The tail of 3I/ATLAS has just yielded a clue about where this interstellar visitor may have been born.
3I/ATLAS did not come from our Solar System. This object was discovered in July 2025 on a trajectory showing that it arrived from interstellar space. It behaved like a comet as it approached the Sun: its ices turned into gas, forming an envelope around it and a long tail.
3I/ATLAS observed by the Hubble Space Telescope on November 30, 2025. Background stars appear as streaks because Hubble was tracking the comet’s motion.
Credit: NASA, ESA, STScI, D. Jewitt (UCLA). Processing: J. DePasquale (STScI). CC BY 4.0.
This is the tail that Lea Ferellec, Cyrielle Opitom and Colin Snodgrass observed with the William Herschel Telescope in the Canary Islands. The measurements were taken on November 30 and December 2, 2025, after 3I/ATLAS passed closest to the Sun. The instrument broke down the light from the tail to identify the substances it contained.
The researchers notably detected molecular nitrogen and carbon monoxide in an electrically charged state. When cometary gas receives energy from the Sun, some molecules lose an electron. They then become ions, which can be detected in the tail through their light.
The relative amount of nitrogen is particularly interesting. The measurements indicate that 3I/ATLAS contains at least 2.3 molecules of nitrogen for every 100 molecules of carbon monoxide. This proportion is high compared with that of comets in our Solar System studied in the same way.
Why does this difference provide information about its past? Molecular nitrogen turns into gas very easily. For a large amount to remain trapped in ice during the birth of a comet, very cold conditions are therefore required. The authors thus link the composition of 3I/ATLAS to formation far from its parent star, in the icy regions of the planetary system where it was born.
This interpretation is consistent with other observations of 3I/ATLAS. Previous measurements of certain forms of hydrogen had already indicated that it probably formed at a temperature no higher than about -243 °C. Other analyses of nitrogen also pointed toward the outer regions of the disk of material surrounding its young star.
The observations also provide information about other gases. The tail contains traces associated with carbon dioxide, water and carbon-based compounds. Their abundance is more difficult to link directly to the original ices, because the chemical reactions taking place around the comet transform these substances after they are released.
It now remains to compare this signature with those of other interstellar visitors. Only three objects from another stellar system have been identified so far, and not all of them allowed their gases to be studied. Each new observable interstellar comet could therefore reveal the conditions under which small bodies form around other stars.