Interstellar Comet 3I/ATLAS Has Dusty Ice Like Worlds Beyond Neptune, Webb Finds

The interstellar comet 3I/ATLAS is long gone from our skies, but the data it left behind are still producing surprises. A new study using NASA’s James Webb Space Telescope has found that the comet’s water ice is not clean and bright like the ice in most comets from our own Solar System. Instead, it is made of tiny grains heavily mixed with dark, rocky dust, much like the ice seen on some frozen worlds beyond Neptune. The result suggests that the icy building blocks of planets around other stars may follow the same basic rules as those around the Sun.

The findings, led by Silvia Protopapa of the Southwest Research Institute (SwRI), were published in The Astrophysical Journal Letters and are now being widely discussed as one of the most detailed looks yet at material from another planetary system.

What is 3I/ATLAS?

3I/ATLAS was discovered on 1 July 2025 by the ATLAS survey telescope in Río Hurtado, Chile. Astronomers quickly realised that its orbit was not closed: it was moving far too fast to be bound to the Sun, more than 200,000 km/h, on a path that came from interstellar space in the direction of Sagittarius. It became only the third interstellar object ever confirmed, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019; the “3I” in its name stands for “third interstellar”.

Unlike ʻOumuamua, which showed no visible coma, 3I/ATLAS behaved like an active comet, releasing gas and dust as it warmed. It passed closest to the Sun on 29 October 2025, at about 1.36 times Earth’s distance from the Sun, and is now heading back out of the Solar System, never to return. Some studies suggest it may have formed billions of years before the Sun, making it potentially one of the oldest objects ever studied up close.

What did Webb find?

Webb’s Near-Infrared Spectrograph (NIRSpec) observed 3I/ATLAS three times over eight months: on 6 August 2025, when the comet was inbound about 3.3 times Earth’s distance from the Sun; on 22 December 2025, after its closest approach; and on 1 April 2026, as it receded to about 5.7 times Earth’s distance. Each time, the telescope split the comet’s infrared light into a spectrum, revealing the fingerprints of the materials in its glowing cloud, or coma.

All three spectra showed a strong water-ice absorption band near 3 micrometres. But two weaker bands near 1.5 and 2.0 micrometres, which are normally obvious in icy comets, were faint or missing. The best explanation, according to the team’s models, is that the ice is locked up in very small grains, around a micrometre across or smaller, each one a blend of water ice and dark, non-volatile “refractory” material such as rock and organic compounds.

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

The spectra also showed gas from water, carbon dioxide and carbon monoxide. Earlier Webb studies had already found that the comet’s coma is unusually rich in carbon dioxide compared with water, one of the highest ratios measured for any comet.

A link to worlds beyond Neptune

The most striking result is the comparison. The spectrum of 3I/ATLAS does not look like the average comet from our Solar System. It looks much more like the average spectrum of a class of icy trans-Neptunian objects: small frozen bodies in the Kuiper Belt beyond Neptune, which also show carbon dioxide ice and organic material on their surfaces.

The authors propose that icy planetesimals, the building blocks of planets, may share a continuum of grain properties across different planetary systems, from nearly pure ice at one end to ice heavily mixed with dust at the other. Comets from our own Solar System tend to sit nearer the clean-ice end; 3I/ATLAS sits at the dusty end. If that is right, the processes that build comets and planets around other stars may be broadly similar to those that worked here 4.6 billion years ago. The team describes the conclusion as tentative and in need of confirmation from future interstellar visitors.

Changes over eight months

The comet did not look exactly the same each time. The April 2026 spectrum hinted at a second population of slightly larger, more ice-rich grains, and a subtle feature that may indicate crystalline ice. Co-author Michael Kelley of the University of Maryland noted that it is not yet clear whether the changes came from uneven heating by the Sun or from fresh material exposed as the comet’s surface eroded. Separating those effects is one of the next goals of the analysis.

Why 3I/ATLAS matters

Interstellar comets are, in effect, free samples from other solar systems delivered to our doorstep. Every star system forms from a disc of gas and dust, and the leftovers are frozen into comets, many of which are later flung out into the galaxy. By studying one up close, astronomers can compare the chemistry of another planetary system with our own without travelling light-years to get there.

3I/ATLAS is by far the best-studied of the three interstellar objects. It was observed by Webb, Hubble and dozens of ground-based telescopes, as well as spacecraft around Mars and elsewhere in the Solar System. Researchers reported methane in its coma, the first time the gas has been confirmed in an interstellar object, and its chemistry has been compared with that of comets from both the inner and outer Solar System.

Will we see more interstellar comets?

Almost certainly. The new Vera C. Rubin Observatory in Chile, which began its survey of the southern sky in 2025, is expected to discover interstellar objects far more often than before, possibly several a year. Astronomers are already preparing to observe the next one quickly, ideally while it is still on its way in. Some have proposed spacecraft that could wait in space and intercept a future interstellar visitor.

In the meantime, you can see which comets from our own Solar System are within reach of binoculars and small telescopes on our comets visible tonight page, and find out more about the telescope behind this discovery on our James Webb Space Telescope tracker. If you are new to Webb, our guide why JWST orbits at L2 explains how it sees so deep into the infrared. Upcoming sky events are listed on our space events page.

Sources: Protopapa et al., “JWST Reveals Refractory-rich Water Ice in Interstellar Comet 3I/ATLAS”, ApJL 1011, L11; Sci.News: Webb finds dust-rich water ice in interstellar comet 3I/ATLAS; Tech Times: JWST detects dusty water ice in 3I/ATLAS.