Interstellar comets are small icy bodies that formed in the disks of gas and dust surrounding other stars and are now passing through our solar system on fleeting visits. When 3I/ATLAS warmed as it approached the Sun, gas escaping from its interior carried ice and dust grains into the surrounding cloud, or coma, that envelops the nucleus. Those grains, suspended in the coma and illuminated by sunlight, hold chemical and physical clues to the conditions in the disk where the comet was born, potentially around a star very different from our own. Because the comet is only the third interstellar visitor ever detected, every measurement of its composition offers a rare window into planet formation elsewhere in the galaxy.

The observations were made as part of a Webb program dedicated to studying interstellar objects, led by Dr. Martin Cordiner of NASA’s Goddard Space Flight Center and the Catholic University of America. The team observed 3I/ATLAS with Webb’s Near-Infrared Spectrograph three times between August 2025 and April 2026, capturing the comet both before and after its closest approach to the Sun. The instrument works by passing the comet’s light through a prism that splits it into its component wavelengths, producing a spectrum that reveals the composition and physical state of the material in the coma. “We designed this program to be ready the moment an active interstellar comet was found and to follow it over time,” Cordiner said. “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.”

The key to the analysis lies in the distinctive fingerprints that water ice leaves in infrared light. Water ice produces a deep dip, or absorption band, near 3 microns, along with two weaker bands near 1.5 and 2.0 microns. In most icy comets studied in our solar system, all three bands appear together. In 3I/ATLAS, however, the 3-micron band is strong while the 1.5- and 2.0-micron bands are very weak or entirely missing. “That combination was a puzzle,” said Dr. Silvia Protopapa of the Southwest Research Institute, lead author of the new paper. “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.”

To resolve the puzzle, the team compared the measured spectra with detailed models of how light scatters off grains of different sizes and compositions. The models favored grains about a micron or smaller, composed of water ice mixed with dust. The physics of light scattering explains why this combination produces the unusual spectral shape: in such small, dusty grains, light travels only a short distance through the ice before being scattered or absorbed, which weakens the 1.5- and 2.0-micron bands while leaving the deeper 3-micron band prominent. Ice grains in other comets are similarly small in size, but they are thought to be relatively pure, containing perhaps only 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 the course of the observing campaign. By the final observation in April 2026, as the comet was receding from the Sun, the data favored an additional population of ice-rich grains, larger than those inferred in the earlier observations but still around a micron in size. The cause of this evolution remains an open question. “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 comparison that gives the finding its broader significance came next. The team placed the 3I/ATLAS spectrum alongside Webb spectra of trans-Neptunian objects, the icy bodies orbiting beyond Neptune that are thought to preserve material from the early solar system in nearly pristine form. The shape of the 3-micron water-ice band in 3I/ATLAS resembles that seen in several groups of these distant objects, especially those whose spectra also show clear signs of carbon dioxide ice and organic compounds. The resemblance lies specifically in the water-ice band, not in the carbon dioxide features. Like 3I/ATLAS, these trans-Neptunian objects also lack strong 1.5- and 2.0-micron water-ice bands, reinforcing the parallel between the interstellar comet’s grains and the icy surfaces of our solar system’s most distant small bodies.

“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.” In this picture, most solar system comets occupy the purer end of the continuum, while 3I/ATLAS sits toward the dust-rich extreme, and some trans-Neptunian objects fall along the same spectrum of grain properties. If confirmed by further observations, this shared range would indicate that the physical form of water ice is established early, within the planet-forming disks around young stars, and is at least partly preserved in the icy bodies that subsequently form there — a memory of the birth environment carried across interstellar space.

The study, titled “JWST Reveals Refractory-rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks,” was published in The Astrophysical Journal Letters. As an observational study based on three epochs of near-infrared spectroscopy, it rests on model-based interpretations of how light interacts with grains, and the authors themselves note that distinguishing between competing explanations for the changes over time remains an open question. Future Webb observations of comets within our solar system, together with spectra of newly discovered interstellar visitors, will help test these interpretations and determine whether 3I/ATLAS is typical of icy bodies formed around other stars or an outlier. For now, the third known interstellar comet has already delivered a message: the tiny grains from which worlds are built may look remarkably familiar, even when they come from a star system light-years away.

Subject of Research: JWST spectroscopy of interstellar comet 3I/ATLAS comparing its dust-rich water ice grains with icy solar system bodies

Article Title: SwRI-led study uses Webb to compare interstellar comet 3I/ATLAS with icy bodies in our solar system

Article References: SwRI-led study uses Webb to compare interstellar comet 3I/ATLAS with icy bodies in our solar system. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: interstellar comet, 3I/ATLAS, James Webb Space Telescope, water ice, cometary dust, trans-Neptunian objects, planetesimals, protoplanetary disks, near-infrared spectroscopy, Southwest Research Institute, The Astrophysical Journal Letters, solar system formation