Alien technosignatures

In early February 2018, New Scientist magazine published a feature on my archaeological work with wild primates and their stone tools. I was excited to read it. But when I picked up the issue at my local supermarket, I quickly forgot all about the monkeys.

From the dark magazine cover leapt a bold, ominous headline: INTRUDERS FROM ANOTHER SOLAR SYSTEM. Behind that headline was an illustration of several mysterious elongated objects, glowing faintly and seemingly hovering in space:

The tag-line was also vaguely menacing, asking ‘how many more are out there?’. That’s not the kind of thing you see every day, outside of 1950s movie posters. I’d planned to read about tool use by non-human animals, instead here was a respected science magazine telling me we may have discovered tool use by non-Earthlings.

The story behind that cover, and the wider emerging field that surrounds it, is the subject of today’s post. We’re going hunting for alien technosignatures. And we’re going to need a bigger telescope.

The Dish

The classic 2000 Australian film The Dish tells a slightly fictionalised story of how the Parkes Radio Telescope tracked the Apollo 11 moon mission, helping bring TV pictures of humanity’s first lunar steps to the world. The film’s main character, the ‘dish master’ played by Sam Neill, was an invention only loosely based on observatory director John Bolton. But one true part of the story was that the enormous 64m wide structure—in 2020 added to Australia’s National Heritage List—sat in the middle of a sheep paddock in rural New South Wales.

A half century after Armstrong and Aldrin went for their famous walk, the upgraded Parkes dish and its receivers are now thousands of times more sensitive (but still in a sheep paddock). The facility continues to generate cutting-edge science too, adding to our knowledge of distant objects such as pulsars and quasars, and giving us the first look at our own galaxy’s magnetic field. Most relevant to us today, though, is its participation in the decade-long, $100m Breakthrough Listen project, the most comprehensive effort ever made to find evidence of alien civilisations.

Here’s the Parkes dish as it looked in the 60s, during the Apollo years:

The search for extraterrestrial (‘outside Earth’) life has gone through several phases. From the 19th century notion that lines on Mars’ surface were constructed canals, through the hunt for other stars with planets in that special Goldilocks zone where water wouldn’t immediately freeze or evaporate, to the fine-tuning of instruments to read an exoplanet’s atmosphere, each advance in our own technology has prompted another surge of interest in finding life elsewhere.

Scanning another planet’s atmosphere—looking for the literal breath of life—falls under the umbrella of tracking biosignatures. The rationale is sound. We know that life on Earth has prompted repeated large scale changes in our own atmosphere, including a dramatic rise in oxygen levels over two billion years ago, caused by bacteria (the ‘Great Oxidation Event’), and more recent human-caused rises in carbon dioxide and radioactive isotope levels. With different molecules and elements absorbing different parts of the electromagnetic spectrum, we have begun to make evidence-based guesses about whether life has contributed to a planet’s gaseous cover.

Some astronomers, however, have set their sights on a more ambitious target. As outlined by Manasvi Lingham and Avi Loeb in their 2021 book Life in the Cosmos:

If lifeforms do exist beyond the Earth, at least some of them might have conceivably attained technological levels comparable to, or surpassing that of, Homo sapiens…The detection of these technological species might therefore be feasible by searching for signatures of their technology.

The idea is that alien life forms use tools to modify their environment, and we use our powers of deduction and extremely sensitive equipment of our own to work out what they’re up to.

The best studied of these technosignatures is that of radio waves, the very thing that the Parkes telescope was built to track. Radio waves are long, making them useful for carrying signals such as music and voices over large distances. We humans have been sending electromagnetic waves not only to each other, but out into the void for over a century, both unintentionally and as part of occasional efforts to contact other planets. In theory, extraterrestrial civilisations may have been doing the same. They may also have attempted to use other parts of the spectrum, such as optical light or infrared, for the same purposes.

In 1964, Soviet astronomer Nikolai Kardashev proposed a scale for alien technologies (or Extraterrestrial Intelligence: ETI), based on how much power they could harness. The more they control the energy of surrounding stars, the more likely we are to be able to detect them. The Kardashev Scale has three main types:

• Type I: ETIs whose technological levels are close to humans’ in the 1960s;

• Type II: ETIs capable of harnessing the energy of a sunlike star; and

• Type III: ETIs that possess energy resources roughly on the scale of the Milky Way.

Galactic civilisations would therefore be easiest to detect, even if our own technological progress is very much still Type I in Kardashev’s scale. Of course, our detection of extraterrestrial civilisations, whatever their size, depends entirely on our ability to pick up what the aliens are laying down. Which returns us to Parkes.

Proxima Centauri and BLC1

In late April, 2019, the Breakthrough Listen project pointed the Parkes observatory at Proxima Centauri, the nearest star to our own sun. Light and radio waves take just over 4 years to travel between the two stars, meaning that anything seen from Earth in April 2019 took place in Proxima Centauri’s system around the start of 2015. That’s close enough in time to be fairly confident that anyone broadcasting from there to here is still around when we get the signal, unlike for stars that are many thousands or millions of light years away. Put another way: anyone on a Proxima Centauri planet with powerful enough receivers was already partway through watching Game of Thrones.

