Exciting Signal Found in Data from the Parkes Radio Telescope

There is perhaps no easier way for astronomers to capture the public’s imagination than by raising the possibility that other intelligent lifeforms exist beyond the blue marble that is our home.

The radio-based Search for Extra-terrestrial Intelligence (SETI) that began in 1960 is the most well-known scientific investigation trying to answer the question “where are they?”, but in over 60 years of looking we are yet to have confirmation of any bona fide alien signals.

Towards the end of 2020, a promising candidate signal was announced that appeared to come from a planet in our closest neighbouring solar system; a habitable planet orbiting the star Proxima Centauri.

Computers sifting through data collected by Australia’s Parkes radio telescope (operated by Australia's national science agency, CSIRO) in 2019 alerted astronomers to the unusual signal late last year. The detection was made by a project called Breakthrough Listen, a 10 year US$100-million funding commitment by Russian entrepreneur and physicist Yuri Milner that will support SETI in its efforts to detect radio messages from advanced cosmic civilisations. The project kicked off its scientific program in 2016, but Milner had already affirmed his intention to keep SETI funded for the long term.

“I am fully committed to this project. If we don’t find anything in 10 years, then we’ll just have to extend [the funding] for another 10 years — and then for another 20, if necessary. We will just keep going until we know the answer,” said Milner in a 2015 interview with popular science magazine Scientific American.

Funding from the Breakthrough Listen program is used by SETI to buy observing time on radio telescopes around the world, with the Parkes radio telescope key to the project because of its access to southern skies. The telescope’s role has been to undertake broad searches of large areas of sky, but it was in data collected while looking for solar flares from Proxima Centauri that the mysterious new signal was discovered. Dubbed Breakthrough Listen Candidate 1, or BLC-1, the signal was found in data taken in multiple scans beginning on April 29, 2019.

Proxima Centauri is a red dwarf star, a type of star that is much smaller and cooler than the Sun but far more common. Red dwarfs have a propensity for giving off gigantic flares that can cause them to double in brightness in a matter of just minutes. This is of interest to astronomers trying to understand the effects of space weather on habitable planets, and at only 4.2 light-years from us, Proxima Centauri is an excellent target for observations.

Twenty-six hours of data from Proxima Centauri were flagged for the Breakthrough Listen program, coming in 30-minute observation intervals over the course of a week. Normally observations like this produce data spread across several frequencies, but this time it was one in a narrow band of the radio spectrum at a frequency of 982.002 MHz that has gotten a lot of people excited, the signal that would later be called BLC-1.

Now, classifying a radio signal from space as the transmissions from an alien species’ intra-galactic communication system should not be done without a lot of overwhelming evidence, and after a few missteps in the past astronomers would rather err on the side of caution. But BLC-1 is unique. It is the first signal to make it through the Breakthrough team’s automated computer testing regime that is designed to filter out the millions of signals that could mistakenly be identified as the signatures of alien technology.

A true signal from our interstellar neighbours needs to have certain characteristics that differentiate it from the constant stream of radio waves constantly bombarding our telescopes. At least, it ‘probably needs to’, or even ‘possibly needs to’, because obviously we remain totally unaware of the thought processes of an alien intelligence. That aside, we expect that as a first requirement it would have to be contained within a narrow frequency band. That would at least distinguish it from naturally occurring noise and transmissions in other neighbouring bands. Centred on a frequency of 982.002 MHz, BLC-1 passes this test with flying colours.

Second, the signal would likely be modulated if said intelligent species wanted to transmit any information to us. The data would be imposed on top of a carrier wave to reduce interference and allow us to conveniently demodulate the signal on Earth. BLC-1 was unmodulated, but that may simply mean that it was more like a beacon, a signal to start a conversation rather than the conversation itself.

Last, and perhaps most obviously, the signal should not be seen when looking away from the target star system. Astronomers use a technique called ‘nodding’, where the telescope spends time looking at the target and then an equivalent period looking elsewhere in the sky to make sure that the signal isn’t the result of a nearby source of interference. This was another test passed by the BLC-1 signal.

