Three looks at a visitor
A team led by the Southwest Research Institute pointed the James Webb Space Telescope at interstellar comet 3I/ATLAS on three dates: 6 August 2025, 22 December 2025 and 1 April 2026. The observations bracket the comet's closest approach to the sun on 29 October 2025, giving the team a record of its ice before and after peak heating.
3I/ATLAS is only the third object confirmed to have arrived from outside the solar system. It was first reported by the ATLAS survey telescope in Chile, which is how it got its name.
One strong band, two missing ones
Water ice leaves a deep absorption band near 3 microns in infrared light, plus two weaker bands near 1.5 and 2.0 microns. In 3I/ATLAS the 3-micron band is strong, and Webb also detected gas emission from water, carbon dioxide and carbon monoxide. The two weaker bands were faint or absent, which is unusual.
In most icy comets all three appear. The team's explanation is grain structure: the ice in 3I/ATLAS sits in aggregates about a micron across, and those grains are mixed through with dust. Light passing through them is scattered before it can produce the cleaner, thinner bands, so the ice hides inside the dust rather than announcing itself.
A match with frozen worlds far out
The dusty signature resembles what astronomers see on frozen bodies beyond Neptune, where ice is thought to range from nearly pure to heavily dust-laden. If that same range holds around other stars, the raw material for building planets may form in a similar way in different systems, starting early in the planet-forming disc and surviving inside the icy bodies that form there.
Measurements reported with the study also found high deuterium levels, a marker of ice that formed in very cold, metal-poor conditions billions of years ago. The team describes the comet as a sample of planet-seeding ice delivered from another star system, and notes that the observations could not have been made from the ground because the relevant infrared bands are blocked by the atmosphere.