Astronomers searching for extraterrestrial intelligence may be overlooking signals in a largely unexplored region of the radio spectrum. Most radio SETI surveys have concentrated on frequencies between 1.42 and 1.66 GHz, a range known as the “water hole.” It lies between the natural emission frequencies of hydrogen and hydroxyl, which together form water. Because this part of the spectrum is relatively quiet and symbolically significant, researchers have long considered it a logical place for advanced civilizations to transmit.

A new study, however, suggests that higher radio frequencies could offer another promising search strategy. University of Manchester PhD researcher Louisa Mason used archived observations from the Atacama Large Millimeter/submillimeter Array in Chile to conduct the first SETI survey with ALMA. Rather than requesting new telescope time, Mason analyzed data originally collected for other astronomical projects. She searched two narrow frequency windows in ALMA’s Band 3 for narrowband signals that might indicate technology rather than natural astrophysical activity. No candidate technosignatures were found above the survey’s detection thresholds. Although the study included only four archived observations, it demonstrates that millimeter and submillimeter wavelengths could become valuable additions to future SETI programs.

The research also examined “stellar bycatch,” the many stars unintentionally captured whenever a telescope observes a target. Instead of relying only on the Gaia catalogue, Mason used the Besançon Galactic Model to estimate stars too faint, distant, or uncertain to appear in existing records. Applied to an earlier survey of 1,327 pointings, this method increased the estimated number of searched stars from about 288,000 to more than 6.1 million.

The findings show that archived observations may cover far more of the galaxy than previously recognized. While no alien signal was detected, the study encourages broader frequency searches and better use of existing radio astronomy data in future searches for intelligent life.

For more details, read tje full article by Royal Astronomical Society.