Subaru Telescope Observes Chemical Evolution in Interstellar Comet 3I/ATLAS

On January 7, 2026, the Subaru Telescope observed interstellar comet 3I/ATLAS (C/2025 N1) after its closest approach to the Sun. By analyzing the colors in the comet's coma—the gas and dust envelope surrounding its nucleus—astronomers estimated the ratio of carbon dioxide to water. This ratio was found to be significantly lower than that inferred from earlier space telescope observations, indicating an evolution in the comet's chemical composition over time.

These findings suggest that the interior composition of 3I/ATLAS differs from its exterior. As the comet approached the Sun and heated up, gases began to escape from different parts of its nucleus, revealing variations in its chemical makeup. This observation provides valuable clues about the structure of interstellar comets and the processes involved in the formation of planetary systems.

Yoshiharu Shinnaka of Kyoto Sangyo University, who led the research, emphasized the significance of these observations:

"With the full-scale operation of survey telescopes in the coming years, many more interstellar objects are expected to be discovered. By applying the observational and analytical techniques we have developed through studies of Solar System comets to interstellar objects, we can now directly compare comets hailing from both inside and outside the Solar System and explore differences in their composition and evolution. Through studies of such objects, we hope to gain a deeper understanding of how planetesimals and planets formed in a wide variety of stellar systems, including our own Solar System."

The study is scheduled for publication in The Astronomical Journal on April 22, 2026.