An SwRI-led study used NASA’s James Webb Space Telescope to compare interstellar comet 3I/ATLAS with icy bodies in our solar system. The comparison offers clues to the early formation of planetary building blocks. At top right, both 3I/ATLAS and the icy solar system comet C/2024 E1 show a strong water-ice absorption band near 3 microns, but the interstellar comet has very weak or missing bands near 1.5 and 2.0 microns. Its spectrum more closely resembles the average spectrum of a class of icy worlds beyond Neptune whose spectra also show carbon dioxide ice and organics (bottom right). The artist’s concept (left, not to scale) compares the relatively pure ice grains with separate dust, as interpreted for most solar system comets, with the dust-rich ice aggregates the study inferred for 3I/ATLAS. Both grain types are around a micron or smaller.
Image Credits: SwRI/Silvia Protopapa.
Spectra: NASA/ESA/CSA JWST. Coma and grains: artist’s concept.
Scientific Frontline: Extended "At a Glance" Summary: Interstellar Comet 3I/ATLAS
The Core Concept: A Southwest Research Institute-led study utilized NASA's James Webb Space Telescope to analyze ice grains streaming from interstellar comet 3I/ATLAS, revealing tiny aggregates of water ice and dust about a micron or smaller.
Key Distinction/Mechanism: Unlike most previously studied comets in our solar system, which show relatively pure ice grains, 3I/ATLAS features grains with significant dust mixed into the ice. This composition causes a strong 3-micron absorption band in infrared light but weak or missing 1.5- and 2.0-micron bands.
Origin/History: 3I/ATLAS is the third object identified from outside our solar system. The Webb observations were conducted between August 2025 and April 2026 as the comet approached and retreated from the Sun.
Major Frameworks/Components:
- Spectral analysis using Webb's Near-Infrared Spectrograph to identify water ice "fingerprints" (absorption bands).
- Comparison of light scattering models for grains of varying sizes and compositions.
- Comparison of 3I/ATLAS spectra with trans-Neptunian objects to identify shared grain properties across protoplanetary disks.
Branch of Science: Astronomy, Planetary Science.
Future Application: Continued Webb observations of both solar system comets and future interstellar objects to verify if the properties of 3I/ATLAS represent a typical standard for icy bodies formed in other star systems.
Why It Matters: The resemblance of 3I/ATLAS's ice grains to those of some trans-Neptunian objects suggests that the icy building blocks of planets, regardless of their star system of origin, share a common range of physical properties established early in planet-forming disks.
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 about 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 the 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. The 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, weakening the 1.5- and 2.0-micron bands while leaving the 3-micron band prominent. Ice grains in other comets are similarly small, but they are thought to be relatively pure, perhaps with a small amount of dust.
"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 properties of the grains in the coma also appeared to change over time. By the final observation in April 2026, as the comet was moving away from the Sun, the data favored an additional population of ice-rich grains, larger than those inferred earlier but still around a micron in size.
"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 Dr. Michael Kelley of the University of Maryland, second author of the paper. "Both are possible, and telling them apart is one of the open questions we aim to address."
The team then compared 3I/ATLAS with Webb spectra of trans-Neptunian objects, icy bodies orbiting beyond Neptune and thought to preserve material from the early solar system. The shape of the 3-micron band in 3I/ATLAS resembles that seen in several groups of these objects, especially those whose spectra also show clear signs of carbon dioxide ice and organic compounds. The resemblance lies in the water-ice band, not in the carbon dioxide. And like 3I/ATLAS, these objects lack strong 1.5- and 2.0-micron water-ice 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. This suggests that the icy building blocks of planets, here and around other stars, may share ice in the form of tiny, micron-scale grains, ranging from relatively pure ice to ice mixed with dust, with 3I/ATLAS toward the dusty end."
If confirmed, this shared range of grain properties could indicate that the physical form of water ice is established early, in planet-forming disks, and is at least partly preserved in the icy bodies that form there.
Future Webb observations of comets in our solar system and of newly discovered interstellar visitors will help test these interpretations and determine whether 3I/ATLAS is typical.
Published in journal: The Astrophysical Journal Letters
Authors: Silvia Protopapa, Michael S. P. Kelley, Martin A. Cordiner, Stefanie N. Milam, Dennis Bodewits, Steven B. Charnley, Maria N. Drozdovskaya, Sara Faggi, Davide Farnocchia, Aurélie Guilbert-Lepoutre, Marco Micheli, Nathan X. Roth, Megan E. Schwamb, Darryl Z. Seligman, and Geronimo L. Villanueva
Source/Credit: Southwest Research Institute
Edited by: Scientific Frontline
Reference Number: astr102626_01
