Astronomers caught something unusual about the ice of a comet that came from another star system.
A Southwest Research Institute team pointed NASA‘s James Webb Space Telescope at interstellar comet 3I/ATLAS three times between Aug. 2025 and April 2026, catching it before and after its closest pass by the sun.
The space telescope spreads a comet’s light into a rainbow, including infrared light that human eyes can’t see. Water ice soaks up light at a few specific spots in the rainbow. That pattern makes it possible to detect water ice the way a fingerprint can identify a person.
Ice normally leaves one deep mark and two faint ones, and most comets around our sun show all three. But in 3I/ATLAS, the two faint marks barely registered, if they showed up at all. Researchers ran computer simulations and found that light scatters off comet grains in a similar way if tiny bits of ice are blended with more dust. Because the dust blocks light before it penetrates far through the ice, the lighter two marks tend to vanish.
“At first glance, 3I/ATLAS looks different from the icy comets we know,” said Silvia Protopapa, lead author of a new paper in The Astrophysical Journal Letters, in a statement. “But when we placed it alongside distant icy worlds in our own solar system, it no longer looked out of place.”
While the alien comet’s dusty ice looked strange compared to the form usually seen in solar system comets, the team noticed that its makeup actually lines up with ice on some frozen worlds far past Neptune. That suggests ice may form in similar ways around different stars, as tiny grains that range from nearly pure to heavily dusty. If so, the raw materials for building planets may look much alike from one star to the next.
Comets like this one start out in planet-forming disks — the flat, spinning clouds of gas and dust that surround young stars. As worlds assemble, gravity can hurl leftover icy chunks clear out of a system. That may explain how some interstellar objects have lost their way from home.
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Astronomers have caught just three passing through our solar system. Telescopes never detected a glowing halo of ice turning into gas, known as a coma, around the first, ‘Oumuamua, causing many to later doubt that it was a comet. But both 2I/Borisov and 3I/ATLAS showed signs of a coma.
The team compared 3I/ATLAS’s halo to existing Webb data on Trans-Neptunian objects, icy bodies that circle the sun beyond Neptune. Many scientists view these distant worlds as time capsules from our solar system’s infancy.
Several groups of them share the comet’s pattern: a strong water-ice signature, with the two faint ones barely detected or missing altogether. The best matches also carry carbon dioxide ice and carbon-rich compounds.
Credit: Southwest Research Institute / Silvia Protopapa infographic
“The icy building blocks 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,” Protopapa said, “with 3I/ATLAS toward the dusty end.”
If more evidence backs up this notion, a young star’s disk may decide early on what form its ice will take, the researchers said. Comets and other icy bodies that grow there would carry at least some of that original material with them.
By April, as 3I/ATLAS raced away from the sun, Webb’s readings pointed to a second crop of ice-rich grains. They ran slightly larger than the earlier ones but still measured roughly a micron — one-millionth of a meter and far thinner than a human hair.
“We can’t yet tell whether the changes over time are the result of the sun heating the grains differently at each observation, or of new material exposed as the comet’s surface eroded near the sun,” said Michael Kelley, a co-author of the paper, in a statement. “Both are possible, and telling them apart is one of the open questions we aim to address.”