Subaru Telescope atop Mauna Kea. (Courtesy Photo: Vera Maria Passegger/National Astronomical Observatory of Japan)
Astronomers using Subaru Telescope atop Mauna Kea on the Big Island found new clues about the makeup of 3I/ATLAS, a rare interstellar comet that originated outside the solar system.
Their observations suggest the comet’s chemistry changed as it passed near the sun, providing insight into how materials in interstellar comets differ from those in comets formed in the solar system.
Comet 3I/ATLAS is the third known interstellar object to visit this part of the cosmic neighborhood, following ‘Oumuamua and Comet Borisov.
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Discovered in 2017 by Pan-STARRS atop Halealakalā on Maui, ʻOumuamua was the first confirmed interstellar visitor. Its Hawaiian name means “a messenger from afar arriving first,” and it appeared to be a rocky, elongated object with no obvious cometary features.
In contrast, 3I/ATLAS clearly behaves like a comet, displaying a bright, glowing coma of gas and dust.
The research team, led by Yoshiharu Shinnaka of Koyama Space Science Institute at Kyoto Sangyo University, observed the comet Jan. 7 with Subaru Telescope after its closest approach to the sun.
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By analyzing light from the comet’s coma — the cloud of gas and dust surrounding its nucleus — the team estimated the ratio of carbon dioxide to water in escaping material from the comet.
That ratio was lower than values inferred from earlier space telescope observations, suggesting different layers of the comet began releasing gas as solar heating changed conditions on and within the nucleus.
For astronomers, that change is especially important because the coma acts as a window into the comet’s nucleus, the frozen central body of the object.
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If the chemistry seen before and after the comet’s closest solar pass is different, it could mean the outer and inner parts of the comet are not the same.
That gives researchers a rare opportunity to learn how small bodies formed and evolved around other stars.
An image of Interstellar comet 3I/ATLAS captured Dec. 13, 2025, by Subaru Telescope atop Mauna Kea on the Big Island. (Courtesy Image: National Astronomical Observatory of Japan)
“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,” Shinnaka said in a release from Subaru.
The findings were published April 22 in The Astronomical Journal. Work was supported in part by grants from Japan Society for the Promotion of Science and Koyama Space Science Institute.
Subaru Telescope is operated by National Astronomical Observatory of Japan.