Three comets, eight years, no statistical significance.

Three comets. Eight years. No statistical significance.

And yet astronomers have spent those eight years using exactly those three rocks to overturn assumptions about Solar System chemistry that stood for decades.

The third rock is called 3I/ATLAS — formally C/2025 N1 (ATLAS) — discovered by the ATLAS survey telescope in Rio Hurtado, Chile on July 1, 2025. On January 7, 2026, the Subaru Telescope caught it at a heliocentric distance of 2.87 AU — outbound, past the Sun. Using the High Dispersion Spectrograph on the 8.2-meter Mauna Kea instrument, a team led by Yoshiharu Shinnaka of Kyoto Sangyo University ran the comet's coma through a spectroscopic analysis of forbidden oxygen lines. What they got back was a green-to-red intensity ratio — a proxy for the carbon dioxide-to-water ratio in the comet's atmosphere — of 0.339, plus or minus 0.027. Shinnaka et al., arXiv:2603.25002

That number matches 2I/Borisov almost exactly. Borisov was the second interstellar comet, detected in August 2019 by Gennadiy Borisov, a Crimean amateur astronomer who built his own 0.65-meter telescope. It remains the only other interstellar comet ever analyzed this way. The match between the two — arriving from completely different stellar systems, different formation histories — is the first chemical link between two interstellar comets. Three confirmed interstellar objects total: 1I/ʻOumuamua (2017, too faint for spectroscopy), 2I/Borisov (2019), 3I/ATLAS (2025). Sci.News

The finding, accepted for publication in The Astronomical Journal on April 22, 2026, carries a significant caveat: three is not a sample. The statistical power is negligible. And yet. Subaru Telescope

Before 3I/ATLAS reached perihelion on October 30, 2025, pre-perihelion observations by JWST and SPHEREx showed it was heavily CO2-enriched — more so than nearly any Solar System comet at similar distances. The post-perihelion Subaru measurement shows a sharply lower CO2/water ratio. The interpretation from Shinnaka's team: the comet's surface was baked during solar passage, releasing CO2 from the outer layers and exposing the less-processed interior. The interior is not like the exterior. That is new. That is the first time anyone has directly observed the interior volatile composition of an interstellar comet differ from its surface. Shinnaka et al., arXiv:2603.25002

The ratio is still higher than typical Solar System comets at the same distance from the Sun. Which raises a question: is this what all interstellar comets look like? Or is our Solar System the outlier?

The field did not exist before 2017. Now it has three objects, two spectroscopy measurements, and the beginnings of an answer to a question nobody had reason to ask before: what does the chemistry of icy planetesimals — the building blocks of planets — look like across the galaxy? The evidence so far is sparse and getting sparser under scrutiny. But it is real.

The Subaru Telescope is operated by the National Astronomical Observatory of Japan, with support from the MEXT Project to Promote Large Scientific Frontiers.