Astronomers using the James Webb Space Telescope have found unusually abundant heavy hydrogen in the methane of interstellar comet 3I/ATLAS. The findings, covered by Phys.org on November 1, suggest its ice formed in very cold surroundings beyond the solar system. The underlying study appeared in Nature Astronomy on October 23, 2026.
Nathan Roth of American University and NASA’s Goddard Space Flight Center led the research. The published paper reports a deuterium-to-hydrogen ratio in methane of 3.18%, with an uncertainty of 0.30 percentage points. Deuterium is a form of hydrogen with an additional neutron; its presence changes a molecule’s infrared signature.
Webb examined gas around the comet on December 22 and 23, 2025, at about 2.4 times Earth’s distance from the Sun, according to the research manuscript. The team separated signals from other molecules to isolate faint features of methane containing deuterium. These measurements probe material released from the icy nucleus rather than sampling its interior directly.
The journal paper puts the ratio at about 13 times the value measured in comet 67P/Churyumov–Gerasimenko by Rosetta. That is the only other comet with a detection of deuterium-bearing methane. The authors say the enrichment fits formation in a cold interstellar cloud or around a young star, with few elements heavier than helium and intense cosmic-ray irradiation.
The result offers a way to test chemical models of how planetary building blocks form elsewhere. It does not identify a particular parent star or prove a unique formation history. The researchers note that direct methane isotope measurements in other star-forming environments remain unavailable. Further comparisons will be needed to understand why this visitor preserved such an unusual chemical record.
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