Breakthrough Listen was a highly ambitious ten-year quest to hunt for ET, costing $100 million —the kind of riches SETI, the search for extraterrestrial intelligence, had never seen before.

When Yuri Milner, the billionaire philanthropist at the head of the Breakthrough Foundation, announced Breakthrough Listen to a packed press conference at the Royal Society in London while flanked by luminaries such as Steven Hawking and Martin Rees, he didn’t outright state that the project had to find aliens to be a success. Still, it must surely have been his greatest hope that it would have succeeded by the end of the initial ten years. This, of course, has not happened, so what has Breakthrough Listen achieved to justify a $100 million outlay, what does its future hold, and how has it changed SETI for the better?

Historically, SETI had always been strapped for cash. The initial enthusiasm that had greeted Giuseppe Cocconi and Philip Morrison’s seminal 1959 paper introducing the concept of using radio for interstellar communication, swiftly followed by Frank Drake’s pioneering first SETI radio search, failed to grab the interest of the wider astronomical community. Shunned by many of their peers, SETI researchers also had to combat political pressure, be it Senator William Proxmire’s Golden Fleece award or the US Congress cancelling federal funding for NASA’s SETI project just as it was about to go live. In response, SETI’s practitioners had learned to live off scraps, begging and dealing for small amounts of time on radio telescopes. Even the great new hope that was the Allen Telescope Array, a proposed 350-dish ensemble, floundered as its construction stalled at 42 dishes and its ownership was forced to change hands as economic pressures threatened to shut it down.

All that changed with Breakthrough Listen. Suddenly, the SETI community had $10 million a year to play with. While not a vast amount, it was far more than they had ever had before.

The task that lay before the Breakthrough Listen team was an ambitious one. Over an initial 10 years, they would conduct long, sensitive observations of several thousand of the closest stars. They would perform a less intense survey of another million stars, and in general listen for signals along the plane of the Milky Way and towards the galactic center, which is the direction in which most of our galaxy’s stars can be found. They would even listen to 97 nearby galaxies, and the trillions of stars in them, for signs of alien life.

To make all this possible, they would book time on the largest radio telescopes, broaden the search to encompass optical laser signals as well as radio, extend SETI from its traditional California base to the rest of the world, adopt new techniques and enlarge the community while training the next generation of SETI scientists. Indeed, at the launch of Breakthrough Listen, Milner talked about introducing a fundamental change in how SETI is done, describing how it would “bring the Silicon Valley approach to the search for intelligent life in the Universe,” and that “Our approach to data will be open and taking advantage of the problem-solving power of social networks.”

Even this statement, just 11 years old, has become dated. Back then, the approach to sifting through big data was citizen science, but in the years since, machine learning and artificial intelligence have seen breakthroughs that the SETI community, and indeed the astronomical community at large, have utilized.

In a recent paper by Breakthrough Listen’s chief engineer David MacMahon and Oxford radio astronomer Daniel Czech, the pair break the first ten years of Breakthrough Listen down into three phases that have come to characterize the history of the project.

The first phase was to apply new wideband recorders to the 100-meter Green Bank and 64-meter Parkes radio telescopes in the United States and Australia, respectively. These recorders would allow the two giant radio telescopes to conduct a survey of thousands of nearby stars. Yet the first actual search under the umbrella of Breakthrough Listen was an optical survey of 200 stars conducted by astronomers led by Shelley Wright at the University of California, San Diego, using the Automated Planet Finder on the 2.34-meter telescope at the Golden State’s mountaintop Lick Observatory to look for the flashes of laser signals.

The second phase saw Breakthrough Listen extend to facilities in other countries, such as the Low Frequency Array (LoFAR) in Western Europe, the 64-meter Sardinia Radio Telescope, and the Westerbork Observatory in the Netherlands, plus the MeerKAT array in South Africa, which is a precursor to the Square Kilometre Array. Currently, Breakthrough Listen is operating on ten telescopes in four continents.

However, it has been the third, most recent, era that has seen one if the most important developments: the introduction of all-sky, all-the-time commensal SETI on the Low Frequency Array (LoFAR) in Western Europe, at the Westerbork Observatory in the Netherlands, and on MeerKAT.

Commensal SETI refers to piggybacking on regular astronomical observations. For example, a radio telescope might be looking at a pulsar or a radio galaxy, but a back-end receiver is also listening for any anomalous radio signals in the telescope’s field of view while it is doing its regular astronomical work.

