This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A Southwest Research Institute–led study used NASA's James Webb Space Telescope to examine ice grains streaming from interstellar comet 3I/ATLAS, just the third object from beyond our solar system ever identified. The observations point to tiny aggregates of water ice and dust, about a micron or smaller.
The grains' spectral characteristics differ from those of ice grains in most previously studied comets but resemble the spectra of some icy worlds beyond Neptune. The results suggest that the icy building blocks of planets around other stars may share a common range of physical properties with those in our own solar system, with tiny grains ranging in composition from relatively pure ice to ice mixed with dust. "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," said SwRI's Dr.
Silvia Protopapa, lead author of a new paper on this research in The Astrophysical Journal Letters. "As 3I/ATLAS warms up, gas escaping from its interior carries ice and dust grains into the surrounding cloud, or coma. Those grains hold clues to the conditions where the comet was born." As part of a Webb program dedicated to studying interstellar objects, the team observed 3I/ATLAS with Webb's Near-Infrared Spectrograph three times between August 2025 and April 2026, before and after the comet's closest approach to the sun.
The instrument uses a prism to split light into its component wavelengths, revealing the composition and physical state of the material it observes. "We designed this program to be ready the moment an active interstellar comet was found and to follow it over time," said Dr. Martin Cordiner of NASA's Goddard Space Flight Center and the Catholic University of America.
"Our goal was to characterize the ice, dust and gas that the comet releases, and so learn about the chemistry and formation of planetesimals—the building blocks of planets—around other stars." Cordiner is principal investigator of the Webb program and a co-author of the paper. Water ice leaves distinct "fingerprints" in infrared light—a deep dip, or 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 1.5- and 2.0-micron bands are very weak or missing, unlike in most icy comets, which show all three.
"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 team compared the spectra with detailed models of how light scatters off grains of different sizes and compositions.
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