Astronomers conducting searches for extraterrestrial intelligence may have been limiting their chances of detection by focusing on a relatively restricted portion of the radio spectrum, according to recent research presented at the Royal Astronomical Society’s National Astronomy Meeting. The majority of radio SETI projects have targeted frequencies between 1.42 and 1.66 GHz, a region known as the ‘water hole’ because it lies between natural radio emissions from hydrogen and hydroxyl. Scientists have long theorized that an advanced civilization might recognize the significance of these elements and establish communication in this quiet part of the spectrum.
A new study suggests that higher radio frequencies warrant exploration as potential venues for detecting signals from distant technological civilizations. Louisa Mason, a PhD researcher at the University of Manchester, conducted the first SETI survey using archived observations from the Atacama Large Millimeter/submillimeter Array in Chile. Rather than requesting new observation time, Mason analyzed data originally collected for unrelated astronomical research, searching for narrowband radio signals that would be more consistent with artificial technology than natural cosmic phenomena. The millimeter and submillimeter radio bands remain largely unexplored territory for SETI efforts, representing what researchers describe as a new parameter space to investigate.
The initial search of two narrow frequency ranges within ALMA’s archived observations did not yield any candidate technosignatures above detection thresholds, despite being limited to four datasets. However, Mason’s work revealed an additional advantage of this approach: telescopes observing a single target typically capture numerous additional stars in their field of view, sometimes referred to as ‘stellar bycatch.’ By applying the Besançon Galactic Model to estimate star distributions across the Milky Way, Mason determined that previous surveys may have examined far more stars than previously calculated. When this method was applied to an earlier SETI survey containing 1,327 telescope pointings, the estimated number of observed stars increased from approximately 288,000 according to traditional star catalogues to more than 6.1 million.
Mason emphasized that the absence of detected signals should not be interpreted as evidence against extraterrestrial intelligence elsewhere in the cosmos. The survey covered only a limited number of observations and two restricted frequency windows. Instead, she expressed hope that the research would motivate the astronomical community to broaden SETI efforts across a wider portion of the radio spectrum and to leverage existing telescope archives for additional technosignature searches without requiring entirely new observation campaigns.
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