The Radio Signal From Proxima Centauri Came From Earth After All

Turns out we were hearing ourselves! Earth can be a noisy place when listening to stars.

Late last year, a story was leaked indicating that the Murriyang radio telescope in Australia had detected a “signal-of-interest”. Dubbed “blc1” (Breakthrough Listen Candidate 1), the signal appeared to originate from the direction of Proxima Centauri, the closest neighbouring star to the Sun. The signal had yet to be fully analyzed when the story was leaked. Now that the analysis is complete, research shows blc1 is in fact “RFI” – radio frequency interference – and not an interstellar signal.

But while it’s not aliens – or “Proxima Centaurians” as lead author on the signal analysis Dr. Sofia Sheikh whimsically refers to them – new methodologies for conducting radio-based SETI (Search for Extraterrestrial Intelligence) have been developed by analyzing blc1; further honing our ability to distinguish future potential ET signals from our own planet.

Simulation of Proxima Centauri b , Rocky World in the Proxima Centauri System – SpaceEngine by author

A Needle in a Hay Stack

BLC1 – Breakthrough Listen’s First “Signal of Interest” – with Dr. Sofia Sheikh

From April 29^th to May 4^th of 2019, the Murriyang telescope targeted ProxCen (Proxima Centauri) to monitor solar flare activity. ProxCen is a red dwarf star located 4.2 light years from Earth. Red dwarfs, are known for their massive solar flares but given that they dominate the stellar population of our galaxy, we want to know how these flares may affect the ability of red dwarfs to harbour life. Mega flares could tear the atmosphere off a planet! ProxCen has demonstrated mega flare activity but also contains at least one rocky planet in the star’s “habitable zone” where it’s warm enough to allow for liquid water.

Murriyang’s scan of ProxCen is intense – 800 million radio frequencies monitored simultaneously spanning the 700Mhz to 4Ghz range. 20TB of information is collected in a matter of days. In addition to flares, Berkeley undergraduate student Shane Smith working with Breakthrough Listen, currently the world’s largest SETI project, used software called turboSETI to scan for signs of alien technology. The software filters signals with certain patterns that may indicate something of interest. After filtering 4 MILLION detection “hits”, one very peculiar signal remained warranting further scrutiny.

Narrowband (technological) signal hits detected in range of the Muriyang telescope. Noted are the registered cellular, satellite, and broadband internet transmitters which can be ruled out immediately as Earth-based signals. c. Smith et al 2021 Figure 2

Smith, found a signal at 982.002571 Mhz, – possibly the SETI winning numbers. Why did this one stand out from the millions of others? Some can be removed straight away as Earth-based signals such as cell-phones and internet transmissions at known frequencies. Another tell is whether the signal still appears when the telescope is pointed “off-source”. At a regular cadence, the scope was pointed away from ProxCen to other calibration sources. If the signal remains when the telescope is moved away from the target, the source of the signal is more likely from Earth and not from the target.

The signals are then checked for “drift.” Drift is the change in a signal’s frequency over time. The hope is that this change in frequency is caused by the source of the signal moving relative to Earth – for example because it is orbiting around a distant star. Its motion creates a Doppler effect similar to the change in pitch of a passing ambulance. After all this filtering. One signal remained – a signal also so narrow in frequency, it couldn’t be created by any known natural phenomenon. A candidate! Blc1! *Cue X-Files whistle.*

A “Waterfall Plot” that shows the presence of blc1. The vertical access is the detection over time from top to bottom. Circled in purple is the detected signal. Circled in yellow are the gaps where the telescope was off-source and pointed to calibration sources. The slope of the signal line is the signal’s drift over time which could indicate a signal source in relative motion to Earth. c. Smith et al 2021 Figure 4

Who Said That?

“For me, it’s the enormity of the challenge that draws me to SETI as a research field. In addition, because there’s so much research still to do in SETI, it allows me to be creative in my work and juggle multiple projects at a time, which I really enjoy!”
-Dr. Sofia Sheikh

But is blc1 Proxima Centaurians!? Dr. Sofia Sheikh, lead writer on the recently published signal analysis, wanted to find out.

