Astronomers using the James Webb Space Telescope have detected water in the interstellar comet 3I/ATLAS with deuterium levels 30 to 40 times higher than in Earth's oceans. This heavy hydrogen ratio exceeds that of any known solar system comet by at least 10 times. The findings suggest the comet originated from a cold, distant region around an ancient alien star.
The interstellar comet 3I/ATLAS, tracked since entering the solar system last year, contains unusually high levels of carbon dioxide and water compared to most comets observed here. Martin Cordiner at NASA’s Goddard Space Flight Center in Maryland and his team used the James Webb Space Telescope to measure its composition, finding deuterium—a heavy isotope of hydrogen—at levels more than 40 times higher than in Earth's oceans and at least 10 times higher than in any previous comet studied. This deuterium-to-hydrogen ratio is exceptional, according to Paul Hartogh at the Max Planck Institute for Solar System Research in Germany, who noted that nobody would have expected it. “3I/ATLAS continues to astonish us with what it reveals about the similarities and differences of its host system compared with our own solar system,” Cordiner said. Such high deuterium levels typically occur in the coldest regions of the Milky Way, indicating the comet likely formed in the outer disc of its original star system, Ewine van Dishoeck at Leiden Observatory explained. “That means it’s probably in the very outer part of the disc around whatever star it was circling, and that makes it also easier to kick it out,” she added. The comet also shows low levels of carbon-13, consistent with formation in a time of fewer supernovae, pointing to an age of 10 billion to 12 billion years—more than twice the sun's age—per Cordiner. However, van Dishoeck cautioned that the precision of carbon measurements limits certainty on the exact age. Early estimates had suggested around 8 billion years. The research appears in preprints on arXiv with DOIs 2603.07026 and 2603.06911.