As reported in two 2021 studies in the journal Nature Astronomy, the international team running the project were interested in a particular planet—known as Proxima Centauri b—orbiting the star at a potentially habitable distance. That planet was discovered in 2016, triggering debates about the likelihood of its having a stable atmosphere, given that it’s vulnerable to large, life-extinguishing flares coming off its star. It is joined by one other confirmed, and one potential planet in the Proxima Centauri system, as seen in this artist’s impression:

As the closest candidate outside our own solar system, the Breakthrough team figured Proxima Centauri b was worth a look. And on 29 April, suddenly it seemed like something might be looking back.

The Parkes telescope was tracking signal frequencies from 0.7 to 4.0Ghz, generating an extremely large amount of data from sources both on and off Earth. To narrow down signals to those possibly from Proxima Centauri b, the scientists focused on only those records that had small and smooth frequency shifts, as would be expected if the transmitter sat on a distant planet moving around its own star (the Earth is moving too, of course). Applying this ‘Doppler drift’ filter still gave over 4.1 million hits. The team then compared signals that only showed up when the telescope was pointed directly at Proxima Centauri—designated ‘on-source’—and were left with 5,160 events.

Those 5,000 plus signals were from the right place (and only the right place), and drifted in the right way. And at that point the real detective work began. The researchers painstakingly eliminated all known Earth-based sources of the signals that had made it through the initial filters. What remained was a single signal that lasted round 5 hours, drifting smoothly from a frequency of precisely 982.0024 MHz to 982.0028 MHz over that time. There’s no known astronomical system that can create a signal with that frequency. And while it’s within a band used for Earth-based aircraft navigation, there’s no such transmitter within 1000 km of the Parkes observatory that could cause interference. The anomalous event was given the name ‘Breakthrough Listen Candidate 1’, or BLC1.

Was this first contact? A Type I signal, proof we’re not alone in the universe, and not even the only species with radios? We could let imagination take over at this point, but it’s precisely here that science and sci-fi diverge. Science depends not on hope or belief or speculation, but on repeated observations. So in November 2020, and again in late April to May 2021, the team re-scanned Proxima Centauri, to see if the signal was still there. They found…nothing.

But they didn’t stop there. They collected data on—and then ruled out as BLC1 sources—ground and air based transmitters, human satellites, and even deep-space probes such as the Voyager craft launched in the 1970s. Then they looked through years of data for signals with the same shift as BLC1, and different frequencies just in case the aliens broadcast multiple signals at once. Each potential extra match to the BLC1 signal—and there were hundreds—turned out to be Earth-based radio frequency interference (RFI) of some kind. Not aliens.

This image shows the drift of the BLC1 signal (the black crosses) going from left to right on 29 April 2019. It doesn’t match any of the satellites in geosynchrounous orbit (green lines), other orbits (magenta and orange), or the named deep space probes:

So what exactly is BLC1? There is still no known source on Earth for that signal. But the presence of many similar RFIs, and the fact that the signal wasn’t re-captured in the subsequent search, dampens the likelihood of it being a real extraterrestrial technosignature. It’s more likely at this point that BLC1 is just a very unusual source of RFI here on our own planet. Sofia Sheikh, from the University of California Berkeley and lead author of one of the BLC1 studies, decided that “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”.

Still, the detective work around BLC1 has given the Breakthrough Listen team a solid process for assessing future potential technosignatures. And we shouldn’t forget that there’s another, even more concrete way that we might encounter alien technologies: they might just show up in our own solar backyard. Remember the glowing intruders on the front of that New Scientist issue?

‘Oumuamua

Its arrival was so unexpected that the International Astronomical Union—namers of planets, moons, comets and more—had to come up with a whole new category. The speeding space object first spotted in October 2017 was already moving away from the Sun, but it was clear that it had recently swung around our star. It was first given the name C/2017 U1, where that C stands for comet, until a lack of ejected gases led to re-classification as asteroid A/2017 U1 (comets are icy and give off gas as they orbit, forming their familiar tails, while asteroids are rocky left-overs from the formation of the solar system). Once the object’s path was backtracked, however, it became clear that this was neither comet nor asteroid. It came from another star system entirely.

And so the space rock became the first object to ever get an ‘I’ designation, for interstellar. More specifically, it was named 1I/2017 U1. For good measure the name ‘Oumuamua (pronounced oh MOO-uh MOO-uh) was added, meaning something like ‘a messenger from afar arriving first’ in Hawaiian, since the first observations were made using the Pan-STARRS1 telescope in Hawaii.

This graphic shows the reconstructed approach of ‘Oumuamua, and its path as it moved through the plane of our orbit and away again in late 2017:

The object’s unusual origin was reason enough for astronomers to be excited. They had literally never before confirmed that something passing through our solar system came from outside that system. Various proposals were made to send a space probe chasing after ‘Oumuamua to learn more. But even from data recorded from Earth and our various space telescopes, there were other puzzling features that hinted at something even more odd.

