Researchers scanning the skies for signals produced by beings on other worlds have provided some of the most sensitive constraints to date on the presence of artificial radio transmitters in our galaxy, and according to the analytical breakthrough, the outlook for finding a technologically advanced planet nearby is not so good.
In 2017, scientists at the Breakthrough Listen Initiative embarked upon a project to study a broad sample of nearby main-sequence stars to either find, or place limits on the chances of finding, a radio signal which was not known to be generated by any natural astrophysical process, I.e. it was generated by an extra-terrestrial lifeform.
The team selected 1709 stars for the survey and began work by studying 692 of the targets with the Green Bank 100-metre Telescope in West Virginia, US.
These stars were all located within 50 parsecs of the Earth; one parsec is about 30 trillion kilometres, or just over three light-years. For comparison, Proxima Centauri, the nearest known star to earth other than the sun, is about 1.3 parsecs (4.24 ly) away.
Although the results from this initial survey identified 11 events that could have come from extra-terrestrial radio sources, a more detailed analysis of each event found that the signals were generated by humans and not from other lifeforms on another far-off planet.
Undeterred, a couple of years later a different team scanned more stars and this time included the Parkes radio telescope in New South Wales, Australia in the search, but again the study concluded that none of the 1327 primary targets that were selected had a powerful enough radio transmitter that humans on Earth could detect.
The trouble is, it is hard for radio telescopes to hone in on a central target star without including large areas of the sky around it; areas that can include a big number of background stars and even a significant number of foreground stars too.
As such, pinpointing a specific signal amongst the myriad of radio signals emitted from a star is very difficult and this is partly caused by not knowing the distance to a particular stellar object in order to weed it out from the results.
Fortunately, in 2013 ESA launched its Gaia telescope which is designed to perform an accurate measurement of the positions, motions, and magnitudes of over a billion stars in the Milky Way.
Using this astrometric data from Gaia, that includes inferred distances to all the stars monitored by the spacecraft so far, a team of researchers at the Manchester of University, UK have been able to dramatically expand the search for extra-terrestrial life via radio signals from 1327 stars to 286,000.
“Knowing the locations and distances to these additional sources [via Gaia], greatly improves our ability to constrain the prevalence of extra-terrestrial intelligence in our own galaxy and beyond,” says Team leader Mike Garrett.
The team also extended their search by selecting stars out to much larger distances (up to about 33,000 light years) than the original sample of nearby stars.
The results, for those hoping to find alien life nearby are disappointing though – less than 0.04 percent of stellar systems have the potential of hosting advanced civilisations with the equivalent or slightly more advanced radio technology than 21st century humans.
“We now know that fewer than one in 1600 stars closer than about 330 light years host transmitters just a few times more powerful than the strongest radar we have here on Earth,” says co-author Bart Wlodarczyk-Sroka, a Masters student at the University.
Beyond a few hundred parsecs though and random fluctuations in the intensity of radio waves, a phenomena known as interstellar scintillation, is likely to cause problems with signals received on Earth, making it harder to discern the real thing.
As well as improving the limits for nearby stars, the team have for the first time placed limits on more distant stars, but it still doesn’t bode well for finding advanced life on the outer fringes of the galaxy.
“Inhabited worlds with much more powerful transmitters than we can currently produce must be rarer still,” adds Wlodarczyk-Sroka.
The technique used by the team can only locate intelligent and technically advanced civilisations that use radio waves as a form of communication; simple life or non-technical civilisations cannot be spotted with this method, says Garrett and colleagues.
Although the prospect of finding intelligent life close to home is looking bleak, the Breakthrough Listen Initiative has plans to survey up to one million nearby stars in its quest to find radio signals from advanced civilisations and this method used by the University of Manchester team could help boost those results on a quicker timescale.
“We expect future SETI surveys to also make good use of this approach” concluded Garrett.
Garrett and colleagues work has now been published in Monthly Notices of the Royal Astronomical Society.