In 2025, astronomers discovered 3I/ATLAS, only the third confirmed interstellar object. 1 year later, its heavy water revealed clues to a cold alien planetary system. The object entered the Solar System on a path that could not close around the Sun. As sunlight warmed its surface, gas escaped from its nucleus and allowed researchers to study material formed around another star. ALMA and the James Webb Space Telescope detected deuterium in its water at levels far above those measured in Solar System comets. The result points to ice that formed at temperatures below about 30 kelvin and retained that chemical record.
