A visitor from another star carries a frozen clue: its methane is heavy with an ancient cold

The interstellar comet 3I/ATLAS is only the third known object to visit our solar system from around another star — and it has brought a souvenir of home. Using the James Webb Space Telescope, Nathan Roth of American University and NASA's Goddard Space Flight Center and colleagues measured the comet's methane — the simple gas in its icy coma — and found it unusually rich in heavy hydrogen: a ratio about 13 times higher than in a familiar solar-system comet, and higher than any methane measured here. Published in Nature Astronomy, the signature points to ingredients that froze in intense cold around another star, or in the cloud before it — a rare, direct sample of how another planetary system began.