But even signals that satisfy all these conditions can often be explained without invoking the alien explanation. When radio pulses were discovered in 1967 all originating from the same patch of sky, the signals were initially (only partly jokingly) labelled LGM-1 – ‘little green men’. Those signals turned out to be the discovery of a new highly magnetised type of compact star called a pulsar, but with so much still unknown about the universe, it is easy to understand how a radio signal like the ones we produce on Earth could be attributed to another intelligent lifeform.

And there have been a lot of false alarms. In May 2015 a team of Russian astronomers reported that a telescope in the Caucasus region had tuned in to a signal consistent with something an alien civilisation might produce. It had been spotted coming from the vicinity of HD 164595, a star about 94 light-years from Earth. Follow-up observations by SETI’s Allen Telescope Array found nothing, and by August of that year the Russian Academy of Sciences confirmed that the origin of the signal had most likely been terrestrial.

One of the most infamous false alarms came in 1997 when astronomers at SETI at the Green Bank Radio Observatory picked up an unusual signal coming from the skies. Before they had a chance to independently verify that the signal was extra-terrestrial, news of the find had been leaked to the press. Within 24 hours the source of the excitement had been traced to a telemetry signal from the SOHO satellite, a solar research satellite, but the lesson here was clear. There was simply no chance that any remotely promising signal in the future could be kept under wraps until it was rigorously confirmed, and so false alarms would forever be a part of our search for other intelligent life.

The Parkes radio telescope has also intercepted baffling signals of its own but signals that ultimately proved to have had a very local origin. The mystery behind strange signals that were first detected in 1998 at ‘The Dish’ confused scientists for nearly two decades before eventually being traced to a microwave oven in a nearby kitchen. Apparently when the door of the microwave was opened during cooking, a burst of interference was detected on the telescope receiver that looked something like fast radio bursts from space. The ambiguous nature of the signals earned them the name 'perytons' after the mythological winged elk that cast the shadow of a man.

The microwave incident might be an amusing anecdote in humankind’s search for a cosmic cousin, but if BLC-1 turns out to be the signal that researchers at SETI are hoping for, the Parkes radio telescope may yet have the last laugh.

The reigning champion of unexplained extra-terrestrial candidate signals is the Wow! signal, a strong narrowband signal received on August 15, 1977, at the Big Ear radio telescope in the United States. Coming from the constellation Sagittarius, it bore the same signatures of alien intelligence as BLC-1, also without any detectable modulation that could potentially have solved this mystery once and for all. The signal received its moniker when astronomer Jerry Ehman annotated the printout of the recorded data by writing Wow! in the margin.

The data itself were fairly innocuous – just a string of 6 alphanumeric characters, 6EQUJ5, that were heard once, and only once, over a period of 72 seconds. Of course, efforts have been made to search for a recurrence of the signal, at the time in 1977 and on several occasions since, but the skies have remained stubbornly silent. Even lengthy observations of the suspect patch of sky using the 26-m radio telescope at the University of Tasmania’s Mount Pleasant Radio Observatory in 1999 failed to turn up any evidence for the signal whatsoever.

Several hypotheses have been put forward to explain the origin of the Wow! signal, but none have achieved widespread acceptance. Was it reflected radio waves from Earth, or did it originate in the depths of space? Could it be explained by some, as yet unknown, natural phenomenon? Even Jerry Ehman was reportedly sceptical of it having been sent by an alien intelligence, saying in an interview that it might well have been a signal from Earth reflecting off a piece of space debris. After further research though he declared that this was, in fact, unlikely, and that even improbable explanations remained possible if they could not be ruled out.

Despite occurring nearly 45 years ago, interest in the Wow! signal has not waned. Recently there was speculation that it may have been due to hydrogen clouds surrounding two comets that are now known to have been in the same region of the sky back in 1977. The trouble is, they were still a long way away (about the same distance as Jupiter), and if they really were the source of the radio waves, we should be picking up signals from comets all the time. Which we don’t. And so, the mystery remains.

The Wow! signal originated from the direction of the constellation Sagittarius, a little below the plane of our galaxy and a little to one side of the galactic centre. Nearby was the globular cluster M55, a collection of a few hundred thousand stars about 17,600 light-years from Earth. But there are no Sun-like stars around which there might be a habitable planet in that direction.

