A groundbreaking discovery has been made by NASA scientists: Water has been detected on the 7-billion-year-old interstellar comet 3I/ATLAS, a finding that could revolutionize our understanding of cometary evolution and the potential for life in distant planetary systems.
This interstellar comet, first spotted on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii, is only the third of its kind ever detected. The others were 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019. What sets 3I/ATLAS apart is its hyperbolic trajectory, indicating it originated from another star system and will not return to our Solar System.
The comet's remarkable age of seven billion years makes it almost twice as old as Earth and potentially the most ancient comet ever observed. This unique characteristic presents a rare opportunity for scientists to study the chemical composition of an alien planetary system before it disappears into interstellar space.
The detection of water's chemical signature on 3I/ATLAS was made by NASA's Neil Gehrels Swift Observatory, in collaboration with Auburn University in Alabama. The observatory picked up a faint ultraviolet signal, caused by the presence of hydroxyl (OH) gas, which forms when sunlight breaks down water molecules. This ultraviolet signature, invisible to ground-based observatories, was captured by Swift's space-based instruments.
The findings, published in a new study, are expected to significantly impact future models of cometary evolution and planetary formation across the Galaxy. The discovery of water on 3I/ATLAS suggests that planet-forming systems beyond our own may have similar chemical compositions, challenging our current understanding of cometary evolution.
Dr. Zexi Xing, the lead researcher, emphasizes the surprising nature of each interstellar comet observed, stating that 3I/ATLAS is no exception. Professor Dennis Bodewits, a co-author of the study, adds that each comet observed has rewritten the rules, highlighting the unexpected presence of water on 3I/ATLAS in conditions where it wasn't anticipated.
