An Enriched Methane D/H Ratio in the Interstellar Object 3I/ATLAS

Interstellar objects like 3I/ATLAS are interlopers from other planetary systems, and their volatile compositions offer a window into planet formation around their host stars. The authors used near-infrared spectra of 3I/ATLAS's coma obtained with the James Webb Space Telescope to measure the deuterium-to-hydrogen (D/H) ratio of methane.

The measured methane D/H ratio is (3.33±0.31)%, which exceeds every methane D/H value measured in the solar system and is a factor of 14±2 higher than that in comet 67P/Churyumov-Gerasimenko, the only other comet with a CH3D detection. Both 3I/ATLAS and 67P show stronger deuteration in methane than in water, matching a broader trend in which organic deuteration exceeds water deuteration by up to an order of magnitude. While the D/H ratio in methane has been observationally unconstrained for extrasolar sources, the enriched ratio in 3I/ATLAS is similar to those seen in methanol and formaldehyde toward primitive environments.

The elevated methane D/H ratio is consistent with ices forming in a cold, low-metallicity interstellar or protostellar environment under a high cosmic-ray irradiation rate, as proposed by Cordiner et al. (2026). This is a rare detection of deuterated organic molecules in an interstellar object and offers new constraints on the conditions of planet formation in other systems.