Comet 3I/ATLAS is packed with an unusually large amount of methanol, an organic molecule that could be considered a pre-cursor for life as we know it.
A study of the interstellar comet using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile reveals its chemical structure is unlike the vast majority of comets we find within our own Solar System.
As such, its slowly revealing what the conditions might have been like in the star system in which it formed, in a far-flung corner of our Galaxy.
More on 3I/ATLAS
A visitor from beyond the Solar System
Comet 3I/ATLAS was discovered on 1 July 2025 and was quickly identified as being an 'interstellar' comet, which means it originated beyond our Solar System.
The vast majority of comets we know about originated in the outskirts of our Solar System, and only three interstellar comets – including 3I/ATLAS – have ever been discovered.
3I/ATLAS is not in orbit around our Sun, but is instead passing through our Solar System and, once it leaves, will be gone from our view forever.
Since its discovery, astronomers have been pointing Earth's largest ground-based and space-based telescopes at it.
It's been observed by the Hubble Space Telescope, by the James Webb Space Telescope, by spacecraft and rovers at Mars and even the Juice spacecraft on its way to Jupiter.
ALMA's observations of 3I/ATLAS
The Atacama Large Millimeter/submillimeter Array is one of the most powerful telescopes on Earth, located under the dark skies of Chile's Atacama Desert.
It's operated by the European Southern Observatory, whose other Chile-based observatory, the Very Large Telescope, was one of the first to observe 3I/ATLAS after its discovery.
New research using ALMAS reveals 3I/ATLAS is full of an unusually large amount of the organic molecule methanol.
In fact, it contains much more methanol than almost all known comets in our own Solar System.
"Observing 3I/ATLAS is like taking a fingerprint from another solar system," says Nathan Roth, lead author of the study and a professor with 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 team behind the study used ALMA’s Atacama Compact Array on multiple dates in late 2025 to observe 3I/ATLAS as it approached the Sun.
This close-approach to the centre of the Solar System caused 3I/ATLAS to heat up, sunlight warming its frozen surface and releasing gas and dust out into space.
The effects of this were the formation of a glowing halo, or coma, around the core of the comet, a well-observed phenomena in Solar System comets that get too close to the Sun.
Astronomers can study light coming from the coma to reveal a comet's chemical fingerprints, and this team were able to decipher the material 3I/ATLAS is composed of.
That gave them the chance to study how comets might form in distant systems.
Here comes the science
The team studying ALMA's data on 3I/ATLAS focused on the faint fingerprints of two molecules: methanol, a type of alcohol, and hydrogen cyanide, an organic molecule commonly found in comets.
ALMA data shows 3I/ATLAS contains more methanol compared to hydrogen cyanide, beyond what's normally seen in comets that formed within our Solar System.
The team measured the ratio of methanol to hydrogen cyanide and found 3I/ATLAS to be among the most methanol‑rich comets ever seen.
That means, the team say, the icy material from 3I/ATLAS was either formed by or underwent entirely different conditions than those that shape comets in our own Solar System.
The James Webb Space Telescope had already found 3I/ATLAS to have a coma dominated by carbon dioxide when it was far from the Sun, but the ALMA results show that methanol is also present.
ALMA also revealed how different molecules move away from the comet, showing differences between methanol and hydrogen cyanide.
The team say hydrogen cyanide appears to mostly come from the comet’s core, and that's typical for comets in our Solar System.
Yet methanol appears to come from both the nucleus and from ice particles in the coma.
These ice particles turn into gas as the comet gets closer to the Sun and release methanol out into space.
This is the first time detailed outgassing has been traced in an interstellar object.
What's clear is that interstellar comets act as frozen time capsules from deep space, giving astronomers the chance to study conditions around distant stars without having to send spacecraft there.
And astronomers believe that, as more powerful observatories like the Vera Rubin Observatory begin to observe the Universe, we'll find many more interstellar comets.
That means we could be on the cusp of a new tool to understanding the complexities of the cosmos.
