PARIS, France — An interstellar comet that blazed past the Sun last year could be nearly three times older than our Solar System and is unlike anything ever before seen in our cosmic backyard, ...

Astronomers have detected water on interstellar comet 3I/ATLAS with a deuterium-to-hydrogen ratio exceeding 6.6 × 10−3, a value more than 40 times that of Earth’s oceans and more than 30 times higher ...