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Interstellar Science
A Visitor From a Colder Star: What 3I/ATLAS's Strange Water Reveals
3I/ATLAS, found in July 2025, is only the third confirmed interstellar object. As it passed the Sun, ALMA measurements led by University of Michigan astronomers found its water's deuterium-to-hydrogen ratio is the highest ever measured in a comet — over 40 times Earth's oceans and at least 30 times typical Solar System comets. That points to a birthplace far colder than the Sun's nursery, and it may be among the oldest comets known. The 'alien' claims did not hold up.