A radio transmission that appeared to come from the Proxima Centauri system, home to the nearest star to the Sun, was likely an artefact produced by human technology, according to researchers with the Breakthrough Listen project.
Two papers published yesterday in Nature Astronomy detail the events and processes that led to the declaration of a promising candidate signal from the Proxima Centauri system, as well as how scientists came to conclude that the signal was of our own making.
“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,” explained Andrew Siemion, an astronomer from the University of California Berkeley, in a press release. “However, this is undoubtedly one of the most intriguing signals we’ve seen to date.”
That radio-transmitting aliens might exist on one of the two known exoplanets in orbit around Proxima Centauri seemed too good to be true. The red dwarf is just 4.2 light-years away, making it the closest star system to our own. For an alien civilisation to exist so close to us in both time and space would’ve been pure bananas and a sure sign that space is not as empty of intelligence as it seems.
But there it was — a signal that lasted several hours, as detected by the 64.01 m radio telescope at the CSIRO Parkes Observatory in Australia. Researchers with the A$133 million Breakthrough Listen project, led by Siemion and funded by Yuri and Julia Milner, were looking for signs of flares on Proxima Centauri when the unexpected signal appeared. The one-off event happened on April 29, 2019 and was later spotted by Shane Smith, an undergrad at Hillside College in Michigan and an intern with Berkeley’s SETI project. The apparent signal, near 982 MHz, could not be easily dismissed as coming from a natural source, so it was labelled BLC01, or Breakthrough Listen Candidate 1 — the first for the project.
And by “candidate,” of course, we’re talking about a potential alien technosignature. A technosignature is anything we can detect on Earth that’s indicative of extraterrestrial technology, from radio and laser signals through to polluted atmospheres and megascale construction projects. We have yet to detect a true technosignature, of course, but SETI scientists are actively on the hunt. Radio signals continue to be a favoured transmission medium, but our civilisation, with its cell phones, radar, satellites, and TV transmitters, has turned our planet into a very noisy place. The challenge for SETI scientists is to rule out this extraneous artificial noise and not mistake it for aliens.
The first of the two Nature Astronomy papers, led by astronomer Danny Price from the University of California Berkeley, describes how the Breakthrough Listen team scanned the target system across a vast frequency range, which the press release likened to tuning into 800 million radio channels at a time. The scan resulted in over 4 million hits, an expected result, given that the cosmos is awash in radio signals.
Smith took this massive dataset and ran it through filters that selected for two primary attributes: doppler shift and persistent locality. A doppler shift happens when the signal changes frequency over time, which is evidence that the target is in motion relative to Earth. The other filter looks for a source signal that’s coming from the direction of the target and not somewhere else; this is done by repeatedly training the telescope at and away from the target in hopes of getting ON and OFF observations.
There’s more to the filtering process than this, but BLC01 trickled through these filters, prompting a more thorough investigation by astrobiologist Sofia Sheikh, also from the University of California Berkeley. In the second paper, Sheikh and her colleagues analysed a larger dataset taken of the Proxima Centauri system, which was gathered at other times. The signals in this expanded dataset did not pass the ON/OFF sniff test, revealing over 60 signals that persisted during OFF observations. The Sheikh paper concludes that the signal isn’t originating from the red dwarf system and is instead the product of a human technology.
“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,” Sheikh explained in the press release. “Taken together, this evidence suggests that the signal is interference from human technology, although we were unable to identify its specific source.”
To which she added: “The original signal found by Shane Smith 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.”
This result might seem like a downer, but there is an upside, as this exercise shows how scientists are able to rule out erroneous signals. As Milner put it, the “significance of this result is that the search for civilizations beyond our planet is now a mature, rigorous field of experimental science.”
We can now add this event to our growing list of things suspected as being of alien origin, only to later find a natural explanation or, as in this case, a local source. The search for a true alien technosignature continues.