Interstellar comet 3I/ATLAS likely formed where its star’s light couldn’t touch it
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It has now been just over a year since the comet 3I/ATLAS first captured the world’s attention as the third known object to fly through our solar system from beyond. Astronomers are still learning new things about where exactly it came from.
For the first time ever, they’ve been able to study charged particles in the tail of the interstellar comet. This has told them that 3I/ATLAS contains more nitrogen than the typical comet from our own solar system — revealing a tantalizing clue about how the strange comet formed.
“Finding that it’s so rich in nitrogen tells us it likely formed in extremely cold conditions, far from its home star,” Lea Ferellec, a research fellow at Northumbria University, said in a statement.
Ferellec’s group used the William Herschel Telescope in Spain’s Canary Islands. One of this telescope’s instruments is a fiber-optic weave named, well, WEAVE. WEAVE lets astronomers do spectroscopy — which splits incoming light into its spectrum. Different chemical elements absorb and emit different wavelengths of light, marking that spectrum with different characteristic fingerprints. With spectroscopy, astronomers can look at a celestial object’s light, pick out the fingerprints, and reconstruct the stuff inside.
Other astronomers have measured 3I/ATLAS with spectroscopy, but they’ve mostly examined the comet’s coma, a shell of gas that forms when the sun’s heat sublimates the comet’s ice into vapor.
Of course, a coma is not the only feature that forms as a comet approaches the sun. Close in, a comet is battered by the torrent of the sun’s solar wind, which pushes the comet’s material away into a distinctive tail. In the process, the solar wind’s high-energy particles strike the comet’s molecules and electrify them into charged ions.
If astronomers can identify a tail’s ions, they can find chemicals and learn information they can’t from the coma alone. This is a tricky task. When fellow interstellar comet 2I/Borisov visited our solar system in 2019, astronomers identified traces of its ions, but they couldn’t make out what those ions actually were.
WEAVE, which switched on in 2023, is powerful and sensitive enough to change that. When Ferellec and colleagues turned WEAVE on 3I/ATLAS in late 2025, they could count ions like nitrogen, carbon dioxide and carbon monoxide in the strange comet’s tail.
Comparing these figures can hint at where a comet formed. For example, a comet with a higher ratio of nitrogen to carbon monoxide tends to have formed in a colder birthplace. Ferellic’s group found that 3I/ATLAS has a much higher ratio than most of the comets that we observe in our solar system.
In particular, Ferellec and colleagues estimate that 3I/ATLAS formed somewhere colder than -240°C. If this comet was born in a distant star system, then, it formed in that system’s outer reaches. In other words, 3I/ATLAS likely formed in a place where little of its star’s light could touch it.
“This object gives us a rare chance to study material that formed somewhere completely different to our own solar system,” Ferellec said in the statement. “Every one of these objects we study helps us understand a little more about how planets form around other stars.”
Ferellec and colleagues published their work in the journal Monthly Notices of the Royal Astronomical Society on Sept. 7.
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