Only three objects from other star systems have ever been caught passing through ours, and 3I/ATLAS is the latest. Because it formed around a different sun, every molecule it sheds is a sample of somebody else’s planetary nursery. A new analysis of its methane in Nature Astronomy, by a group headed by Nathan Roth, who works at American University in Washington and at NASA’s Goddard centre, suggests that nursery was bitterly cold.
The team pointed the James Webb Space Telescope at the comet on 22 and 23 December 2025, when it sat roughly 2.4 times Earth’s distance from the Sun. Webb’s infrared spectrograph mapped the glowing gas cloud around the nucleus. After stripping out the contributions of other molecules, the researchers hunted in the gaps between bright methane lines for the much fainter fingerprint of methane carrying a deuterium atom.
They found plenty. Roughly one hydrogen atom in thirty in that methane turns out to be deuterium, some 3.2%. For comparison, the home-grown comet 67P carries only about a thirteenth as much. In the paper’s words, no methane ratio “measured in the solar system” comes close.
Deuterium builds up in molecules at very low temperatures, helped along by cosmic rays that kick-start the relevant chemistry. So the result points to an origin in a frigid gas cloud or the surroundings of a newborn star poor in heavier elements. There is a hint about timing too: the ratio in methane is roughly three times that earlier found in the comet’s water, so the methane may have formed later, once the heavy isotope had piled up.
The authors stress this is a plausible story, not a pinpointed address. Nobody has yet measured methane in distant star-forming regions for comparison, and existing models predict lower values.
