to 1 compared to water, dramatically higher than anything in our own comets. It also contained carbon monoxide, methane, and carbonyl sulfide. The ALMA radio telescope in Chile detected methanol at concentrations 70 to 120 times higher than hydrogen cyanide. Nickel vapour appeared in the gas around the comet, but with very little iron alongside it, which is odd because those two normally show up together in our solar system.
These aren’t just numbers on a spectrometer readout. They’re clues about the conditions in whatever distant system formed this object. Different temperatures, different radiation environment, different raw ingredients. A recipe from a kitchen we’ve never visited.
What billions of years of wandering does to you
Here’s the part that stops me: this comet may be older than our Sun. It could have been drifting through the galaxy before Earth, before Jupiter, before the cloud of gas that became our solar system had even started to collapse.
During that immense journey, cosmic radiation slowly altered its outer layers. Scientists believe the unusual carbon dioxide richness is partly a result of this processing: cosmic rays converted carbon monoxide into carbon dioxide over billions of years. What we’re seeing isn’t necessarily what the comet looked like when it formed. It’s what billions of years of solitude did to it.
A brief visit, then gone
3I/ATLAS swung closest to the Sun on 29 October 2025, passing between the orbits of Earth and Mars. ESA’s Juice spacecraft photographed it. NASA’s MAVEN orbiter observed it in ultraviolet. Even the Perseverance rover on Mars pointed its cameras skyward to catch a glimpse. A telescope in Chile first spotted it in July as a faint smudge in the constellation Sagittarius, moving too fast and at the wrong angle to be orbiting our Sun.
It’s now heading back into interstellar space, carrying with it a chemical fingerprint from a place we’ll never see.
Talking about this with your kids
A snowball made around a different star, possibly before our Sun was born, rolled through our solar system. We couldn’t touch it, couldn’t land on it, could only watch. But the light it gave off told us what it’s made of, and it contains some of the same chemical building blocks that scientists think are important for life. Ask your kid: if you could send a tiny spacecraft to grab a piece before it disappeared forever, what do you think you’d find inside?
We live in one small corner of a very large galaxy. Most of the time, the universe stays at a distance. But for a few months in 2025, a piece of it came to visit. And we were paying attention.
Sources and further reading
Wikipedia overview of 3I/ATLAS: https://en.wikipedia.org/wiki/3I/ATLAS
SETI Institute perihelion update: https://www.seti.org/news/comet-3iatlas-perihelion-update
NOIRLab Gemini South observations: https://noirlab.edu/public/news/noirlab2525
JWST composition analysis (Astrobiology, November 2025): https://www.liebertpub.com/journal/ast
