A Changing Visitor from Another Star

Interstellar comet 3I/ATLAS — born around a distant star and now passing through our solar system — has been found to have changed its chemical composition following its closest approach to the sun. A research team led by Yoshiharu Shinnaka of Japan's Koyama Space Science Institute observed the object on January 7, 2026, using the Subaru Telescope, an 8.2-meter optical-infrared telescope near the summit of Maunakea, Hawaii.

By analyzing the colors of 3I/ATLAS' coma — the gas bubble that envelopes comets regardless of origin — the team estimated the ratio of carbon dioxide to water surrounding the comet. They found that ratio had shifted since the comet made its closest approach to the sun on October 29, 2025.

Why It Matters

3I/ATLAS is only the third interstellar object ever detected, offering scientists a rare window into raw materials formed around other stars. The chemical change found after perihelion goes beyond surface-level activity: it also provides clues about the comet's internal structure.

"By applying techniques developed through solar system comet studies to interstellar objects, we can now directly compare comets from inside and outside the solar system," Shinnaka said.

A Growing Field of Interstellar Science

The study of interstellar objects began in earnest with the 2017 discovery of 1I/'Oumuamua, which puzzled scientists with its unusual acceleration. In 2019, 2I/Borisov became the second such object, yielding richer data and showing compositional similarities to solar system comets. 3I/ATLAS marks the third, and the first whose perihelion passage has been closely tracked for chemical evolution.