Interstellar comet 3I/ATLAS has ice like worlds beyond Neptune, Webb finds

The ice streaming off interstellar comet 3I/ATLAS does not look like the ice of the comets we know. James Webb Space Telescope spectra show a strong water-ice signal with two of its usual companion signals almost gone, and that pattern matches some of the icy worlds that orbit far beyond Neptune.

The match matters because 3I/ATLAS was born around another star. If its ice grains share their form with material preserved at the edge of our own solar system, the raw ingredients of planets may come out of planet-forming disks in a similar state across the galaxy. The authors offer that as a possibility to test, not a settled result.

The team, led by Silvia Protopapa of the Southwest Research Institute in Texas, found that the comet’s coma, the cloud of gas and dust around its nucleus, is best explained by tiny grains of water ice mixed with dust. Each grain is about a micron across or smaller, less than a fiftieth of the width of a human hair.

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“Interstellar comets are small icy bodies that formed in the disks of gas and dust around other stars and are now passing through our solar system,” Protopapa said. Only two such objects were known before this one: ‘Oumuamua, which showed no visible coma, and the comet 2I/Borisov.

Two ice signals that went missing

Water ice leaves fingerprints in infrared light. The deepest is an absorption band near 3 microns. Two weaker bands sit near 1.5 and 2.0 microns, and most icy comets show all three. In 3I/ATLAS the 3-micron band is strong, cutting the reflected light by 77% at its deepest point in the first Webb visit and by 94% in the last. The two shorter bands are very weak or missing.

“That combination was a puzzle,” Protopapa said. “The strong 3-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 explanation is path length. The weak bands only show up when light travels a long way through ice, the way a thin sliver of tinted glass looks almost clear while a thick block shows its colour. In grains this small and this dusty, light crosses very little ice before it scatters back out. The weak bands fade, and the strong one survives.

How Webb read the comet three times

“We designed this program to be ready the moment an active interstellar comet was found and to follow it over time,” said Martin Cordiner of NASA’s Goddard Space Flight Center and the Catholic University of America, the program’s principal investigator.

Webb pointed its Near-Infrared Spectrograph at the comet three times over eight months, before and after its closest pass to the Sun. In prism mode it records 0.6 to 5.3 microns at once, so dust, ice and escaping gas land in a single spectrum.

Only the first and third visits were modelled. The middle one, taken nearest the Sun, was contaminated by glowing water vapour, organic molecules, nickel atoms and warm dust.

The team compared the spectra with models of light scattering off grains of different sizes and recipes. Pure ice grains, and ice mixed with carbon or olivine, either overpredicted the weak bands or failed to reproduce the strong one. The best fit was ice blended with pyroxene, a silicate mineral common in rocks and meteorites. By the last visit, as the comet headed away from the Sun, the data also required a second population of larger, ice-rich grains still around a micron in size, plus a faint feature near 3.1 microns consistent with crystalline ice.

Unlike our comets, like the worlds past Neptune

Grain size is not what sets 3I/ATLAS apart. Comet Hartley 2 sheds grains of nearly pure ice of similar size, and comet Catalina’s icy grains were modelled with at most 1% dust. The interstellar comet sits at the dusty end of that line.

“The grains of 3I/ATLAS are about the same size as those in our own comets,” Protopapa said. “What seems to set them apart is how much dust is mixed into the ice.”

The surprise came from the trans-Neptunian objects, icy bodies that orbit beyond Neptune and are thought to preserve material from the solar system’s birth. Of the spectral classes defined by Webb’s DiSCo survey of mid-sized ones, the 3-micron band of 3I/ATLAS looks most like the class rich in carbon dioxide ice, and those objects also lack strong 1.5 and 2.0 micron bands. The resemblance lies in the water-ice band, not in the carbon dioxide.

“At first glance, 3I/ATLAS looks different from the icy comets we know,” Protopapa said. “But when we placed it alongside distant icy worlds in our own solar system, it no longer looked out of place.”

The comparison runs the other way, too. For those classes of distant objects, the 3-micron dip had mostly been attributed to organic compounds and methanol, because the weaker water bands were absent. The comet shows that dusty water-ice grains can produce the same shape on their own, so some of that signal may be water ice after all.

What one comet cannot settle

The result rests on laboratory data. The model needs measured optical properties of water ice, and two standard datasets give noticeably different band shapes. The authors cannot say whether the ice was crystalline at both visits or turned from a disordered, amorphous form into crystals, and they call for new laboratory measurements of ice across temperatures and phases.

The change between visits also has two explanations. “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 of the University of Maryland, the paper’s second author.

Grains in a coma are not a direct sample of the nucleus either, and may be fragments of larger clumps. The surfaces of trans-Neptunian objects have been reworked for billions of years by radiation and impacts, so the authors warn against reading the match as proof of identical birthplaces. Other differences in its chemistry suggest the disk that made 3I/ATLAS was not a copy of ours. An earlier analysis had instead compared the comet with a reddish, organics-rich class of distant objects, so the field has not converged.

Then there is the sample size. This is one interstellar comet, seen three times.

Common questions about 3I/ATLAS and its ice

What is 3I/ATLAS?

It is the third confirmed object to enter our solar system from interstellar space, and a true comet with a coma and tails. Its open, hyperbolic path means it will never return. It passed between the orbits of Earth and Mars at its closest to the Sun and never came near Earth.

Does 3I/ATLAS contain water?

Yes. Webb has detected water vapour and water ice in its coma. It is also unusually rich in carbon dioxide: in Webb’s first look the coma held about eight times more carbon dioxide than water, one of the highest ratios measured in a comet.

What are trans-Neptunian objects?

They are icy bodies orbiting the Sun beyond Neptune, more than about 30 times the Earth-Sun distance, a population that includes Pluto. Kept cold for billions of years, they preserve clues to how the planets formed.

How old is 3I/ATLAS?

Nobody knows. From its speed and direction, one study estimated it is between 7.6 and 14 billion years old if it came from the Milky Way’s thick disk of old stars, which would make it older than the 4.6-billion-year-old solar system. That estimate is statistical, not a measurement.

What comes next

The comet was discovered on 1 July 2025 by the ATLAS survey telescope at Río Hurtado, Chile. Webb observed it on 6 August 2025, 22 December 2025 and 1 April 2026, at 3.3, 2.4 and 5.7 times the Earth-Sun distance from the Sun. Its closest approach to the Sun came on 29 October 2025, at 1.36 times that distance. The paper appeared in The Astrophysical Journal Letters on 26 October 2026, signed by 15 researchers from the United States and Europe.

More Webb observations of solar system comets and new interstellar visitors will test whether 3I/ATLAS is typical. The Vera C. Rubin Observatory, whose ten-year sky survey began in June 2026, is expected to find more of them. Tomorrow marks one year since 3I/ATLAS rounded the Sun, and it is already on its way back out of the solar system. The ice it carries looks less foreign than it first seemed.

Reference: Protopapa et al., “JWST Reveals Refractory-rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks,” The Astrophysical Journal Letters, 2026. DOI: 10.3847/2041-8213/aea60d

Tags: astronomy, James Webb Space Telescope, trans-neptunian objects, 3I/ATLAS, comets, interstellar objects

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