First, ‘Oumuamua was tumbling as it flew. The way that light reflected off its surface during those tumbles showed that it was much longer than it was wide, perhaps ten times longer than its width. This indicated either a cigar-shaped or perhaps disc-shaped object, a few hundred metres long, not a rough ball of lumpy rock like most asteroids. And second, there was an unusual acceleration after ‘Oumuamua passed the sun. That’s fairly typical for comets, whose lost gases can act as little jets that push the main body around. But for a rocky body that’s not expected.

You can guess where this is going. Harvard astronomer Avi Loeb (co-author of the book mentioned earlier) made the provocative suggestion that ‘Oumuamua might be a space probe from another star. That extra push in its trajectory? Possible evidence that the object had a solar sail, surfing our sun’s output. But we needed more evidence.

Loeb convinced the Breakthrough Listen project to point the US Green Bank Radio Telescope at the object, to check for any emitted signals. It was perhaps the best chance humanity had ever had for direct contact with aliens. Any sign that this thing was transmitting either to us or back to its distant home planet would have been a major milestone in our species’ history. Over a period of eight hours—long enough to cover a full tumbling rotation—the telescope scanned for signs across several frequencies.

And again…nothing. From this and other observations, it seems that ‘Oumuamua is simply a lonely space rock, reddish in colour and anywhere up to several billion years into its travels across a largely empty universe. The elongated shape is unusual, but not impossible for a natural object, and that extra acceleration might have been from some kind of undetected natural emissions from the object’s surface (or not: that one remains a topic of some debate).

One other argument in favour of ‘Oumuamua being a messenger from the stars has taken a hit as well. It turns out that the sheer rarity of finding an interstellar object might rely on the quality of our detection systems more than the actual scarcity of such floating objects. In August 2019, a second interstellar object—named 2I/Borisov after its Crimean discoverer—was observed as it approached our sun. This time, the object was much more like a regular comet, producing a gaseous tail from its icy core that spread out to around 14 times the size of the Earth. With the first two interstellar visitors now logged, we can except more in the years to come.

This 2-minute video from NASA explains more about 2I/Borisov, including its composition, likely formation, and comparisons with other comets:

Down to Earth

The likelihood of some kind of life elsewhere in the universe is high, but whether we’ll ever encounter it is another matter. Even a galactic-scale, Kardashev Type III civilisation could slip past us if don’t listen the right way, or at the right time. And if we do come across an extraterrestrial visitor cutting briefly through our solar system, we’d have to be quick to launch an intercept probe, or to pick up whatever electromagnetic or other signals it was putting out.

In reality, though, we don’t need to look that far for to find un-human intelligences. The animals that share our planet right now are challenging and fascinating enough to engage with, in all their multi-sensory and multi-technology variations. Animals in the past were likely just as weird, and we can search back in time here—via archaeology, genetics and ecological reconstruction—even more easily than searching back through the expanding universe with hyper-powered telescopes. When it comes to the search for alien thought, we may have just as much to learn from monkeys using stone tools on a Thailand beach as from a slow drifting burst of radio waves from Proxima Centauri.

Look again at the image of the 1960s Parkes telescope, and those sheep in the foreground. What are they thinking? They are definitely alive and active and complex creatures, but despite thousands of years of living side by side with their species, we still know little about how and why they do what they do. As we reach ever further into the stars, we need to remember that it doesn’t hurt to pay attention to what’s in our own paddocks too.

Sources: Smith, S. (2021) A radio technosignature search towards Proxima Centauri resulting in a signal of interest. Nature Astronomy 5:1148–1152. || Sheikh, S. et al. (2021) Analysis of the Breakthrough Listen signal of interest blc1 with a technosignature verification framework. Nature Astronomy 5:1153–1162. || Anglada-Escudé, G. et al. (2016) A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536:437–440. || Koren, M. (2018) Aliens Didn't Send That Mysterious Object to Our Solar System After All. The Atlantic 10 January 2018; https://www.theatlantic.com/science/archive/2018/01/oumuamua-seti-breakthrough-listen-yuri-milner/550070/ || Jewitt, D. et al (2020) Outburst and Splitting of Interstellar Comet 2I/Borisov. The Astrophysical Journal Letters 896 L39.

Main image: NASA; https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/oumuamua/in-depth/ || Second image: New Scientist, 3 February 2018 || Third image: CSIRO, 1961; https://www.csiro.au/en/News/News-releases/2020/CSIRO-Parkes-radio-telescope-added-to-National-Heritage-List#photosection || Fourth image: Breakthrough Listen project; http://seti.berkeley.edu/blc1/overview.html || Fifth image: Sheikh et al. (2021) || Sixth image: NASA; https://www.nasa.gov/feature/jpl/small-asteroid-or-comet-visits-from-beyond-the-solar-system