The search for alien life has mostly listened in on a narrow slice of the radio spectrum, leaving vast higher frequencies almost untouched. A new study using archived data from the ALMA telescope in Chile suggests that promising signals could be hiding where astronomers rarely look.
The water hole that became a blind spot
For decades, most radio SETI projects have focused on frequencies between 1.42 and 1.66 gigahertz. Astronomers call this range the water hole because it sits between the natural radio emissions of hydrogen and hydroxyl, the two ingredients that form water. The logic was simple: an advanced civilization might also recognize the importance of water and choose to broadcast there. But that narrow focus may have caused researchers to miss signals at higher frequencies that could be just as promising.
A first SETI survey with ALMA
Louisa Mason, a PhD researcher at the University of Manchester, decided to test what might be found outside the water hole. She turned to the Atacama Large Millimeter/submillimeter Array in Chile, a telescope built for studying cold gas and dust, not for hunting aliens. Mason combed through archived observations that had originally been collected for unrelated astronomy projects. She searched for narrowband radio signals, the kind that could come from artificial technology rather than natural cosmic processes. The search covered two narrow frequency ranges within ALMA's Band 3 observations. No candidate signals turned up above the survey's detection thresholds. But the project was limited to just four archived observations, a tiny sample of what ALMA could offer.
Stellar bycatch and a new frontier
Mason's work also highlights a quirk of radio astronomy that could accelerate future searches. When a telescope points at one target, its field of view often captures many additional stars in the surrounding area. These unintentionally observed stars are sometimes called stellar bycatch. Even a handful of ALMA observations may quietly include millions of stars, meaning SETI surveys could piggyback on routine astronomy without needing dedicated telescope time. The millimeter and submillimeter radio bands remain almost completely unexplored for SETI, and Mason says telescopes operating at these higher frequencies could become valuable tools in the hunt for alien technology.
The study was presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham. No signals were found, but the research opens a new region of the radio spectrum for future searches, one that has been hiding in plain sight.
