Using the James Webb Space Telescope (JWST), scientists have found that the interstellar comet 3I/ATLAS was built from tiny grains of ice mixed with dust that are quite different from the solar system’s comets.
These ice grains do, however, bear striking similarities to some icy bodies located at the edge of the solar system, beyond the orbit of Neptune, so-called trans-Neptunian objects (TNOs).
3I/ATLAS was discovered entering the solar system in the summer of 2025. It was only the third object found entering the solar system from farther afield in the Milky Way after 1I/’Oumuamua, seen in 2017, and 2I/Borisov, detected in 2019. This meant the interstellar comet offered a rare chance to glimpse the conditions under which such bodies form in other star systems. Thus, the JWST was quickly on the trail of 3I/ATLAS.
“We designed [JWST] to be ready the moment an active interstellar comet was found and to follow it over time,” Martin Cordiner of NASA’s Goddard Space Flight Center and the Catholic University of America said in a statement.
Cordiner leads the JWST program to study objects such as 3I/ATLAS, and he is a co-author on a new research paper describing some of the findings.
The unclear origins of 3I/ATLAS
Researchers know that 3I/ATLAS wasn’t born in the solar system, but nobody knows around which star 3I/ATLAS formed. What scientists are more sure of is the fact that this interstellar comet was produced within a planet-forming disk of gas and dust around a young star, possibly as long ago as 10 to 12 billion years. That’s over twice the age of our solar system. It was then ejected from its home system somehow and left to wander interstellar space before encountering our solar system.
Like the familiar comets in our solar system, 3I/ATLAS is composed of grains of ice less than a micron – a millionth of a meter – in size. When the surface of the comet is warmed by the sun, gas erupts outwards, carrying ice grains into the cloud of ice and dust surrounding the comet, its coma, and its tail, where JWST can detect them.
When viewed spectroscopically, the water-ice in comets native to our solar system creates a strong absorption line in a comet’s spectrum at a wavelength of three microns, as well as less-prominent absorption lines at 1.5 and two microns.
Yet when the JWST’s Near-Infrared spectrograph (NIRSpec) instrument viewed 3I/ATLAS three times between August 2025, before its closest approach to the sun, and after closest approach in April 2026, scientists found the 1.5- and two micron absorption lines were missing, though the three-micron line remained strong.
“That combination was a puzzle,” research lead author Silvia Protopapa of the South-west Research Institute said. “The strong three-micron band points to water-ice in the coma, but the weak or absent shorter-wavelength bands suggest the ice is not in the form we usually see in solar system comets.”
The JWST’s muddy 3I/ATLAS findings
Whenever presented with an unusual observational signature such the JWST delievered for 3I/ATLAS researchers turn to modeling to find the physical characteristics that best fit the spectrum.
The modeling suggested to Protopapa’s team that 3I/ATLAS was composed of sub-micron grains of dirty water-ice, rich in dust. In comparison, the grains from the solar system’s indigenous comets are much cleaner, with almost pure water-ice relatively uncontaminated by dust.
“What seems to set [3I/ATLAS’ grains] apart is how much dust is mixed into the ice,” said Protopapa. “Those grains hold clues to the conditions where the comet was born.”
Comets are made of primordial material found in the planet-forming disk of gas and dust that surrounds a young star, and therefore the composition of comets can provide a window into the conditions in which planets form.
Whereas solar system comets tell us about the birth of Earth, an interstellar comet can tell us about conditions in which planets formed around another star, and how similar or dissimilar those conditions were to the birth of the solar system.
Yet comets are not the only icy remnants that date back to such primordial times. There are various families of bodies in the outer solar system, and 3I/ATLAS’ spectrum bears a resemblance to the spectrum of some medium-sized TNOs that are rich in carbon-dioxide ice and organic compounds (organic meaning containing carbon atoms). Indeed, in a separate 2025 study led by Cordiner, the JWST found 3I/ATLAS to have one of the greatest abundances of carbon dioxide ever seen in a comet.
TNOs are sizable bodies that are located far beyond the orbit of Neptune, The TNOs in question would have formed in a different part of the Sun’s protoplanetary disk than most comets and their composition reflects that.
On the other hand, the results imply that 3I/ATLAS was formed around its star of origin from similar icy material to that which produced some of the TNOs in our solar system, showing that the building blocks of planets around other stars share a common range of properties with the building blocks that formed our solar system. Indeed, 3I/ATLAS could even be a fragment of a TNO-like body that was shattered in a collision.
“At first glance, 3I/ATLAS looks different from the icy comets we know,” said Protopapa. “But when we placed it alongside distant icy worlds in our own solar system, it no longer looked out of place.
“This suggests that the icy building block of planets, here and around other stars, may share ice in the form of tiny, micron-scale grains, ranging from relatively pure ice to ice mixed with dust, with 3I/ATLAS toward the dusty end.”
What this tells us is that these sub-micron-sized icy grains must form first in planet-forming disks, to then be incorporated into large bodies such as comets and protoplanets.
The team’s findings were published on Monday (Oct. 26) in The Astrophysical Journal Letters.