Observations by the James Webb Space Telescope suggest that interstellar comet 3I/ATLAS formed from sub-micron grains of water ice heavily mixed with dust. The composition differs from that of the Solar System’s known comets, whose ice grains are generally much cleaner, but it resembles the material found in some icy bodies beyond Neptune.
3I/ATLAS was discovered entering the Solar System in the summer of 2025. It is only the third confirmed object observed arriving from outside the Solar System, following 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. Its interstellar origin offers researchers a rare opportunity to study material formed around another star.
The JWST’s Near-Infrared Spectrograph observed the comet three times between August 2025 and April 2026, covering the period before and after its closest approach to the Sun. The observations detected a strong spectral signature associated with water ice at three microns, while two other features commonly seen in Solar System comets, at 1.5 and two microns, were weak or absent.
According to research lead Silvia Protopapa of the Southwest Research Institute, this combination indicates that the water ice in 3I/ATLAS is not arranged in the same way as the ice in familiar Solar System comets. Modeling instead points to very small grains of “dirty” water ice containing substantial amounts of dust.
The researchers say this composition preserves information about the environment in which the comet formed. Comets contain primordial material from the gas-and-dust disks surrounding young stars, making their composition a record of the conditions present during planet formation.
Although the star around which 3I/ATLAS formed is unknown, researchers believe the comet originated in a young planetary system and may have formed as long as 10 to 12 billion years ago. It was later expelled into interstellar space before passing through the Solar System.
The comet’s spectrum also resembles that of some medium-sized trans-Neptunian objects, which orbit beyond Neptune and are rich in carbon-dioxide ice and organic compounds. A separate 2025 JWST study led by Martin Cordiner found that 3I/ATLAS had one of the highest carbon-dioxide abundances measured in a comet.
These similarities suggest that the icy material used to build planets around other stars may share properties with the material that formed parts of our own Solar System. The researchers also say 3I/ATLAS could be a fragment of a larger, trans-Neptunian-object-like body broken apart in a collision.
The findings indicate that tiny ice grains form early within planet-forming disks before becoming incorporated into larger objects such as comets and protoplanets. The study was published on October 26 in The Astrophysical Journal Letters.
Sources: Space.com.