When astronomers point a telescope at a distant galaxy, they often catch far more than they bargained for. Stars they never intended to study slip into the field of view — what researchers call "stellar bycatch." Now, a new study suggests this accidental cosmic collection might be much larger than anyone realized, and that hidden stash could change how scientists search for alien life.
Louisa Mason, a PhD researcher at the University of Manchester, recently completed the Atacama Large Millimeter/submillimeter Array's first-ever SETI survey. SETI stands for Search for Extraterrestrial Intelligence — basically, scientists listening for signals from alien civilizations. But instead of requesting new telescope time, Mason did something clever: she dug through old observations that had already been collected for other astronomy research.
She focused on ALMA, a powerful telescope array in Chile that detects radio waves at very high frequencies, measured in millimeters and submillimeters. These frequencies sit far above the range that most SETI projects have traditionally searched. Most previous alien hunts have looked between 1.42 and 1.66 GHz — a region called the "water hole" because it falls between the natural radio signals given off by hydrogen and hydroxyl, two chemicals that combine to make water. Scientists liked this quiet band because an advanced civilization might recognize its significance and choose to broadcast there.
Mason wanted to know what else might be out there.
"For decades, SETI searches have concentrated on a relatively small part of the radio spectrum," 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."
Her scan of four archived ALMA observations found no signs of alien technology. But the real revelation came when she estimated how many stars those observations had actually captured. Traditional star catalogs like Gaia had listed roughly 288,000 stars in an earlier SETI survey with 1,327 telescope pointings. But when Mason used a different method — the Besançon Galactic Model, which simulates where stars are distributed across the Milky Way — the number jumped to an astonishing 6.1 million.
"One of the most exciting things about this work is realizing that we've surveyed many more stars than initially thought," Mason said. "Even a very small observation can contain a huge number and diversity of stars that we might never have intended to study."
The finding, presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham, suggests that existing telescope archives could be reused to expand the hunt for technosignatures — potential signs of alien technology — without building new observatories. Mason emphasizes that finding nothing in her limited search does not mean intelligent life is not out there. It simply means scientists have barely scratched the surface of what the radio spectrum has to offer. Her hope is that the work will inspire astronomers to cast a wider net — and maybe, just maybe, pick up a signal hiding in plain sight.