In their paper, MacMahon and Czech note how Breakthrough Listen has inherited a long tradition of commensal SETI from Berkeley astronomers, dating back to 1979 and the first version of Project SERENDIP, which stood for ‘Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations’. Led by Berkeley astronomers Dan Werthimer, Stuart Bowyer, Charles Donnelly and others, SERENDIP went through six iterations and was deployed on a number of large telescopes, including Green Bank and Arecibo before the latter was destroyed. There was also a collaboration with University of Oxford astronomers, who used SERENDIP VI’s data after it was installed on Arecibo in 2014 to search for fast radio bursts. This collaboration with Oxford would become important later.

The serendipitous nature of SERENDIP’s chances of success meant that while it was a cost-effective way of doing SETI without having to book expensive and exclusive telescope time, its dependence upon how other astronomers were using the telescope on which it was operating meant that its scope was always limited.

Today, however, Breakthrough Listen talks about another kind of commensal SETI, one that is ‘all sky, all the time’.

“People had been talking about doing all-sky, all-the-time for a while,” Steve Croft, a Breakthrough Listen radio astronomer jointly at the University of Oxford and the University of California, Berkeley, tells Supercluster.

This is why it is so important, because it means SETI has less chance of missing a signal just because a telescope wasn’t pointed in the right direction at the right time.

Green Bank’s 100-meter dish has a single-pixel receiver with a fairly narrow field for detailed work on specific targets. Croft likens it to looking through a drinking straw, which is great for a targeted SETI search, for example, if we wanted to listen to a particular star, but not great for doing large, indiscriminate surveys.

What’s needed for all-sky, all-the-time SETI is an array of smaller wide-field telescopes that can combine to provide great resolution across a large swathe of sky. To that end, Breakthrough Listen has moved to deploy commensal SETI experiments on arrays such as LoFAR and MeerKAT, the latter with an eye on scaling up the experiment for the full Square Kilometre Array-Mid when it comes online in the early 2030s.

The commensal SETI experiment on MeerKAT is called BLUSE, standing for ‘Breakthrough Listen User Supplied Equipment’. Because the field of view is so large (the primary beam of just one of MeerKAT’s 64 dishes is about a degree across) BLUSE has the freedom to observe any target in the field of view, even if it isn’t the target that the astronomers using the telescope are focusing on.

“We have our own back-end on there [BLUSE] that is picking out individual targets in the field of view,” says Croft. “Someone can be using the facility to look at a pulsar, and we can tick off a few SETI targets that are also in the field.”

BLUSE also employs machine learning at the receiver rather than afterwards when the data has all been downloaded. This allows for real-time alerts whenever a promising candidate signal is detected.

All-sky surveys do produce a lot of data, and handling all that raw information is challenging. MacMahon explains that the team originally planned to ship data from telescopes on hard disks, but this quickly proved impractical, and now all Breakthrough Listen data is downloaded over the Internet. It feels like the rest of the astronomical community has really been playing catch-up with SETI in recent years in terms of embarking on large surveys, big data and anomaly detection, all facilitated by machine learning and allowing the two communities to dovetail in their interests and specialities.

Many might have hoped that Breakthrough Listen would have answered the question of whether we are alone in the Universe by now. Alas, that hasn’t come to pass, but now that we are in year 11 of the 10-year project, it has become evident that the real value of Breakthrough Listen to the SETI community is in its investment in people and equipment that will allow the search to continue for many more years to come.

“One thing that I have been very involved in and very passionate about is our student internship program,” says Croft. He cites several undergraduate interns to Breakthrough Listen, including Sofia Sheikh, Karen Perez and Isabel Angelo, who have all since gone on to achieve PhDs either in SETI or a related field.

“You could count on the fingers of one hand, or close to it, the number of PhDs awarded in SETI before Breakthrough Listen came along!” says Croft. “So we are building this community and convincing the broader community. There are people sitting on [telescope-time allocation or funding] panels who previously might have looked down their noses at a SETI proposal but don’t anymore, and you can stand up at a meeting of the American Astronomical Society and talk about SETI, and you get a decent audience and people coming up to you afterwards offering to share their datasets and algorithms. We’re not out on a limb doing this weird thing by ourselves anymore; we’ve come into the heart of the scientific community.”

Another legacy of Breakthrough Listen is how it has made SETI international. For the first few decades of SETI, it had been a mostly American and Soviet affair, with the occasional input from a researcher in Europe or South America. When the Berlin Wall fell, so did state funding for Russia’s SETI work, but it continued in the United States as a privately funded project. For SETI to prosper, however, it has to become a global initiative.

