Only three objects from beyond our solar system have ever been caught passing through it. The first, 1I/’Oumuamua, arrived in 2017 looking like nothing astronomers had seen before and left more questions than answers. The second, 2I/Borisov, carried familiar chemistry and behaved like a comet from our own neighborhood. The third, 3I/ATLAS, is turning out to be something stranger still.
New observations using the Atacama Large Millimeter/submillimeter Array reveal that 3I/ATLAS carries an extraordinary amount of methanol relative to other molecules. The ratios measured place the comet among the most methanol-rich objects ever studied, including every comet that formed around our own Sun. Whatever star system built this object, it operated under conditions that left a very different chemical signature in the ice.
“Observing 3I/ATLAS is like taking a fingerprint from another solar system,” said Nathan Roth, lead author of the study published inThe Astrophysical Journal Letters 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.”
ALMA Tracked the Comet as the Sun Began to Warm It
Roth’s team used ALMA’s Atacama Compact Array, a cluster of smaller dishes within the larger observatory in Chile’s Atacama Desert, to observe 3I/ATLAS across multiple dates in late 2025. As the comet drew closer to the Sun, solar heating caused ices on its surface to sublimate directly into gas, creating a glowing envelope of material around the nucleus called a coma.
That coma became the measurement target. By reading the faint submillimeter signals it emitted, the team identified the chemical fingerprints of two organic molecules: methanol (CH₃OH) and hydrogen cyanide (HCN), a nitrogen-bearing compound commonly detected in solar system comets.
The numbers from two separate observing dates were striking. The team measured methanol-to-HCN production rate ratios of approximately 70 and 120. In comets that formed around our Sun, those ratios are far lower, and the methanol enrichment alone was enough to place 3I/ATLAS in a category apart from anything previously studied.
The Two Molecules Escape the Comet Through Different Routes
The ALMA data revealed something beyond the raw abundance: methanol and hydrogen cyanide do not leave the comet the same way. HCN appears to originate almost entirely from the solid nucleus, which is standard behavior for solar system comets.
Methanol, however, comes from two sources simultaneously: the nucleus and the icy dust grains suspended within the coma itself. Those grains act as secondary emitters. As sunlight warms them, the methanol ice they carry converts to gas and feeds the surrounding coma directly.
The process is called extended outgassing, and while it has been documented in some solar system comets before, the ALMA Observatory notes this is the first time it has been spatially mapped in an interstellar object. That distinction matters because it means the team could show where the methanol was coming from, not just that it was present.
A Second Instrument Had Already Flagged Something Unusual
The extreme methanol content is not the first chemical oddity tied to 3I/ATLAS. Earlier observations with the James Webb Space Telescope found that when the comet was still far from the Sun, its coma was dominated by carbon dioxide rather than water, which is the norm for solar system comets at comparable distances.
The ALMA data now add methanol to that unusual inventory, building a chemical picture that departs from familiar patterns at more than one point. Two separate instruments, looking at the same object under different conditions, have each returned results outside the range of what local comets typically show.
Methanol forms on the surfaces of cold interstellar dust grains through a process that adds hydrogen atoms to carbon monoxide at very low temperatures. The efficiency of that conversion depends on the temperature, radiation exposure, and elemental makeup of the environment where the grains first condensed. An outsized methanol fraction relative to molecules like HCN can indicate that the material spent time in an exceptionally cold or chemically distinct region of a protoplanetary disk, though the study stops short of identifying a specific origin.
What the Third Visitor Adds to a Very Short List
With only three confirmed interstellar objects on record, each one carries disproportionate scientific weight. ‘Oumuamua produced no detectable coma and accelerated in a way that water ice alone could not explain. Borisov showed chemistry close enough to solar system comets to suggest planet formation produces similar results in many places.
3I/ATLAS now complicates that picture. It arrives with a composition well outside the range of anything observed locally, showing that interstellar comets can also carry chemistry shaped by genuinely different conditions.
