Using the James Webb Space Telescope (JWST), scientists have found that the interstellar comet 3I/ATLAS was built from tiny grains of ice mixed with dust that are quite different from the solar system’s comets.
These ice grains do, however, bear striking similarities to some icy bodies located at the edge of the solar system, beyond the orbit of Neptune, so-called trans-Neptunian objects (TNOs).
3I/ATLAS was discovered entering the solar system in the summer of 2025. It was only the third object found entering the solar system from farther afield in the Milky Way after 1I/'Oumuamua, seen in 2017, and 2I/Borisov, detected in 2019. This meant the interstellar comet offered a rare chance to glimpse the conditions under which such bodies form in other star systems. Thus, the JWST was quickly on the trail of 3I/ATLAS.
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