Observations from the Subaru Telescope on January 7, 2026, indicated an unexpectedly low ratio of carbon dioxide to water in the interstellar comet 3I/ATLAS, implying alterations in its composition as it neared the Sun.
Subaru Telescope image of interstellar comet 3I/ATLAS. Image credit: National Astronomical Observatory of Japan.
The interstellar comet 3I/ATLAS was identified on July 1, 2025, by the NASA-funded ATLAS survey telescope located in Rio Hurtado, Chile.
This cosmic traveler, also known as C/2025 N1 (ATLAS) and A11pl3Z, approached from the constellation Sagittarius.
On October 30, 2025, 3I/ATLAS reached perihelion, marking its closest point to the Sun.
“Interstellar comet 3I/ATLAS is the third confirmed interstellar object,” stated Yoshiharu Shinnaka, an astronomer at the Koyama Institute for Space and Astronautical Science in Japan.
“This presents a unique opportunity to examine the physical and chemical characteristics of icy planetesimals formed in protoplanetary systems beyond our solar system.”
Since its identification by the Asteroid Earth Impact Last Alert System (ATLAS) on July 1, 2025, subsequent observations have promptly confirmed its cometary activity and analyzed the characteristics of its dust coma, including core size and dust distribution.
Dr. Shinnaka and his team utilized the Subaru Telescope to examine 3I/ATLAS on January 7, 2026, over two months following its perihelion.
This analysis included applying specialized techniques honed through the study of solar system comets.
The research estimated the carbon dioxide to water ratio in the comet’s coma, the gaseous cloud enveloping the comet’s nucleus.
The composition of the coma gas offers valuable insights into the nature of the nucleus itself.
The calculated carbon dioxide to water ratio from Subaru Telescope findings was lower than previous estimates derived from data from the Webb Space Telescope and SPHEREx Space Telescope.
This variation supports the theory that the internal makeup of the nucleus differs from its outer layers, suggesting that as 3I/ATLAS approached the Sun, gases began to release from different regions of its nucleus.
“In the upcoming years, we anticipate the full-scale operation of the observing telescope, leading to the discovery of even more interstellar objects,” Dr. Shinnaka remarked.
“By leveraging observation and analytical methods developed from solar system comet research, we can now directly compare comets from both inside and outside the solar system, allowing us to investigate differences in their composition and evolution.”
“Studying these celestial bodies enables us to deepen our understanding of planetesimal and planet formation processes across diverse star systems, including our own.”
The team’s research paper is set to be published in Astronomy Magazine.
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Yoshiharu Shinnaka and colleagues. 2026. Post-perihelion constraints on CO2/H2O ratio of interstellar comet 3I/ATLAS [O I] Forbidden line. A.J. in press. arXiv: 2603.25002
Source: www.sci.news
