Webb Telescope Finds Interstellar Comet's Ice Is Unusually Dusty, Hinting at a Shared Recipe for Planet-Building Blocks

Astronomers studying the interstellar comet 3I/ATLAS with the James Webb Space Telescope have found that its water ice is unusually rich in rocky dust, a signature that differs from comets native to our solar system but resembles icy bodies found far beyond Neptune. The finding, published October 26 in The Astrophysical Journal Letters, adds a new data point to scientists' efforts to understand how ice forms around other stars, not just our own.

The study was led by Silvia Protopapa of the Southwest Research Institute, working with a 15-member team that included researchers from NASA's Goddard Space Flight Center, the University of Maryland, and several European institutions. The team drew on three rounds of observations by Webb's Near-Infrared Spectrograph, taken on August 6, 2025, December 22, 2025, and April 1, 2026, spanning the comet's closest approach to the sun on October 29, 2025. The full dataset and modeling details are available in the team's preprint on arXiv.

What the spectra showed

Comets are built largely from ice that froze out of the gas and dust disk around a forming star, and their infrared spectra carry a kind of fingerprint of that ice's physical state. In solar-system comets, water ice typically produces three absorption features in Webb's instrument range, near 1.5, 2.0, and 3 microns. 3I/ATLAS showed a strong, broad absorption near 3 microns, but the two shorter-wavelength bands were weak or missing entirely, alongside separate gas emission lines from water, carbon dioxide, and carbon monoxide escaping the comet's nucleus.

That pattern is the spectral calling card of ice grains that are extremely small, at or below roughly a micron across, each one blended intimately with dark, non-icy "refractory" material rather than existing as separate clumps of pure ice and dust. The researchers modeled several grain configurations and found the dust-blended, submicron-to-micron picture fit best. Critically, the grain sizes themselves were not unusual: solar-system comets have similar-sized ice grains. What set 3I/ATLAS apart, according to the paper, was simply how much dust was mixed directly into those grains.

"What seems to set them apart is how much dust is mixed into the ice," said Silvia Protopapa, the study's lead author, as quoted in coverage of the findings.

The team also found the spectrum was not static. By the final observation in April 2026, roughly six months after perihelion, the data hinted at a second, separate population of larger, more ice-rich grains, along with a faint feature near 3.1 microns consistent with crystalline ice. Co-author Michael Kelley of the University of Maryland noted that it is not yet possible to say whether that shift reflects the sun heating different grain populations differently as the comet receded, or some other process altering the coma over time.

Why a dusty comet matters

3I/ATLAS is only the third interstellar object ever confirmed passing through the solar system, following 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. The NASA-funded ATLAS survey telescope in Chile first spotted it on July 1, 2025, and earlier images from the Zwicky Transient Facility later extended its observed path, revealing a strongly hyperbolic trajectory that confirmed it came from outside the solar system. NASA has put its speed at roughly 137,000 miles per hour at discovery, rising to about 153,000 miles per hour at its closest approach to the sun, and estimated its nucleus at somewhere between 1,400 feet and 3.5 miles across based on Hubble observations. Because it formed around a different star entirely, long before arriving in our neighborhood, its composition offers a rare direct sample of how planetary building blocks assemble elsewhere in the galaxy.

Solar-system comets are thought to have formed from ice grains that condensed relatively cleanly, with dust and ice segregating into separate clumps as they coalesced in the young sun's disk. Icy bodies in the Kuiper Belt and other distant, cold reaches of the outer solar system, by contrast, often show spectra more like what Webb found at 3I/ATLAS: heavily dust-infused ice rather than cleanly separated grains. The new paper's authors argue that rather than treating "solar-system-like" and "exotic" ice as two separate categories, it may be more accurate to think of a continuum of grain properties that spans many protoplanetary disks, with 3I/ATLAS sitting toward the dust-rich end of that spectrum.

That reframing matters for planetary science more broadly. If icy planetesimals across different star systems share a common range of physical states rather than falling into sharply distinct solar-system versus interstellar categories, it suggests the basic physics and chemistry of ice formation in young, dusty disks may operate similarly around many stars, with local conditions, such as a disk's dust content or a particular region's temperature history, pushing the outcome toward one end of the spectrum or the other.

Who is affected and what comes next

The immediate audience for the finding is the community of planetary and cometary scientists who have been racing to extract as much information as possible from 3I/ATLAS before it recedes permanently out of range; the comet is now headed back out of the solar system and will not return. Mission teams for NASA's and ESA's other solar-system-watching instruments, including Hubble and ground-based observatories that have tracked the comet since its discovery, will continue to fold their own data into the broader picture building up around the object.

The paper's authors say the clearest next step is more modeling work to pin down what drove the apparent change in grain population between the December 2025 and April 2026 observations, and whether similar shifts would be detectable if astronomers get the chance to study a future interstellar visitor. Given that only three such objects have ever been confirmed, each new one remains a rare, non-repeatable opportunity, underscoring why researchers moved quickly to analyze and publish the Webb data as the comet was still observable.