To that end, in 2023 the Breakthrough Listen project announced that it was moving its headquarters to the University of Oxford, while keeping ties with Berkeley as well as the SETI Institute and the Allen Telescope Array – many of its researchers, including Croft, now split their time between sunny California and leafy, rainy Oxfordshire.

The reasons for the move were myriad. “There was a pull from Oxford as well as a push from us,” says Croft.

SETI has often been seen as a California-only ‘cult’, which, while not strictly true, is a perception that the Breakthrough Listen team were keen to shed, hence reaching out to scientists around the world to get different nations involved in the search. There was an eye on getting SETI onto the Square Kilometre Array, which will be the most powerful radio telescope ever built but of which the United States is not a member.

To get SETI on the SKA therefore necessitates the involvement of nations who are members of the SKA consortium. The United Kingdom is not only a member but also hosts the SKA’s headquarters at the University of Manchester’s Jodrell Bank Observatory. Oxford astronomers also have strong affiliations with the SKA and MeerKAT, and there was also a connection from the days of SERENDIP VI and a common interest in employing such techniques on MeerKAT and later the SKA. This all made Oxford an attractive choice for the future of Breakthrough Listen.

Despite searching thousands of nearby stars, millions more stars in the Milky Way and almost a hundred other galaxies, there is still no sign of extraterrestrial life. Has Breakthrough Listen brought us any closer to finding that elusive answer to the question of whether technological life exists beyond Earth?

“What it has done is place new constraints on the prevalence of technology in our galaxy,” says Croft.

Constraints are not as sexy as actually finding alien life, but they are important because they help us to manage our expectations and show us how much the net is closing around our extraterrestrial quarry. The Drake equation tries to estimate the number of technological civilizations in the galaxy at any one time, and it is useful for highlighting all the things that we don’t yet know, such as the fraction of planets that have life or the average lifetime of technological species.

As a predictive tool, however, the Drake equation leaves a lot to be desired since its estimates for the number of technological civilizations range from one – us – to a million or more; such is the wide uncertainty in many of its factors. Because we don’t know what the answer to the Drake equation should be, it is difficult for us to know what to expect. Should we have heard from someone by now, or should we not be surprised that ET hasn’t given us a call yet?

The constraints that Breakthrough Listen is providing do what the Drake equation cannot do: they help us to manage our expectations ahead of the next phase of SETI.

Back in the 1960s, when things really began kicking off for SETI and luminaries such as Frank Drake and Carl Sagan started to become household names, the expectation was that aliens would be transmitting powerful radio beacons towards us. Yet more than 60 years have now passed without detecting one of these radio-loud beacons, and SETI scientists were already beginning to think such beacons might be rare. Now Breakthrough Listen has been able to place a quantitative expectation on the existence of such powerful beacons.

“We can put some numbers on that, particularly for the nearby stars that we have studied in detail with big telescopes such as Green Bank,” says Croft. “We’re sensitive to beacons that are similar in strength to the Arecibo radar, and we can say that fewer than one in a thousand stars has a transmitter like that.”

There are caveats to all this. An always-on, omnidirectional beacon would soak up huge amounts of power and be expensive to run, so perhaps instead they cycle the transmission beam from star to star or have downtime, and when we just happen to be listening, they are either switched off or pointing in a different direction. Or, perhaps, they are operating at a different frequency to the ones that we are listening on, or are not using radio at all – Breakthrough Listen has made optical SETI a permanent fixture in response to this.

That doesn’t mean they are undiscoverable, however. Their own radar and deep-space communications might still be picked up, although without any regularity to their transmissions, we might find them difficult to confirm. Alternatively, we could potentially eavesdrop on their radio leakage. While the inverse-square law means that signals not intended for broadcast to space quickly become hard to hear, the Square Kilometre Array will extend the range at which we can pick up Earth-level radio leakage a little further. However, our own terrestrial leakage plaguing SETI as radio frequency interference really makes it difficult to discern a real discovery from interference, as the example of the biggest false alarm in Breakthrough Listen’s history, BLC-1, shows.

So even with Breakthrough Listen’s millions, the odds are still stacked against us. Nevertheless, despite now being 11 years into the 10-year plan, the search continues.

Breakthrough Listen is still funded to this day by Yuri Milner and the Breakthrough Foundation. They are joined by others, including the SETI Institute, which received a $200 million bequest from the late philanthropist Franklin Antonio in 2023, and research grants from the National Science Foundation.