When a comet leaves the Sun's warmth, astronomers typically anticipate its slow dimming into the darkness of space. However, the behavior of 3I/ATLAS has surprised the scientific community: this visitor from another system experienced a resurgence in brightness several weeks after its closest approach to our star.
Discovered in July 2025, this comet is only the third confirmed interstellar object after 1I/'Oumuamua and 2I/Borisov. Its rapid trajectory through our planetary neighborhood makes it a rare sample of material from other stellar systems. Scientists were able to take advantage of its passage to study its structure with state-of-the-art instruments, thus obtaining unprecedented chemical details.
NASA's SPHEREx infrared observations in December 2025 show dust, water, organic molecules, and carbon dioxide in the coma of comet 3I/ATLAS.
Credit: NASA/JPL-Caltech The SPHEREx telescope captured images in December 2025, as the comet was moving away from the Sun. Contrary to expectations, it displayed intense activity, with a bright coma rich in water vapor, carbon dioxide, and organic compounds. These infrared data allow for the precise identification of released molecules, such as methane and cyanide, providing a detailed inventory of the ingredients present.
The late activity of 3I/ATLAS intrigues researchers, as comets are supposed to be most active near the Sun. One phenomenon could explain it: solar heat likely penetrated slowly beneath the surface before causing the sublimation of buried ices. This mechanism released ancient materials, which had remained hidden for billions of years, thus explaining the sudden brightness increase recorded by the instruments, like a delayed "flash-bomb."
Among known interstellar objects, 3I/ATLAS stands out for its long period of activity and the diversity of its chemical composition. While 'Oumuamua and Borisov offered more fleeting glimpses, this comet allowed for prolonged analysis. Its similarities with comets in our Solar System indicate that the fundamental materials for planet formation might be widespread across the galaxy.
These observations help scientists understand how planets form around other stars. By examining the chemistry of 3I/ATLAS, they can compare the processes at work in different stellar environments. This supports the hypothesis that conditions suitable for the emergence of life could exist elsewhere, based on organic compounds analogous to those found on Earth.