The search for extraterrestrial intelligence (SETI) is often criticised for covering only a tiny fraction of the Milky Way. However, new research shows that astronomers have already surveyed far more stars than previously believed, reports Space.com.

How many stars have actually been surveyed

Louise Mason, a postgraduate researcher at the University of Manchester, used the Besançon Galaxy Model and compared it with data from the Green Bank and Parkes radio telescopes. According to star catalogues, about 288,000 objects had been recorded across 1,327 observations. However, the model revealed that the actual field of view contained more than 6.1 million stars—most of them too faint to have been detected by optical telescopes.

"One of the most exciting findings of this work is realising that we have surveyed far more stars than we initially thought. Even a very small observation can contain a huge number and variety of stars that we never intended to study," Mason noted.

When a radio telescope is pointed at a specific target, it simultaneously "listens" to all objects within its field of view. This enables so‑called commensal SETI—the search for signals in parallel with routine astronomical observations.

Why no signals have been found after all

Despite the larger number of stars, the listening time per star remains very short, and the frequency range is limited. A signal could easily have been missed if it occurred at a different time or on a different frequency.

Traditionally, SETI has focused on the so‑called "water hole"—the range between 1,420 and 1,666 MHz. These are the emission frequencies of hydrogen and hydroxyl. They were chosen because they pass well through Earth's atmosphere, are widely used in astronomy, and are symbolically linked to water—the basis of life.

A new range: millimetre waves

Mason conducted the first‑ever SETI search using the ALMA array in Chile. She analysed archival data in the millimetre and submillimetre ranges—at higher frequencies than usual.

"For decades, SETI researchers have concentrated on a relatively small portion of the radio spectrum. We wanted to see what would happen if we looked in a completely different direction," she said.

Higher frequencies are less affected by dispersion—the distortion of signals as they pass through the interstellar medium. Although no interesting signals were found in the ALMA archival data, the approach opens up a virtually unexplored region.

"The millimetre and submillimetre radio bands are almost entirely unexplored in SETI, so this is a genuine expansion of the search parameter space," Mason emphasised.

The results were presented at the Royal Astronomical Society's National Astronomy Meeting and are described in two papers published in the Monthly Notices of the Royal Astronomical Society.

In brief

New research has shown that the Green Bank and Parkes radio telescopes have already covered more than 6 million stars—several times more than catalogue‑based estimates suggested. This expands the scale of the search for extraterrestrial signals. At the same time, scientists have for the first time applied the millimetre range—using ALMA—to SETI, opening a nearly unexplored domain. Although no detections were made, the work demonstrates that the search can be conducted far more broadly and deeply than previously thought.