Astronomers looking for evidence of intelligent life beyond Earth may have missed promising signals simply because they were focused on a relatively narrow part of the radio spectrum.
Most wireless SETI (Searching for Extraterrestrial Intelligence) projects target frequencies between 1.42 and 1.66 GHz. This region is often referred to as the “water hole” because it lies between the natural radio emissions produced by hydrogen and hydroxyls that combine to form water.
Researchers have long thought that this quiet region of the spectrum could be a logical gathering place for interstellar communications. Advanced civilizations may recognize the importance of hydrogen and hydroxyl and choose to send or monitor signals there.
Searching for the other side of the water hole in space
New research suggests that higher radio frequencies may provide another valuable venue for searching for technological signals from distant civilizations. The findings will be presented this week at the Royal Astronomical Society’s National Astronomical Conference in Birmingham.
Postdoctoral researcher Luisa Mason from the University of Manchester conducted the first telescopic SETI survey using archived observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile.
Instead of requesting time for a new telescope, Mason examined observations originally collected for unrelated astronomical research. She searched for data on narrowband radio signals. This may be more consistent with artificial technology than with natural processes in the universe.
“For decades, SETI searches have focused on relatively small parts of the radio spectrum. We wanted to see what would happen if we looked in completely different locations,” Mason said.
“The millimeter-wave and sub-millimeter-wave radio bands remain almost completely unexplored for SETI, so this really explores a new area of parameter space.”
ALMA opens new frontiers for SETI
Mason looked at two narrow frequency ranges within ALMA’s Band 3 observations. This search did not identify any candidate technosignatures (alien signals) that exceeded the investigation’s detection threshold.
This project was limited to only four archived ALMA observations. Still, Mason said the results show that telescopes operating at higher radio frequencies could be useful tools in future SETI efforts.
This study also draws attention to an often overlooked feature of radio astronomy. When a telescope is pointed at one celestial object, its field of view usually includes many more stars in the surrounding area.
These unintentionally observed stars are sometimes called “star bycatch.”
Millions of stars hidden in telescope data
Astronomers have traditionally used catalogs of stars such as Gaia to estimate the amount of “stellar bycatch” in their observations. However, these catalogs do not include all the stars in the field, especially objects that are very faint, distant, or difficult to identify with confidence.
Mason instead used the Besancon Galaxy Model, a simulation designed to estimate the distribution and characteristics of stars throughout the Milky Way. This allowed us to calculate the number of stars that could be captured in each observation, including stars that are not included in existing catalogs.
When this method was applied to an initial SETI survey involving 1,327 telescope pointings, the estimated number of stars included in the search increased dramatically. Gaia data identified about 288,000 stars, but galaxy models suggest that more than 6.1 million stars may have actually been observed.
Mason said the new estimates provide a more complete picture of how much of the Milky Way galaxy has already been examined for technosignatures.
“One of the most exciting things about this study was the realization that we investigated far more stars than we originally thought,” she said.
“Even very small observations can contain vast numbers of diverse stars that we never intended to study. By combining high-frequency observations with galaxy simulations, we can better understand what to explore and where to look next.”
Just because there’s no signal doesn’t mean there’s no life
Mason cautions that the lack of a 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 signals were found within these specific ranges.
Instead, she hopes this study will encourage astronomers to expand SETI surveys to wider parts of the radio spectrum. We also show how existing telescope archives can be reused to search for possible signs of technology without requiring entirely new observational campaigns.
The research was carried out in collaboration with Professor Michael Garrett, Dr Andrew Siemion and Dr Kelvin Wandia.
The poster “Strategies for using high-frequency interferometric data to explore the SETI parameter space” is part of the Statistical Challenges for Next Generation Astronomical Surveys session at NAM2026.
