Astronomers studying the rare interstellar visitor 3I/ATLAS have discovered that the comet is unusually rich in methanol, a type of alcohol, offering a unique chemical snapshot of how planets may form in distant star systems.
The object, only the third confirmed interstellar comet detected passing through our Solar System, was analysed using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
The observations revealed that the comet contains extraordinarily high levels of methanol compared with hydrogen cyanide, a pattern rarely seen in comets formed within our own Solar System.
“Observing 3I/ATLAS is like taking a fingerprint from another solar system,” said Nathan Roth, the study’s lead author and a professor at American University. “The details reveal what it’s made of, and it’s bursting with methanol in a way we just don’t usually see in comets in our own Solar System.”
The research team used ALMA’s Morita Array, also known as the Atacama Compact Array, to observe the comet multiple times in late 2025 as it moved closer to the Sun. As sunlight heated the icy surface of the comet, frozen material began to vaporise, releasing gas and dust that formed a glowing halo called a coma.
By analysing faint submillimeter signals from molecules within this coma, astronomers identified chemical signatures of two important compounds: methanol (CH3OH) and hydrogen cyanide (HCN). The results were striking.
On two observation dates, scientists measured methanol-to-HCN ratios of around 70 and 120, placing 3I/ATLAS among the most methanol-rich comets ever studied.
Earlier observations with the James Webb Space Telescope had already shown that the comet’s coma contained large amounts of carbon dioxide. The new findings add methanol as another major component of its unusual chemical makeup.
ALMA’s high-resolution imaging also revealed how these molecules escape from the comet. Hydrogen cyanide appears to originate mainly from the comet’s nucleus, typical behaviour for most Solar System comets.
Methanol, however, seems to be released both from the nucleus and from tiny icy grains drifting within the coma.
These grains act like miniature comets themselves: as sunlight warms them, the ice they contain vaporises, releasing methanol gas into space.
Comet 3I/ATLAS follows two other interstellar visitors detected in recent years, 1I/‘Oumuamua and 2I/Borisov.
Scientists say studying such rare objects provides valuable clues about the chemical conditions and planet-forming processes occurring in planetary systems across the Milky Way galaxy.
