Reanalysis of Breakthrough Listen Data to Include Other Stellar Objects in the Field Produces Most Comprehensive SETI Search to Date

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Green Bank

Breakthrough Listen (the initiative to find signs of intelligent life in the universe) and the University of Manchester announced today a reanalysis of existing data that represents a new milestone in the search for extraterrestrial intelligence (SETI).

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 (and the minds that constructed them).

Breakthrough Listen's previous strongest limits came from two analyses published in 2017[1 ] and 2020[2] by the Listen science team. Based primarily at the University of California, Berkeley, SETI Research Center, the team is responsible for carrying out Listen's primary program of observations on the Green Bank Telescope (GBT) in West Virginia and the CSIRO Parkes Radio Telescope in Australia, in addition to other facilities around the globe.

In their earlier papers, the Berkeley team looked for technosignatures in radio data gathered when GBT and Parkes were pointed in the directions of 1327 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. Masters student Bart Wlodarczyk-Sroka and his advisor Prof. Michael Garrett at the University of Manchester in the United Kingdom, collaborating with Berkeley SETI Director Dr. Andrew Siemion (who is also a Visiting Professor at Manchester) 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.r

Combing through the catalog produced by the European Space Agency's Gaia spacecraft, which measured the distances to over a billion stars, the researchers recalculated limits on the prevalence of transmitters around additional stars within the GBT and Parkes 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 1327 to 288,315. A small fraction of these additional stars (around 196 in total) were closer than the 160 light year range of the original sample (some of them in binary systems, and some unrelated to the original primary targets). Most, however, were more distant than 160 light years, meaning that only more powerful transmitters could be visible at these increased ranges.

1 https://breakthroughinitiatives.org/news/10
2 https://breakthroughinitiatives.org/news/25

However, the sheer number of stars studied enabled Wlodarczyk-Sroka 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, the team 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.

Team leader Mike Garrett had always been troubled that SETI searches didn't usually take into account the many other cosmic objects that fall within the range of sky a telescope is sensitive to, in addition to the main target. According to Garrett, Gaia has changed all that. "Knowing the locations and distances to these additional sources," he says, "greatly improves our ability to constrain the prevalence of extraterrestrial intelligence in our own galaxy and beyond. 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 21st century technology," remarked Wlodarczyk-Sroka. "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. Inhabited worlds with much more powerful transmitters than we can currently produce must be rarer still."

"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 paper, "Extending the Breakthrough Listen nearby star survey to other stellar objects in the field", has been accepted for publication in Monthly Notices of the Royal Astronomical Society. A preprint is available at https://arxiv.org/pdf/2006.09756.pdf. Supplementary material, including the catalog of Gaia stars used in the analysis, along with an accompanying video and artwork, are available at http://seti.berkeley.edu/deeper.

Jodrell Bank Center for Astrophysics: http://www.jodrellbank.manchester.ac.uk/
Berkeley SETI: http://seti.berkeley.edu
Breakthrough Initiatives: https://breakthroughinitiatives.org/

Breakthrough Listen is a scientific program in search for evidence of technological life in the Universe. It aims to survey one million nearby stars, the entire galactic plane and 100 nearby galaxies at a wide range of radio and optical bands.

The Breakthrough Initiatives are a suite of scientific and technological programs, founded by Yuri Milner, investigating life in the Universe. Along with Breakthrough Listen, they include Breakthrough Watch, an optical search for Earth-like planets in the habitable zones of nearby stars; and Breakthrough Starshot, the first significant attempt to design and develop a space probe capable of reaching another star.

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