Interstellar Comet 3I/ATLAS Likely Formed Far From Its Star's Light, Nitrogen Clue Suggests

It has been just over a year since comet 3I/ATLAS first captured attention as the third known object to fly through our solar system from beyond it, and astronomers are still learning where it came from. For the first time, researchers have studied the charged particles, or ions, in the interstellar comet's tail, using the William Herschel Telescope in Spain's Canary Islands and an instrument named WEAVE that performs spectroscopy. The findings were published in Monthly Notices of the Royal Astronomical Society on September 7.

Spectroscopy splits incoming light into its spectrum, letting astronomers pick out the chemical fingerprints of different elements. Earlier studies of 3I/ATLAS mostly examined its coma, the shell of gas that forms as the sun's heat turns the comet's ice to vapor. A tail forms separately, when the solar wind's high-energy particles strike a comet's molecules and electrify them into ions — a much harder signal to read. When astronomers studied ions in the tail of interstellar comet 2I/Borisov in 2019, they detected traces but could not identify them; WEAVE, switched on in 2023, was sensitive enough to do so for 3I/ATLAS.

Lea Ferellec, a Northumbria University research fellow who led the late-2025 observations, and colleagues counted ions including nitrogen, carbon dioxide and carbon monoxide in the comet's tail. 3I/ATLAS showed a much higher ratio of nitrogen to carbon monoxide than most solar-system comets — a sign, since higher ratios point to colder birthplaces, that it formed somewhere colder than -240 degrees Celsius.

'Finding that it's so rich in nitrogen tells us it likely formed in extremely cold conditions, far from its home star,' Ferellec said. If the comet formed in a distant star system, that would place its origin in that system's outer reaches, where little of its star's light could reach. 'This object gives us a rare chance to study material that formed somewhere completely different to our own solar system,' she said. 'Every one of these objects we study helps us understand a little more about how planets form around other stars.'