Last year, the Breakthrough Listen Initiative – the largest scientific research program to look for evidence of civilizations beyond Earth – released its deepest catalog yet on the search for extraterrestrial intelligence (SETI). Searching for radio signals or technosignatures, it looked at 1,327 individual stars located within 160 light-years from Earth. However, a team of researchers realized this catalog could be massively expanded without requiring any new observations.
The original radio observations were conducted with the Green Bank Telescope (GBT) in West Virginia and the CSIRO Parkes Radio Telescope in Australia. Researchers from the University of Manchester worked out that by combining that data with observations from the Gaia observatory they could increase the number of stars in the catalog by almost 220 times, going from 1,327 stars analyzed to over 288,000.
The beams of the telescopes didn’t cover a huge area in the sky but once you consider how far signals might come from, you suddenly have quite a lot of depth. Gaia measured the distances to almost 1 billion stars, so the researchers matched those locations to the region probed by the radio telescopes.
The new catalog has expanded the numbers of stars analyzed to 288,315 stars in total with distances up to 33,000 light-years. The increase in distance would require that any possible aliens transmitting signals have better and more powerful equipment than what humanity currently possesses. This dramatic expansion allowed astronomers to put some stringent values on how common intelligent life is out there. Their findings are detailed in a preprint available on the ArXiv, accepted for publication in the Monthly Notices of the Royal Astronomical Society.
“Our results help to put meaningful limits on the prevalence of transmitters comparable to what we ourselves can build using 21st-century technology,” lead author Bart Wlodarczyk-Sroka said in a statement. “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.”
Despite the tight constraints, there might be intelligent civilization in the nearby universe. There are tens of thousands of stars within 330 light-years.
“This work shows the value of combining data from different telescopes,” noted Siemion. “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, Bart’s 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?”
The answer as always will need more observations.