On the other hand, astronomers do have a good idea of where BLC-1 originated from, if not how or why, because the Parkes radio telescope was pointed directly at Proxima Centauri when the call came through. And we know that Proxima Centauri happens to have at least one planet (named Proxima Centauri b) orbiting within its habitable zone. So, is that it? We have an alien-looking signal originating from a planet that may be inhabited by aliens. Case closed.

Not so fast.

As already mentioned, Proxima Centauri was being studied at the time because astronomers want to understand whether the variability and extreme weather associated with M-dwarf stars would be an impediment to the existence of life. It may be that despite temperatures on Proxima Centauri b allowing for the presence of liquid water, a necessary part of life as we know it, the variability and violence of its star precludes life of any kind.

And even if it could withstand this violent onslaught, one side of the planet would likely be permanently locked facing its sun, just as the Moon is tidally locked to the Earth. The cooler temperatures of red dwarf stars compared to Sun-like stars mean that their habitable zones are closer, and as a result one side of the planet could experience an eternal day while the other is forever in darkness. Extreme temperature differences between the two sides result, leaving few places on the planet for a comfortable existence.

As the research of Proxima Centauri was taking place at Parkes, other astronomers were analysing space weather around the star by observing it at optical as well radio wavelengths. Using NASA's orbiting Transiting Exoplanet Survey Satellite (TESS) as well as the Australian Square Kilometre Array Pathfinder (ASKAP) and other Australian telescopes, a team led by Dr Andrew Zic managed to observe a massive flare on the star that was accompanied by a spectacular radio outburst. The flare could have powered Australia for the next 17 million years, and these are not exactly rare events, occurring every few weeks on Proxima Centauri.

Though the Sun can exhibit similar behaviour, it occurs less frequently, and as far as we know the Earth is also better equipped than Proxima Centauri b to withstand such powerful blasts of radiation with its thick atmosphere and powerful magnetic field.

“Our own Sun regularly emits hot clouds of ionised particles during what we call coronal mass ejections. But given the Sun is much hotter than Proxima Centauri and other red-dwarf stars, our habitable zone is far from the Sun’s surface, meaning the Earth is a relatively long way from these events. Further, the Earth has a very powerful planetary magnetic field that shields us from these intense blasts of solar plasma,” said Dr Zic.

All of which means that it remains a long shot that BLC-1 is a callout from our nearest neighbours on Proxima Centauri b.

In reality, despite the Parkes 64-m dish pointing at Proxima Centauri during its observations, we cannot localise the BLC-1 signal any more accurately than to say that it came from an area roughly half the diameter of the Moon centred on the star. Literally, hundreds of thousands of other stars were also present in the field of view, any one of which could have been the source of BLC-1. This is an unfortunate limitation of using a single dish for SETI rather than combining several widely spaced dishes using a technique known as very long baseline interferometry (VLBI).

Australia will be well placed for future SETI surveys should VLBI become the preferred approach to alien hunting, with part of the Square Kilometre Array, the world’s largest radio telescope, to shortly begin construction in Western Australia. One of its predecessors, the Murchison Widefield Array telescope, has already been used to search for signs of extra-terrestrial life. In 2020 it conducted the broadest ever scan of the sky, centred around the constellation Vela, in a search for emissions at frequencies similar to FM radio frequencies. An enormous number of perhaps 10 million stars turned out to be completely silent.

There is also the question of what finding another intelligent species living so close to us would actually mean. Are the chances of two civilisations being neighbours so astronomically small that it would imply that the galaxy is literally teeming with life? Or is Proxima Centauri just like our closest cell phone tower, the last link in a galaxy-wide communication network? For some, questions like these may well go beyond the bounds of science.

And the chances are, we are not going to know the answers any time soon. But to uncover the mystery of BLC-1 astronomers will need to see if the signal repeats while they consider every possible signal source – and rule them out, one by one.

So, is BLC-1 the signal that SETI researchers (and many, many others) have long been hoping for?

Probably not. But maybe.

Because when you have eliminated the impossible, whatever remains, no matter how improbable, must be the truth.

Welcome to the science of SETI.

We would like to acknowledge the Wiradjuri People as the traditional owners of the Parkes Observatory site. We would also like to acknowledge the Wajarri people as the traditional owners of the ASKAP site.