As a unique discovery, blc1 required new methodology for testing which was developed by Sheikh and her team. The first tests rule out the most local of possibilities – the Murriyang facility itself. Diagnostics of the telescope on the days of recording were analyzed. The team also verified the scope didn’t simply detect local RFI from the surrounding complex. For example, was blc1 seen only when the telescope dish was pointed at a building coinciding with ProxCen’s direction? Reobservations of ProxCen were also made which yielded no signal.

Deeper analysis sought to explain blc1’s drift rate. The signal could still be from Earth but in motion relative to the telescope- originating from a car, airplane, or satellite. However, none of these possibilities accounted for the measured drift over the duration the signal was detected. Deep space objects like probes were not in the direction of ProxCen during the detections. But neither did the drift match what would be expected if the signal were moving relative to Earth within the ProxCen system itself.

So what else could cause the drift if not a moving object? Drift can also be created by an electronic device designed to change frequency like tuning a knob on a dial. To the telescope, such a device may appear to be moving even while fixed to the Earth’s surface.

A sampling of the RFI set that blc1 ultimately belongs too. The drift slope shape is similar across all these signals that appeared on-source as well as off-source indicating they are all part of a set of RFI likely belonging to a single source or created by the interference of various sources. c. Sheikh et al 2021

As a final step in the analysis, the team revisited the data to see if other signals similar to blc1 could be found. 60 signals, across a range of frequencies and all with incredibly similar drift to blc1, were discovered in the data. Each signal appeared to the telescope at the same time as blc1. Why were these signals initially filtered out? Because they appeared off-target and so were determined to be RFI. Blc1’s similarity to these other signals indicates it is most likely one of a set of signals that are all RFI. And, as a result of the way these signals mixed and interacted, blc1 appeared localized in space toward ProxCen even though it wasn’t. Furthermore, the spacing in frequency between the signals is in multiples of 2Mhz, a common spacing created by known Earth-technology – a clock oscillator. So, because blc1 so closely matches this set of signals, and the set can be identified as RFI, blc1 itself can also be ruled out as RFI. No calls from ET…(yet).

Listening to the Future

“As Freeman Dyson once said “Every search for alien civilizations should be planned to give interesting results even when no aliens are discovered.” Our “interesting results” are often cutting-edge astronomy in their own right!”
-Dr. Sofia Sheikh

The actual source of blc1, and its companion set of RFI, has yet to be determined. While not aliens, working to verify the signal helps us better detect aliens in the future. No signal has ever been ruled out as RFI by comparing it to other RFI before. That’s because no signal like blc1 had ever been seen before! This whole process was the result of the most sensitive search of ET radio signals ever conducted on a single target star. The results also demonstrate that software tools like turboSETI were doing their job. The software detected that 1 in 4 million signal that MIGHT be extraterrestrial triggering the flurry of analysis.

Two Confirmed Planets at Proxima Centauri – Video by Fraser Cain of Universe Today

Breakthrough Listen is expanding operation to include MeerKAT, a telescope array in South Africa that can observe targets with Murriyang simultaneously. The advantage of simultaneous observation is that a true interstellar signal will show up in both telescopes while RFI would be unique to one – a technique used by some past SETI initiatives such as Project Phoenix.

Ultimately, a civilization or other life on Proxima b could still be possible. Given the system’s proximity to Earth, it remains a target of interest for future observation. We could also just GO to Proxima Centauri to find out! Another component of the Breakthrough initiative is Breakthrough Starshot, a future mission to send a miniature probe all the way to our neighbouring star. Attach that probe to a massive solar sail accelerated by a few gigawatts of laser power and you have a craft headed to ProxCen at 20% the speed of light – thousands of times faster than our current probes. It would arrive in just 20 years – interstellar flight within a human lifetime!

Until then, we keep listening to the stars.

“It would be very, very cool if life, technology, and even intelligence were features of our galaxy more broadly, and not just confined to our “pale blue dot” of Earth. But we’ll just have to see what the data holds in the future!”
– Dr. Sofia Sheikh

Feature Image: The three telescopes at CSIRO’s Murriyang Observatory. The primary dish is 64 meters wide and one of the largest moving dishes in the world. Credit: Red Empire Media/CSIRO.

Big thanks to Dr. Sofia Sheikh for contributing comments to this piece!