Webb finds interstellar comet 3I/ATLAS carries dusty ice grains like those on icy worlds beyond Neptune

As of 8:42 a.m. ET, Wednesday, Oct. 28, 2026

Ice grains streaming from interstellar comet 3I/ATLAS look different from those in most comets studied so far but closely resemble the ice on some bodies beyond Neptune, according to a new study using NASA's James Webb Space Telescope.

The study, led by Silvia Protopapa of the Southwest Research Institute (SwRI), appears in The Astrophysical Journal Letters, SwRI announced. 3I/ATLAS is only the third object from outside the solar system ever identified.

"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 in the SwRI release. "Those grains hold clues to the conditions where the comet was born."

What Webb saw

The team observed the comet three times with Webb's Near-Infrared Spectrograph, before and after its closest approach to the Sun: on Aug. 6, 2025, Dec. 22, 2025, and April 1, 2026, according to the paper. The comet reached perihelion on Oct. 29, 2025.

Water ice usually leaves three marks in infrared light: a deep absorption band near 3 microns and two weaker bands near 1.5 and 2.0 microns. In 3I/ATLAS, the 3-micron band is strong, but the two shorter-wavelength bands are very weak or missing, unlike in most icy comets, SwRI said.

"That combination was a puzzle," Protopapa said.

The team's models favor grains about a micron or smaller, made of water ice mixed with dust. In such small, dusty grains, light travels only a short distance through the ice, which weakens the shorter-wavelength bands while leaving the 3-micron band prominent, according to the release.

"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."

By the final observation in April, the data also favored a second population of larger, ice-rich grains, still around a micron in size. Michael Kelley of the University of Maryland, the paper's second author, said the team cannot yet tell whether that change came from the Sun heating the grains differently or from new material exposed as the comet's surface eroded.

A match beyond Neptune

When the researchers compared the comet with Webb spectra of trans-Neptunian objects, icy bodies thought to preserve material from the early solar system, the shape of the 3-micron water-ice band resembled several groups of those objects. The closest resemblance was to objects whose spectra also show carbon dioxide ice and organic compounds, SwRI said. Like 3I/ATLAS, those objects lack strong 1.5- and 2.0-micron bands.

"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."

If confirmed, the shared range of grain properties could indicate that the physical form of water ice is set early in planet-forming disks and partly preserved in the icy bodies that form there, the release said. The authors caution in the paper that the surfaces of trans-Neptunian objects have also been changed over time by radiation, impacts and heat, and that the similarities do not mean the comet's home disk had the same composition as the early solar system.

Martin Cordiner of NASA's Goddard Space Flight Center and the Catholic University of America leads the Webb program and is a co-author. NASA has not issued its own release on this study. Its June release covered a separate Webb study of the comet's chemistry.