In 2019, astronomers with the Breakthrough Listen project observed Proxima Centauri using CSIRO’s Parkes ‘Murriyang’ radio telescope. They detected an intriguing narrowband signal with characteristics consistent with a extraterrestrial technosignature near 982 MHz. Using a new procedure for the analysis of potential engineered signals, they have now found that the signal is not an extraterrestrial technosignature, but rather an artifact of Earth-based interference from human technologies.
An artist’s impression of the Proxima Centauri system. Image credit: Lorenzo Santinelli.
“The significance of this result is that the search for civilizations beyond our planet is now a mature, rigorous field of experimental science,” said Yuri Milner, founder of Breakthrough Inititatives.
Proxima Centauri is an astrobiologically fascinating target due to its proximity: (i) it is the closest star to the Sun at 4.22 light-years; (ii) it hosts at least two exoplanets, Proxima b and c; and (iii) it has even featured as the target of a proposed in situ search of extraterrestrial life via the Breakthrough Starshot Initiative.
Despite being our nearest stellar neighbor, few technosignature searches have been conducted towards Proxima Centauri.
In the 1990s, two SETI programs were conducted in the southern hemisphere towards nearby stars: the Project Phoenix search of 202 solar-like stars and a search for technosignatures from 176 of the brightest stars.
From April 29 to May 4, 2019, the Breakthrough Listen astronomers conducted a search for signs of technologically advanced life from the direction of Proxima Centauri using the Parkes ‘Murriyang’ radio telescope.
They scanned the planetary system across a frequency range of 700 MHz to 4 GHz, with a resolution of 3.81 Hz — in other words, performing the equivalent of tuning to over 800 million radio channels at a time, with exquisite detection sensitivity.
They detected a total of 4,172,702 hits — that is, narrowband signals detected above a signal-to-noise threshold.
Of these, only one signal — at 982 MHz — passed all rounds of filtering and visual inspection and didn’t lie within the frequency range of any known local radiofrequency interference.
Dubbed BLC1 (short for ‘Breakthrough Listen Candidate 1’), it had many characteristics consistent with a putative transmitter located in another stellar system.
“The original signal is not obviously detected when the telescope is pointed away from Proxima Centauri — but given a haystack of millions of signals, the most likely explanation is still that it is a transmission from human technology that happens to be ‘weird’ in just the right way to fool our filters,” said Dr. Sofia Sheikh, a postdoctoral researcher in the Department of Astronomy at the University of California, Berkeley, and a member of the Breakthrough Listen team.
The signal of interest BLC1 from Proxima Centauri. Image credit: Smith et al., doi: 10.1038/s41550-021-01479-w.
Dr. Sheikh and colleagues found that BLC1 is not an extraterrestrial technosignature, but rather an electronically drifting intermodulation product of local, time-varying interferers.
They also identified dozens of instances of radio interference with similar morphologies to the BLC1 signal at frequencies harmonically related to common clock oscillators.
“In the case of this particular candidate, our analysis suggests that it’s highly unlikely that it is really from a transmitter out at Proxima Centauri,” said Dr. Andrew Siemion, an astronomer in the Department of Astronomy at the University of California, Berkeley, SETI Institute, the Department of Physics and Astronomy at the University of Manchester, and a member of the Breakthrough Listen team.
“However, this is undoubtedly one of the most intriguing signals we’ve seen to date.”
“While we were unable to conclude a genuine technosignature, we are increasingly confident that we have the necessary tools to detect and validate such signatures if they exist,” said Dr. S. Pete Worden, executive director of the Breakthrough Initiatives.
The team’s results appear in two papers in the journal Nature Astronomy.
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S. Smith et al. A radio technosignature search towards Proxima Centauri resulting in a signal of interest. Nat Astron, published online October 25, 2021; doi: 10.1038/s41550-021-01479-w
S.Z. Sheikh et al. Analysis of the Breakthrough Listen signal of interest blc1 with a technosignature verification framework. Nat Astron, published online October 25, 2021; doi: 10.1038/s41550-021-01508-8