This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The interstellar comet 3I/ATLAS carries a chemical clue to conditions in the planetary system of its origin, locked away long before it reached our sun. Its methane signature is unusually rich in heavy hydrogen, suggesting that the comet's frozen ingredients took shape in an intensely cold environment around another star—or in the cloud that preceded it.
Using the James Webb Space Telescope, Nathan Roth of American University and NASA's Goddard Space Flight Center and colleagues measured a heavy-to-ordinary hydrogen ratio in 3I/ATLAS that's about 13 times as high as that of comet 67P/Churyumov-Gerasimenko (which likely originated in our own solar system). Their findings, published in Nature Astronomy, give scientists a rare opportunity to investigate another planetary system's early chemistry. The challenge is working backward from the escaping gas to the conditions that made its ice.
Heavy hydrogen, called deuterium, has an extra neutron in its nucleus. It can take the place of ordinary hydrogen in a molecule of methane, which consists of one carbon atom joined to four hydrogen atoms. That substitution changes the molecule's infrared signature, allowing astronomers to distinguish between the two forms, but the deuterium-bearing version produces a much fainter signal.
Webb observed 3I/ATLAS on Dec. 22 and 23, 2025, when it was about 2.4 times as far from the sun as Earth is. The telescope examined infrared light from the coma, the cloud of gas surrounding the comet's icy nucleus. The researchers modeled and subtracted light from other gases, then looked between ordinary methane's much brighter features for the faint traces of its heavier counterpart.
Those signals yielded a deuterium-to-hydrogen ratio, or D/H ratio, of approximately 3.2% in methane. "This D/H ratio exceeds any other value for methane measured in the solar system," the authors write. The comparison with comet 67P, which is a Jupiter-family comet that probably originated in the Kuiper Belt, is especially useful because it is the only other comet in which deuterium-bearing methane has been detected.
That measurement required a close-up investigation by the Rosetta spacecraft. Webb detected the molecule from afar in 3I/ATLAS, a visitor born outside our solar system. Why should an extra neutron reveal anything about a comet's birthplace?
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