Potential Extraterrestrial Signal Attenuated by Common Molecules, Harvard Astronomer Reports
CAMBRIDGE, MA – November 2025 - A compelling radio signal detected emanating from interstellar object 3I/ATLAS, initially considered a potential indicator of extraterrestrial technology, is now understood to have been substantially weakened by absorption from hydroxyl (OH) radicals present in space, according to research led by Avi Loeb, head of the Galileo Project at Harvard University. The findings, published this month, offer a natural explanation for the signal’s unusual characteristics and underscore the challenges of identifying genuine technosignatures amidst cosmic noise.
The initial detection in 2023 sparked intense speculation about the possibility of a directed transmission from an alien civilization. However, Loeb’s team’s analysis reveals the signal’s frequency – 982 MHz – aligns with a known absorption line for OH radicals, common molecules found throughout interstellar space. This absorption would explain the signal’s narrow bandwidth and its weakening as 3I/ATLAS moved further from Earth. The discovery highlights the necessity of accounting for natural phenomena when searching for evidence of extraterrestrial intelligence and emphasizes the importance of rigorous scientific scrutiny.
Loeb, also founding director of Harvard University’s Black Hole Initiative and director of the Institute for Theory and computation at the Harvard-Smithsonian Center for Astrophysics, explained that the team’s observations where crucial in understanding the signal’s behavior. “The signal was unusual, but not necessarily indicative of extraterrestrial origin,” Loeb stated.”Our analysis demonstrates that the observed attenuation is consistent with absorption by OH radicals, a common constituent of interstellar gas.”
The Galileo Project, dedicated to the search for extraterrestrial technological signatures, continues to analyze data from 3I/ATLAS and other interstellar objects. The team is developing advanced algorithms and observational strategies to differentiate between natural signals and potential technosignatures, focusing on identifying signals that exhibit characteristics inconsistent with known astrophysical processes.
Loeb is the bestselling author of “Extraterrestrial: The First Sign of Bright Life Beyond Earth” (2021) and co-author of “Life in the Cosmos” (2021),and recently released the paperback edition of his book,”Interstellar” (August 2024). He previously served on the President’s Council of Advisors on Science and Technology and chaired the Board on Physics and Astronomy of the National Academies.
