Are aliens running out of places to hide? It turns out we've searched much more of the galaxy than we realized

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When people grow impatient at the lack of success in the search for alien life, researchers often point to how few of the Milky Way's 100 billion stars or so have been searched so far. But it may be that the search for extraterrestrial intelligence (SETI) has surveyed more stars than astronomers had realized.

Louisa Mason, a PhD student at the University of Manchester, had a recent epiphany that astronomers have actually searched much more of our galaxy for radio signals than we thought. Mason came to the conclusion by using a specific simulation of the Milky Way, called the Besançon Galactic Model, and then comparing it to surveys of the sky conducted by the Green Bank and Parkes radio telescopes. Based on star catalogues, it was calculated that the two telescopes had seen 288,315 stars seen in 1,327 observations of the sky. However, by applying the Besançon Galactic Model, Mason discovered that the survey had actually included more than 6.1 million stars, most too faint to be obvious.

"One of the most exciting things about this work is realizing 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." But while Mason's calculations show that we've actually surveyed many more of those stars than we realized, there's no need to grow disheartened by the lack of a detection.

When a radio telescope points towards a target star, it isn't just listening to that star, but is also eavesdropping on all the other stars in the field of view. This is very useful for a practice called commensal SETI, whereby a SETI instrument piggybacks on a radio telescope's usual day-to-day observations.

Typically, the number of stars in a radio telescope's field of view is estimated by comparing it to detailed star catalogues such as those compiled by the European Space Agency's Gaia mission. However, these catalogues are limited because they only include stars bright enough to be seen; there are many more stars too faint to be seen by our optical and infrared telescopes. Yet if a radio telescope is pointed in their direction, it could still detect a radio signal from them even if we cannot see the star.

Although the search may be bigger in terms of the number of stars, it is still very small in terms of the time spent listening to each star, and is still limited in the radio frequency range covered. We could very easily have missed a signal by observing between broadcasts, or at the wrong frequency.

So Mason has also looked to expand the range of radio frequencies that can be searched. Ever since modern SETI began in 1960, the bulk of SETI searches have focused on a frequency range known as the 'water hole', between the emission frequency of atomic hydrogen at 1,420 MHZ, and hydroxyl (a molecule composed of one atom of hydrogen and one atom of oxygen) at 1,666 MHz. The name 'water hole' comes about because if you combine hydrogen with hydroxyl you get a water molecule (two hydrogen atoms and one oxygen atom).

One of the reasons the water hole has been so favored is because radio waves in this frequency range are not absorbed by Earth's atmosphere. Furthermore, a lot of astronomy is conducted around the hydrogen frequency, so there's a good chance of a coincidental SETI detection. Plus, aliens might recognize the importance of water to life and the symbolism of transmitting in that frequency range.

However, there are many other frequencies that could be looked at, and in that vein Mason has conducted the first ever SETI search with ALMA, the Atacama Large Millimeter/submillimeter Array in Chile.

"For decades, SETI researchers have concentrated on a relatively small part of the radio spectrum," said Mason. "We wanted to ask what might happen if we looked somewhere very different."

Mason didn't perform new observations with ALMA, but searched a small amount of archive data for any narrowband signals that could easily have been missed. ALMA operates at shorter radio wavelengths and therefore correspondingly higher frequencies than most regular radio telescopes.

One of the advantages of looking at higher frequencies is that they suffer less from dispersion, which is when lower frequency waves interacting with electrons in space are delayed and arrive at their destination a little later than higher frequencies, therefore distorting the signal overall.

Although Mason didn't come up with any promising new signals in her ALMA search, she remains optimistic about using higher frequencies.

"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," she said.

Mason presented her work at the Royal Astronomical Society's National Astronomy Meeting in Birmingham in July. The research is also described in two papers – one on the ALMA survey, the other on the Besançon Galactic Model, in Monthly Notices of the Royal Astronomical Society.