Interstellar comet 3I/ATLAS likely formed where its star's light couldn't touch it

Click for next article Hubble captured this image of the interstellar comet 3I/ATLAS on July 21, 2025, when the comet was 277 million miles from Earth. 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 '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. Another view of comet 3I/ATLAS. ( Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist. Rodriguez (International Gemini Observatory/NSF NOIRLab)/T. Rector (University of Alaska Anchorage/NSF NOIRLab)/M. Zamani (NSF NOIRLab).)Comparing these figures can hint at where a comet formed.