Astronomers searching for signs of extraterrestrial intelligence may have been missing alien signals in part of the radio spectrum that has not recently been explored.
Most radio SETI (Search for Extraterrestrial Intelligence) surveys have focused on frequencies between 1.42 and 1.66 GHz. This range is known as the 'water hole' because it lies between the natural radio frequencies emitted by hydrogen and hydroxyl, two molecules whose combination forms water.
Scientists have long thought that this relatively quiet part of the radio spectrum would be a logical place to communicate, as a technologically advanced civilisation might recognise the significance of hydrogen and hydroxyl and be likely to transmit and listen there.
But a new study suggests that higher radio frequencies could provide an important new avenue in the search for possible technological signals from other civilisations. The research is being presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham .
Using archived observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, astronomer Louisa Mason, a PhD researcher at the University of Manchester, carried out the first-ever SETI survey using the telescope. Rather than making new observations, she analysed existing data originally collected for other astronomical purposes.
She looked for narrowband radio signals that might indicate the presence of technology rather than natural astrophysical processes.
"For decades, SETI searches 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.
"The millimetre and submillimetre radio bands remain almost completely unexplored for SETI, so this is really about opening up a new area of parameter space to search."
Mason searched two small frequency windows in ALMA's Band 3 observations but found no candidate technosignatures (alien signals) above their thresholds.
Although the survey examined only four archived ALMA observations, Mason says the work demonstrates that high-frequency radio telescopes could play an important role in future SETI programmes.
The research also highlights an overlooked opportunity hidden within every radio observation: when astronomers point a telescope at a single target, they also capture many other stars within the telescope's field of view.
Traditionally, researchers have estimated this 'stellar bycatch' using catalogues such as Gaia. Instead, Mason used the Besançon Galactic Model to estimate the full stellar population contained within each observation, including stars too distant, too faint or too difficult to identify reliably in existing catalogues.
Applying the technique to a previous SETI survey of 1,327 telescope pointings increased the estimated number of stars included in the search from around 288,000 identified using Gaia to more than 6.1 million using the galactic model.
Mason says this provides a much more realistic picture of how much of the galaxy has actually been surveyed for technosignatures.
"One of the most exciting things about this work is realising that we've surveyed many more stars than initially thought," she said.
"Even a very small observation can contain a huge number and diversity of stars that we might never have intended to study. By combining high-frequency observations with galactic simulations, we can better understand exactly what we've searched and where we should look next."
Mason emphasises that the absence of any detected signal does not mean intelligent life does not exist, only that no candidate signal was found within the small frequency ranges examined in this study. Instead, she hopes the work will encourage future SETI surveys to search more widely across the radio spectrum and make better use of existing astronomical observations.
The work was done in collaboration with Professor Michael Garrett, Dr Andrew Siemion and Dr Kelvin Wandia.
