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NASA's James Webb Space Telescope has captured the first mid-infrared chemical fingerprint of interstellar comet 3I/ATLAS, detecting methane and revealing carbon dioxide-to-water ratios far exceeding any known solar system comet. The data — combined with extreme deuterium enrichment and anomalous carbon isotope ratios — suggest the object formed in a frigid, chemically distinct planetary system up to 12 billion years ago, raising new questions about the diversity of planetary formation environments across the Milky Way.