Astronomers used NASA’s James Webb Space Telescope in December 2025, as interstellar comet 3I/ATLAS moved away from the Sun, to analyze its chemistry. The study, published in Nature, found unexpectedly high levels of deuterium, or heavy hydrogen, and unusual carbon ratios in a comet estimated at about 2.6 kilometers across.
Lead author Dr. Martin Cordiner of NASA’s Goddard Space Flight Center said the object offered a rare chance to study “an ancient object from a distant galaxy” that “probably existed before the Sun and our solar system.” Using Webb’s NIRSpec infrared spectrograph, the team measured the comet’s light and identified chemical composition, temperature, speed and distance. They found deuterium at roughly 30 times the level seen in comets from our solar system, suggesting formation in an extremely cold environment, around minus 243 degrees Celsius, with long exposure to radiation but not prolonged heat.
NIRSpec also detected only traces of carbon-13 relative to carbon-12, another sign of great age. The researchers said stars enrich their surroundings with carbon-13 over time, and the solar system, which formed about 4.5 billion years ago, contains more of it than 3I/ATLAS. The exact origin of the comet within the Milky Way is unknown, but the team believes interstellar objects are expelled during violent planet formation, similar to comets in our system. Previous interstellar visitors, 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019, were too dim for isotope analysis.
The team estimates 3I/ATLAS could be 10 to 12 billion years old, making it possibly the oldest object ever observed in our solar system and dating to a time when the universe was only about 13% of its current age. Dr. Stephanie Milam said such studies help determine how common the conditions for life may be elsewhere in the universe. The comet is now approaching Saturn’s orbit, is expected to cross Pluto’s orbit in 2029, and should leave the outer solar system around 2035.
