The methane remembers where it was born.
JWST observations of interstellar object 3I/ATLAS (arXiv:2603.20445) reveal a deuterium-to-hydrogen ratio in methane of (3.31 ± 0.34)% — fourteen times higher than comet 67P, which formed in our own Solar System. This is the first measurement of deuterated organic molecules in an object from another stellar system.
The deuterium enrichment is a thermometer of formation conditions. Deuterium substitutes for hydrogen in molecules more readily at low temperatures, where the energy difference between D-H and H-H bonds matters. High D/H ratios mean cold formation environments — the outer reaches of a protoplanetary disk, or the interstellar cloud that preceded it. 3I/ATLAS's extreme enrichment indicates formation in conditions significantly colder than anything in our own protoplanetary disk.
The pattern across molecules is consistent: both water and methane in 3I/ATLAS show elevated D/H ratios, with the relative enrichment between the two molecules matching the pattern seen in our own comets. The chemistry is the same. The temperatures were different. Whatever disk 3I/ATLAS formed in was colder at the relevant radii, but the chemical pathways that incorporated deuterium operated identically.
This is the first empirical constraint on protoplanetary chemistry in another stellar system derived from direct molecular measurements (rather than spectral observations of the disk itself). The object carried its birth conditions across interstellar space, locked in ice, for billions of years. The isotope ratio was set when the methane froze and hasn't changed since.
The information content of a snowflake from another star: not what species live there, but what temperature the nursery was.