Astronomers using the William Herschel Telescope on La Palma detected unusually high levels of nitrogen in the plasma tail of interstellar comet 3I/ATLAS. The finding reveals that the comet formed at temperatures below approximately –240°C, far from its parent star in another planetary system.
The research team, led by Lea Ferellec of Northumbria University, used the WEAVE spectrograph on the 4.2-metre WHT to analyze gases streaming from 3I/ATLAS as it moves away from the Sun. When sunlight heats a comet's nucleus, released gases become ionized and swept by solar wind into a plasma tail. By analyzing the light spectrum of this tail, the astronomers identified five ions, including ionised molecular nitrogen (N₂⁺) and ionised carbon monoxide (CO⁺).
The ratio of nitrogen to carbon monoxide in a comet's ices is particularly temperature-sensitive. The unusually high nitrogen ratio in 3I/ATLAS indicates formation at temperatures below 33 kelvin, placing its birthplace in the frozen outer reaches of its original planetary system. The researchers also traced how different ions changed with distance along the tail, marking the first time this level of detail has been achieved for an interstellar object.
3I/ATLAS was discovered in July 2025 and ranks as only the third confirmed interstellar comet to pass through our Solar System, after 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. Because these objects formed around distant stars before ejection into interstellar space, their chemistry allows direct comparison of planetary system materials beyond our own.
"Every one of these objects we study helps us understand a little more about how planets form around other stars," Ferellec said.
The research appears in Monthly Notices of the Royal Astronomical Society.