|In a new study to be published in the Monthly Notices of the Royal Astronomical Society, a research team led by University of Manchester astronomers extended a sample of 1,327 stellar systems recently observed by the Breakthrough Listen Initiative by including additional 288,315 stars that also reside within the target fields of the Robert C. Byrd Green Bank Telescope in West Virginia and CSIRO’s Parkes radio telescope in Australia — increasing the number of stars analyzed by a factor of more than 200. Their results suggest that less than 0.04% of stellar systems have the potential of hosting advanced civilizations with the equivalent or slightly more advanced radio technology than 21st century humans.
SETI scientists search for technosignatures — indicators of technology developed by extraterrestrial intelligence — using cutting-edge instruments at some of the world’s most powerful telescopes.
No technosignatures have yet been detected, but as more and more comprehensive searches are carried out, astronomers can place tighter and tighter limits on how many stars in our neighborhood might be home to powerful radio transmitters.
In their earlier studies, astronomers from the Breakthrough Listen Initiative looked for technosignatures in radio data gathered when Green Bank and Parkes radio telescopes were pointed in the directions of 1,327 individual stars.
Their search focused on relatively nearby stars — within about 160 light-years from our Sun — because less powerful transmitters would become more easily detectable the closer they are to our telescopes.
However, as anyone who has looked at images of deep space knows, even small regions of the sky are full of stars at a range of distances from Earth.
When Breakthrough Listen searches for technosignatures coming from a nearby star, it is also sensitive to more powerful potential technosignatures from other stars within the telescope’s beam.
University of Manchester astronomers Bart Wlodarczyk-Sroka and Professor Michael Garrett and Berkeley SETI Director Dr. Andrew Siemion took advantage of this fact to determine new, more stringent limits on the prevalence of technosignatures, without the need to gather any new telescope data.
Combing through the catalogue produced by ESA’s Gaia spacecraft, which measured the distances to over a billion stars, they recalculated limits on the prevalence of transmitters around additional stars within radio telescopes’ fields of view.
By selecting stars out to much larger distances — up to about 33,000 light-years — than the original sample of nearby stars, they were able to expand the number of stars studied from 1,327 to 288,315.
“Knowing the locations and distances to these additional sources greatly improves our ability to constrain the prevalence of extraterrestrial intelligence in our own Galaxy and beyond,” Professor Garrett said.
“We expect future SETI surveys to also make good use of this approach.”
“Our results help to put meaningful limits on the prevalence of transmitters comparable to what we ourselves can build using twenty first century technology,” Wlodarczyk-Sroka added.
“We now know that fewer than one in 1,600 stars closer than about 330 light-years host transmitters just a few times more powerful than the strongest radar we have here on Earth.”
“Inhabited worlds with much more powerful transmitters than we can currently produce must be rarer still.”
The sheer number of stars studied enabled the researchers to place some of the most stringent limits to date on the prevalence of powerful radio transmitters in this region of our Galaxy.
In addition, for the first time, they have been able to do this as a function of stellar type — the extended sample includes not only a wide range of main-sequence stars, but also numerous giant stars and white dwarfs.
“This work shows the value of combining data from different telescopes,” Dr. Siemion said.
“Expanding our observations to cover almost 220 times more stars would have required a significant investment of our telescope time, not to mention the computing resources to perform the analysis.”
“By taking advantage of the fact that we already had radio scans of stars in the background of our primary targets, and by reading their positions and distances from the Gaia catalog, our analysis has extracted additional information from the existing dataset.”
“Work like this gets us one step closer to the goal of knowing the answer to humanity’s most profound question: Are we alone?”
B.S. Wlodarczyk-Sroka et al. 2020. Extending the Breakthrough Listen nearby star survey to other stellar objects in the field. MNRAS, in press; arXiv: 2006.09756