Interstellar comet 3I/ATLAS, on its way out, surprises with something strange about our Solar System

3I/ATLAS, the third known interstellar object to pass through our Solar System in its intergalactic journey, has baffled scientists aperiodically. Now, on its way out of the solar system, forever, after its closest approach to the Sun last October, the ice grains from the algid piece of rock have shown some new characteristics never found in any previously studied comets.

A Southwest Research Institute(SwRI)-led study that examined the streaming ice grain from interstellar comet 3I/ATLAS, which is likely as old as the universe, with NASA’s James Webb Space Telescope, found some unusual spectral characteristics of the tiny aggregates of water ice and dust, about a micron or smaller.

Comet 3I/ATLAS is believed to have come from a different star system located in the general direction of the constellation Sagittarius, where the central region of our galaxy, the Milky Way, is located. But remarkably, the ice grains on the interstellar comet resemble the spectra of some icy worlds found in the outer boundary of our solar system beyond Neptune.

Origin of 3I/ATLAS gives new clues about planet formation

The results suggest that planets in other star systems may share a common range of physical properties like our own solar system, ‘with tiny grains ranging in composition from relatively pure ice to ice mixed with dust,’ the university wrote in a media release.

“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,” Dr. Silvia Protopapa, lead author of the new study about this research published in The Astrophysical Journal Letters, said in a statement.

“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.”

How did they characterize the ice, dust and gas that the comet releases

With Webb’s Near-Infrared Spectrograph, Protopapa and her team observed 3I/ATLAS three times between August 2025 and April 2026, before and after the perihelion.

The instrument possesses a prism that splits light into its component wavelengths, which reveals 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,” Dr. Martin Cordiner of NASA’s Goddard Space Flight Center and the Catholic University of America, and co-author of the study, said in a statement.

Adding that their 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.”

When infrared light shines on water ice, it leaves a distinct fingerprint: a deep absorption band near 3 microns alongside two weaker bands near 1.5 and 2.0 microns.

While most icy comets display all three, 3I/ATLAS presents a unique anomaly; its 3-micron band is pronounced, but the 1.5- and 2.0-micron bands are remarkably faint or entirely absent.

“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.”

When infrared light shines on water ice (H₂O), the molecules absorb specific wavelengths of light, causing them to vibrate. Because it blocks or absorbs these exact wavelengths, it leaves behind a distinct visual signature.

“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.”

Properties of the ice grains changed before and after their closeness to the Sun

When the comet was moving away from the Sun, the researchers finally observed it in April 2026, and the data “favored an additional population of ice-rich grains, larger than those inferred earlier but still around a micron in size,” the University noted.

“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,” Dr. Michael Kelley of the University of Maryland, another author of the paper, said. “Both are possible, and telling them apart is one of the open questions we aim to address.”

Interstellar comet 3I/ATLAS’s icy grains resemble our own solar system

The team then compared 3I/ATLAS with other trans-Neptunian objects, which are icy bodies orbiting beyond Neptune. Scientists consider that they preserve material from the early solar system.

As per the researchers, the shape of 3I/ATLAS’s 3-micron band resembles that of several groups of these objects found at the outer wall of our solar system, whose spectra also show signs of carbon dioxide ice and organic compounds. And just like 3I/ATLAS, these objects from our solar system 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 noted. “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,” she explained.

If confirmed by further studies, these findings from the grain properties of 3I/ATLAS could indicate that the physical form of water ice was there at the time of formation of the universe, in planet-forming disks, and partly preserved in the icy bodies that form there.