Astronomers looking for evidence of intelligent life beyond Earth may have overlooked promising signals simply because they have concentrated on a relatively narrow portion of the radio spectrum.
Most radio SETI (Search for Extraterrestrial Intelligence) projects have targeted frequencies between 1.42 and 1.66 GHz. This region is often called the ‘water hole’ because it falls between the natural radio emissions produced by hydrogen and hydroxyl, which combine to form water.
Researchers have long considered this quiet region of the spectrum a potentially logical meeting place for interstellar communication. An advanced civilization might recognize the importance of hydrogen and hydroxyl and choose to send or monitor signals there.
Searching Beyond the Cosmic Water Hole
New research now suggests that higher radio frequencies could offer another valuable place to search for technological signals from distant civilizations. The findings are being presented this week at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham.
Louisa Mason, a PhD researcher at the University of Manchester, used archived observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to conduct the telescope’s first SETI survey.
Instead of requesting new telescope time, Mason examined observations that had originally been collected for unrelated astronomy research. She searched the data for narrowband radio signals, which could be more consistent with artificial technology than with natural processes in space.
“For decades, SETI searches have concentrated on a relatively small part of the radio spectrum. We wanted to ask what might happen if we looked somewhere very different,” Mason said.
“The millimeter and submillimeter radio bands remain almost completely unexplored for SETI, so this is really about opening up a new area of parameter space to search.”
ALMA Opens a New SETI Frontier
Mason examined two narrow frequency ranges within ALMA’s Band 3 observations. The search did not identify any candidate technosignatures (alien signals) above the survey’s detection thresholds.
The project was limited to only four archived ALMA observations. Even so, Mason says the results show that telescopes operating at higher radio frequencies could become useful tools in future SETI efforts.
The study also draws attention to an often overlooked feature of radio astronomy. When a telescope is aimed at one object, its field of view usually includes many additional stars in the surrounding area.
These unintentionally observed stars are sometimes described as ‘stellar bycatch.’
Millions of Stars Hidden in Telescope Data
Astronomers have traditionally estimated the amount of ‘stellar bycatch’ in an observation by using star catalogues such as Gaia. However, those catalogues may not include every star in the field, particularly objects that are extremely faint, distant or difficult to identify with confidence.
Mason instead used the Besançon Galactic Model, a simulation designed to estimate the distribution and characteristics of stars throughout the Milky Way. This allowed her to calculate the likely number of stars captured in each observation, including many that do not appear in existing catalogues.
When the method was applied to an earlier SETI survey containing 1,327 telescope pointings, the estimated number of stars included in the search rose dramatically. Gaia data had identified about 288,000 stars, while the galactic model suggested that more than 6.1 million stars may actually have been observed.
According to Mason, the new estimate provides a more complete picture of how much of the Milky Way has already been examined for technosignatures.
“One of the most exciting things about this work is realizing that we’ve surveyed many more stars than initially thought,” she said.
“Even a very small observation can contain a huge number and diversity of stars that we might never have intended to study. By combining high-frequency observations with galactic simulations, we can better understand exactly what we’ve searched and where we should look next.”
No Signal Does Not Mean No Life
Mason cautions that the lack of a detected signal should not be interpreted as evidence that intelligent life does not exist elsewhere. The search covered only a small number of observations and two limited frequency windows, and no candidate signal was found within those specific ranges.
Instead, she hopes the work will encourage astronomers to expand SETI surveys across a broader portion of the radio spectrum. It also shows how existing telescope archives could be reused to search for possible signs of technology without requiring entirely new observing campaigns.
The work was done in collaboration with Professor Michael Garrett, Dr. Andrew Siemion and Dr. Kelvin Wandia.
The poster ‘Strategies Utilising High-Frequency Interferometric Data to Explore SETI Parameter Space’ is part of the Statistical Challenges for Next-Generation Astronomical Surveys session at NAM2026.
