Water D/H in 3I/ATLAS as a probe of formation conditions in another planetary system
John J. Tobin
Nature Astronomy, Published online: 23 April 2026; doi:10.1038/s41550-026-02850-5 ALMA observations of the interstellar comet 3I/ATLAS provide a limit on the deuterium-to-hydrogen ratio in water. This ratio is a sensitive probe of temperature, suggesting that comet 3I formed in an ultracold environment with minimal thermal processing in its home system.