Webb finds dust-rich water ice in interstellar comet 3I/ATLAS By Luís Henrique Costa
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The James Webb Space Telescope detected tiny grains of ice mixed with dust inside 3I/ATLAS, the interstellar comet that came closest to Earth on December 19, 2025. The structure of the object differs from that of comets born in the Solar System. Its estimated age is more than 10 billion years, over twice the age of our planetary system.
The findings were published on November 2, 2026, in The Astrophysical Journal Letters. Researchers from the Southwest Research Institute, based in the United States, led the work. Infrared data from the observatory indicate the visitor formed in the disk of gas and debris around a young star during the early Milky Way, before strong gravitational interactions threw it into deep space.
Only one water-ice signal appears in the comet’s light
The light reflected by the cloud of gas and dust surrounding the nucleus behaved in an unexpected way. Comets native to the Solar System show three clear absorption marks from pure water ice. The visitor from another star showed just one of them.
Computer simulations built by the team explain the difference. The single signal appears because the ice particles of 3I/ATLAS are heavily contaminated by cosmic dust, while the frozen bodies near the Sun keep much purer ice.
See also: James Webb Space Telescope detects methane gas on interstellar comet 3I/ATLAS for the first time
Five traits Webb measured in the interstellar visitor
- Age estimated at more than 10 billion years, over twice the age of the Solar System.
- Ice grains at microscopic scale, with a high share of embedded dust.
- Spectrum showing a single water band, instead of the usual pattern seen in comets.
- High concentration of frozen carbon dioxide and carbon-rich organic compounds.
- Third object from outside the Solar System recorded by astronomers, after 1I/’Oumuamua and 2I/Borisov.
The chemical signature closely resembles that of trans-Neptunian objects, a group of frozen bodies in the outer limits of the Solar System, beyond the orbit of Neptune. The high amount of carbon dioxide suggests the comet condensed its materials in the most remote and cold regions of its birth star. A separate Webb study, published on October 23, 2026, in the journal Nature Astronomy, found methane with about 3.33% of its hydrogen replaced by deuterium, the heaviest share measured in the Solar System.
Check out: Cosmic rays build 20-meter shell on interstellar comet 3I/Atlas
Astronomers say 3I/ATLAS may be a fragment thrown off after the collision and destruction of a much larger icy body. According to the researchers, the way these small dusty grains came together points to the basic process of building planets following similar rules in different regions of the galaxy.
