Astronomers from the Southwest Research Institute (USA) have studied the interstellar comet 3I/ATLAS using the James Webb Space Telescope. This is only the third object to arrive from beyond the Solar System. The results were published in The Astrophysical Journal Letters.
Comet 3I/ATLAS is a small icy body that formed in the gas and dust disk around another star. As it approaches the Sun, ice and dust from its interior burst outward, forming a cloud around the nucleus. By studying these particles, it is possible to learn under what conditions the comet was born.
A team led by Dr. Silvia Protopapa observed the comet three times between August 2025 and April 2026 - before and after its closest approach to the Sun. For this, a near-infrared spectrograph was used: the instrument splits light into wavelengths, which makes it possible to determine the composition and physical state of the material.
In the infrared spectrum, water ice leaves characteristic "fingerprints" - dips at certain wavelengths. 3I/ATLAS has a strong dip at a wavelength of 3 microns, but the other two, at 1.5 and 2.0 microns, are very weak or absent. Most known icy comets show all three.
The scientists compared the data with models of light scattering on particles of different sizes and compositions. Tiny grains - about a micron (one millionth of a meter) and smaller, consisting of water ice mixed with dust - fit best. In such small dusty grains, light passes through the ice only a short distance, so the weak bands disappear while the band at 3 microns remains noticeable. Other comets also have small ice grains, but these are considered purer.
"The grains of 3I/ATLAS are about the same size as those of our comets. The difference seems to be in how much dust is mixed into them," Protopapa explained.
The properties of the particles changed over time. By the last observation in April 2026, when the comet was moving away from the Sun, the data indicated an additional population of larger, but still micron-sized, icy grains. It is not yet possible to say whether this is due to heating or to the exposure of new layers as the surface breaks down - both options are possible.
The astronomers then compared 3I/ATLAS with objects of the Kuiper Belt - icy bodies beyond Neptune that preserve material from the early Solar System. The shape of the band at 3 microns in the comet turned out to be similar to that seen in some such objects, especially those rich in carbon dioxide ice and organics. The similarity concerns specifically water ice, not carbon dioxide.
According to the authors, this suggests that the icy "building blocks" of planets - both in our system and around other stars - may have a common range of properties: from nearly pure ice to ice with dust. However, this is so far an interpretation, not a proven fact. Future observations will help test whether comet 3I/ATLAS is typical.