7 mins read 26 Oct 2021

Candidate Breakthrough Listen Signal at Parkes – Not Aliens

In 2020 the Breakthrough Listen project detected a strange signal coming from the closest star system to Earth, Proxima Centauri, using the Parkes radio telescope. Now, astronomers have figured out what caused it.

An artist’s impression of the Alpha Centauri stellar system as viewed from the surface of the habitable-zone planet Proxima Centauri b. The planet is just a bit more massive than Earth and orbits in the habitable zone. Proxima Centauri is part of the triple star system Alpha Centauri. Credit: ESO / M. Kornmesser.

At the end of 2020, astronomers who form part of the Breakthrough Listen project were using powerful computers to sift through the large volumes of data collected with the Parkes radio telescope (owned and operated by Australia’s national science agency, CSIRO), when they detected what appeared to be an interesting signal.

It wasn’t the typical kind of radio signals that the iconic telescope observes on a regular basis (like that of pulsars, or emissions from the huge masses of cold hydrogen gas in the Milky Way) that tend to be spread across a range of frequencies. Instead, this signal was very focused in a narrow band of the radio spectrum, tuned into exactly 982.002 MHz.

Even more, interestingly, the signal appeared to come from a nearby location outside of our own Solar System – from the closest exoplanet to Earth, in fact, known as Proxima Centauri b.

At the time, excitement started building and the signal was put through a variety of rigorous testing regimes, to ensure it wasn’t some form of background noise or anomaly that could be jumping out from the data – and yet, it passed all tests.

The team scanned the target across a frequency range of 700 MHz to 4 GHz, with a resolution of 3.81 Hz - performing the equivalent of tuning to over 800 million radio channels at a time, with exquisite detection sensitivity.

Why was this narrowband radio signal from the nearest exoplanet to us appearing in the data when observed at this frequency? And how could it have passed such rigorous examinations and still remain a candidate?

Well, it turns out that the interesting signal was probably generated a little closer to home, rather than originating as an astrophysical source, and according to the astronomers as part of the Breakthrough Listen project – was likely radio frequency interference (RFI) detected by Parkes.

RFI to radio telescopes can be thought of as the equivalent of ‘light pollution’ to optical telescopes – terrestrial-based noise and interference generated by our everyday activities (satellites, mobile phones, TV transmissions, aircraft communication, even microwave ovens). When RFI occurs relatively local to a radio telescope, there’s a chance it can get caught in the telescope’s receiver and appear in the data. Radio telescopes are extremely sensitive instruments as well, picking up signals even from large distances away.

Postdoctoral researcher Dr Sofia Sheikh from the Breakthrough Listen team at UC Berkeley sifted through a larger dataset collected by Parkes from observations taken at other times and found at least 60 signals that shared the same characteristics as the candidate signal reported in 2020.

“We can therefore confidently say that these other signals are local to the telescope and human-generated,” says Sheikh.

“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.”

“Taken together, this evidence suggests that the signal is interference from human technology, although we were unable to identify its specific source. 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.”

Proxima Centauri

An artist’s depiction of a coronal mass ejection event on Proxima Centauri. Red dwarfs like Proxima Centauri give off enormous flares, causing them to double in brightness in just minutes. This also generates a lot of radio signals that can be detected from Earth. Credit: Mark Myers / OZGrav.

Whilst all technosignature candidates (from any star being observed) is interesting, what made this signal so intriguing is that it appeared to originate from the closest star to Earth – Proxima Centauri – a star that has been studied for decades.

Proxima itself is a red dwarf star – slightly smaller (about 12.5% the mass) and cooler (approximately, 3000 Kelvin) than our Sun, emitting a lot of its light in the red wavelengths. The system features not only Proxima Centauri b but also a secondary planet known as Proxima Centauri c.

Of the two planets, Proxima Centauri b is in an orbit around its host star at just the right distance in which the temperatures allow liquid water to exist on the surface, a key requirement for any form of life (as we know it) to develop.

However, the Proxima Centauri system is not always a friendly place – and like most M-class red dwarfs, Proxima Centauri often lashes the surrounding regions with powerful stellar flares that could harm and erode the atmosphere of any nearby planets. Of course, this would also depend on the planet’s own magnetic field strength.

Proxima Centauri is itself also part of a larger stellar system that features three stars – two of which are very prominent for southern sky observers. These are known as Alpha Centauri A (Rigel Kentaurus) and Alpha Centauri B (Toliman). These stars are also famous for being the pointer star on the Australian flag, as they reside very close to the Southern Cross (Crux) constellation.  

The Back and Forth of Science

Credit: Breakthrough Listen.

Whilst some may be disappointed that the Breakthrough Listen signal is not of extra-terrestrial origin and of the more mundane terrestrial RFI nature – this case showcases another excellent example of how science progresses.

With interesting observations announced, further investigations undertaken and new results produced – the back and forth of questioning, testing and analysing continues as science progresses forward with each step.

And although the topics of aliens might seem jovial, it’s actually a fascinating and deeply studied field of astronomy, that features a number of branches – such as the search for technosignatures from potential other societies amongst the stars. Many telescopes, including other Australian telescopes like the MWA, continue to undertake study in these fields.

There’s even a global field of sub-astronomy, known as the Search for Extra-Terrestrial Intelligence (SETI) which began in 1960 and has been running for decades, scanning the stars for any hints of radio transmissions that can validate if we’re alone in this vast cosmos, or if other societies are out there too. To date – there has been no confirmation of any strong evidence to suggest the existence of extra-terrestrials.

The Breakthrough Listen Project

Credit: Breakthrough Listen.

Breakthrough Listen (a program of the Breakthrough Initiatives) is an astronomical science program searching for technosignatures - signs of technology that may have been developed by extra-terrestrial intelligence.

Listen’s science team, led by Dr Andrew Siemion at the University of California, Berkeley, uses some of the largest radio telescopes in the world, equipped with the most capable digital processing systems, to capture data across broad swaths of the radio spectrum in the direction of a wide range of celestial targets.

The search is challenging because Earth is awash with radio signals from human technology and searching for a faint signal from a distant star is akin to picking out a needle in a vast digital haystack – and one that is changing constantly over time. 

CSIRO’s Parkes radio telescope in New South Wales, Australia (one of the largest telescopes in the Southern Hemisphere, known as ‘Murriyang’ in Wiradjuri) is among the facilities participating in Breakthrough Listen’s search.