Last December news leaked that the Breakthrough Listen project, part of the Search for Extraterrestrial Intelligence (SETI), had picked up an unexplained signal from the direction of Proxima Centauri. Although everyone involved stressed how unlikely it was that our first evidence for alien intelligence would come from the nearest star to our Sun, some dared to hope. Further research, however, has made Earth-based interference a near-certain explanation.

There are many reasons to study Proxima Centauri besides the possibility of technological radio emissions. Australia’s giant Murriyang radio telescope was pointed towards the star primarily to study stellar flares, but in the process collected a wealth of other data. In October 2020, Breakthrough Listen intern Shane Smith noticed something very odd at 982 MHz in recordings made in April 2019, which was subsequently given the name BLC1. Having ruled out all the most common explanations, Smith and colleagues were still looking into more exotic options in December when news leaked. 

Now, however, two papers have been published in Nature Astronomy revealing the disappointing truth. Not only is there no evidence for aliens in our cosmic backyard, but BLC1 wasn’t even the product of a previously unknown natural astronomical phenomenon, which would have been an excellent compensation prize.

More than 4 million narrowband signals were detected during the time Murriyang was pointed at Proxima Centauri and a comparison point in the sky, the first paper reports. However, most of these were immediately ruled out since they were detected in both directions, indicating the cause was local interference. A remaining 5,160 so-called “events” were picked while pointed at Proxima, but not the comparison point, but only one survived filtering tests that eliminate easily explained cases.

“The event does not lie within the frequency range of any known local radiofrequency interference (RFI), and has many characteristics consistent with a putative transmitter located in another stellar system,” the paper notes. This made it the most promising candidate for an alien signal since the Wow! signal in 1977, but from the start, few thought this was the real thing.

The second paper confirms that skepticism was wise. It describes the processes undertaken to investigate possible explanations for BCL1, including seeking similar signals in historical data and follow-up observations of Proxima.

Approximately 60 BLC1 “look-alikes” were found in other directions and by other telescopes. “The signals are spaced at regular frequency intervals in the data, and these intervals appear to correspond to multiples of frequencies used by oscillators that are commonly used in various electronic devices,” Dr Sofia Sheikh of UC Berkeley, lead author of the second paper, said in a statement. 

BLC1 is thought to have more complex origins than most of the counter-parts, otherwise the signal would have remained when Murriyang took data from its reference point. It is thought likely different sources of local radio-frequency interference mixed together to create BLC1, although exactly what these were remains unsolved.

“This is undoubtedly one of the most intriguing signals we’ve seen to date,” said Breakthrough Listen's Dr Andrew Siemion. Nevertheless, “It’s highly unlikely that it is really from a transmitter out at Proxima Centauri.”

Breakthrough Listen focused their attention on Proxima Centauri in the first place because it is known to host at least one planet in what is usually considered the habitable zone, where temperatures are right for liquid water. However, Proxima also erupts in intense flares and coronal mass ejections, which many astronomers think would strip nearby planets of their atmospheres, and sterilize any life that somehow emerged. The project was designed to investigate these flares in greater detail to explore whether there is any hope for an atmosphere to survive.