Though the researchers are not done with their study, there has been speculation that the signal is a "technosignature" produced by an alien civilization. Proxima Centauri has at least one planet that may be habitable to life, and while mundane explanations are far more likely, the scientists have yet to identify a terrestrial cause such as ground-based equipment or a passing satellite.
"[For] the moment, the only source that we know of is technological," lead investigator Andrew Siemion, an astrophysicist at the University of California, Berkeley, told Scientific American.
The Breakthrough Listen team has been scanning skies for technosignatures – indicators of technology developed by advanced alien civilizations. Prior to their discovery, the team's effort had been fruitless, as many promising detections turned out to be man-made.
But one signal, labeled Breakthrough Listen Candidate 1 (BLC1), stood out among the several radio signals the researchers picked up while examining data taken using the Parkes Observatory in Australia during a 2019 search for stellar flares from Proxima Centauri. The star has two known planets in orbit: an inhabitable gas giant and an Earth-like rocky planet, called Proxima b, located in the star system's habitable zone, an orbital region around a star where liquid water can exist on a planet.
The researchers determined that all of the signals except BLC1 were caused by human-made technology like satellites. The outlier signal, however, lasted about three hours and was concentrated in a narrow range of wavelengths – 982 megahertz – which isn’t generally used for satellites and spacecraft. There is also no known natural astronomical event that could generate a signal at such wavelengths.
"We don’t know of any natural way to compress electromagnetic energy into a single bin in frequency," Siemion said.
Moreover, the signal reappeared after the telescope was shifted to another part of the sky and turned back to Proxima Centauri during a calibration procedure to check for interference from local sources such as microwave ovens. This hinted that the signal did originate from Proxima Centauri – or some other deep-space source in that part of the sky – before making its way to Earth.
BLC1 was the first signal to pass through the researchers' first round of checks, which are mainly designed to eliminate signals that actually originate from Earth. (Related: Mysterious blobs of radio emissions found floating in space.)
"It’s the most exciting signal that we’ve found in the Breakthrough Listen project because we haven’t had a signal jump through this many of our filters before," said astronomer Sofia Sheikh of Pennsylvania State University, who led an analysis of the signal for Breakthrough Listen which will be detailed in an upcoming paper.
Though the Breakthrough Listen team still has no explanation for the signal, they have been reminding that an alien origin is highly unlikely and that the signal could very well be caused by a natural or man-made phenomenon that remains poorly understood.
Pete Worden, executive director of the Breakthrough Initiatives, said: "The Breakthrough Listen team has detected several unusual signals and is carefully investigating. These signals are likely interference that we cannot yet fully explain. Further analysis is currently being undertaken."
Scientists also reiterated that Proxima b sits too close to Proxima Centauri. It completes an orbit every 11 days and is tidally locked like the moon, meaning one side of it is always bombarded by radiation and the other plunged into perpetual darkness.
"It’s hard to imagine how you can have a stable climatic system and all the things you need to get from bacteria, which are hardy, up to intelligent animal life forms, which certainly are not," said astrobiologist Lewis Dartnell of the University of Westminster.
Read more stories about the search for alien life at Cosmic.news.