Astronomers have discovered that the interstellar comet 3I/ATLAS contains isotopic signatures unlike any Solar System body, suggesting it formed in the early Milky Way. Analysis of its water and carbon compounds reveals extreme deuterium enrichment and unusual carbon isotope ratios that point to formation in a metal-poor environment.

The comet's water shows deuterium-to-hydrogen ratios of 0.95%, more than ten times higher than typical comets. Its carbon isotope ratios range from 141-191 for CO2 and 123-172 for CO, exceeding values found in Solar System objects, nearby interstellar clouds, and protoplanetary disks. These measurements indicate formation at temperatures below 30 Kelvin.

When interpreted through models of Galactic chemical evolution, the data suggests 3I/ATLAS formed roughly 10-12 billion years ago during an early period of intense star formation. This makes it a preserved fragment of an ancient planetary system, providing direct evidence for ice chemistry and volatile-rich planetesimal formation in the young Milky Way galaxy.