Astronomers may have unknowingly searched far more of Milky Way for alien signals

“One of the most exciting things about this work is realising that we’ve surveyed many more stars than initially thought,” Mason said in a statement. “Even a very small observation can contain a huge number and diversity of stars that we might never have intended to study.”

The discovery stems from the way radio telescopes operate. When aimed at a target star, they simultaneously receive signals from every star within their field of view. This makes them particularly useful for commensal SETI, where searches for alien signals are conducted alongside routine astronomical observations.

Traditional estimates rely on visible star catalogues, such as those produced by the European Space Agency’s Gaia mission. However, these catalogues exclude countless dim stars that remain invisible to optical telescopes but could still produce detectable radio signals.

Despite the larger search area, researchers caution that the hunt remains limited. Most stars have only been observed for brief periods, meaning any transmission outside those observation windows would have gone unnoticed. In addition, SETI searches have largely focused on a narrow section of the radio spectrum.

Symbolic significance related to water

For decades, scientists have concentrated on the so-called water-hole frequencies between 1,420 MHz and 1,666 MHz, a region considered favourable because radio waves there travel efficiently through Earth’s atmosphere and carry symbolic significance related to water, a key ingredient for life.

Mason is now exploring frequencies beyond that range. She recently carried out the first SETI search using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, which operates at much higher radio frequencies than conventional SETI instruments.

“For decades, SETI researchers 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.

Although the ALMA search did not uncover any promising technosignatures, Mason believes the higher-frequency bands remain largely untapped.

“The millimetre 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,” she added.

The findings were presented at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham and are detailed in two papers published in Monthly Notices of the Royal Astronomical Society.