SETI May Have Spent 60 Years Listening on the Wrong Channel

For most of its history, the search for extraterrestrial intelligence has concentrated on a narrow strip of the radio dial. New work presented at the Royal Astronomical Society's National Astronomy Meeting in Birmingham argues that choice may have been a serious blind spot.

Since the 1960s, SETI surveys have focused on frequencies between roughly 1.42 and 1.66 GHz — a band astronomers call the "water hole," because it sits between the natural emission lines of hydrogen and hydroxyl, which together make water. The reasoning was that any water-based civilization would recognize the same landmark and broadcast there. It is an elegant argument, and it has never been tested against the alternatives, because the millimeter and submillimeter bands above it have gone almost entirely unsearched.

Louisa Mason, a PhD researcher at the University of Manchester, went looking there. Rather than competing for scarce new telescope time, she pulled archived observations from the Atacama Large Millimeter/submillimeter Array in Chile — data collected years ago for unrelated astrophysics projects and never examined for artificial signals. She analyzed four archived ALMA datasets, searching two narrow frequency ranges within the array's Band 3. The work was done with Professor Michael Garrett, Dr. Andrew Siemion and Dr. Kelvin Wandia.

No candidate technosignatures turned up above the detection thresholds. That is the expected outcome of nearly every SETI search ever run, and it is not the interesting part of the result.

The interesting part is what the archive turned out to contain. When Mason applied her method to an earlier SETI survey covering 1,327 telescope pointings, Gaia catalogue cross-matching identified about 288,000 stars in the field of view. But a simulation using the Besançon Galactic Model — which accounts for the far larger population of stars too faint for Gaia to catalogue individually — estimated that more than 6.1 million stars had actually been observed. The surveys had been sampling more than twenty times as many stars as their own published counts claimed.

"Even a very small observation can contain a huge number and diversity of stars that we might never have intended to study," Mason said.

Two conclusions follow. The first is that high-frequency radio observatories, built for entirely different science, are viable SETI instruments — and that the parameter space of frequencies searched can be widened without asking any telescope committee for a single hour of new time. The second is that the archives of the world's radio observatories represent an enormous, already-paid-for SETI dataset sitting unexamined on disk.

The null result stands. But the search space just got substantially larger, and the cost of covering it just got substantially smaller.