Scientists studying 3I/ATLAS have detected unusually high levels of heavy water, indicating it formed in an extremely cold environment far beyond our solar system. As only the third known interstellar object, it provides a rare opportunity to study material from another star system.
An illustration comparing the water content of 3I/ATLAS to that of Earth.
Photo Credit: NASA
An interstellar intruder has been discovered that will change everything we know about how planets form in the galaxy. The comet 3I/ATLAS, which belongs to an exclusive group of three known interstellar objects that entered our solar system, was first observed on July 1 2025, through the ATLAS telescope, which received funding from NASA and operates from Chile. Scientists identified the object as an extraterrestrial space body because it moved at a speed of 137000 miles per hour.
An April 23, 2026, study published in Nature Astronomy by Luis E. Salazar Manzano et al. from the University of Michigan utilized the ALMA array to record the very first detection of "semi-heavy" water, or deuterated water, in any interstellar body. The comet contains around 30 times as much heavy water as solar system comets and 40 times as much as the Earth's oceans. As this enrichment is observed only at temperatures below 30 Kelvin (–406°F), the results indicate that the comet originated in an exceedingly cold star system compared to ours.
3I/ATLAS is the latest addition to the small group of comets that have hailed from beyond our Solar System. While previous examples have not revealed their composition, 3I/ATLAS represents the first interstellar comet whose D/H ratio was measured, an analysis that goes back to its birth in a distant star system that could be so old that it might not even exist anymore. “Every interstellar object is carrying information about its own history,” explains co-author Teresa Paneque-Carreño. With telescopes such as ALMA, scientists can finally study the chemistry of distant planetary cradles.
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