A radio signal detected by an Australian telescope in 2019, which seemed to be coming from the star closest to the sun, was not from aliens, researchers reported in October 2021 in two papers in Nature Astronomy. “It is human-made radio interference from some technology, probably on the surface of Earth,” says Sofia Sheikh, an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute and a co-author of both papers.

But the disturbance, detected by Breakthrough Listen—an ambitious and privately funded $100-million effort—looked intriguing enough at first that it sent astronomers on a nearly yearlong quest to understand its origins. It was the first time that data from Breakthrough Listen triggered a detailed search, and the experience put scientists in a better position to study future candidate detections.

“It's really valuable for us to have these dry runs,” says Jason Wright, an astronomer at the Pennsylvania State University. “We need these candidate signals so we can learn how we will deal with them—how to prove they are extraterrestrial or human-made.”

Mysterious Blips

Since 2016, Breakthrough Listen has used telescopes around the world to listen for possible broadcasts from alien civilizations. The program has picked up millions of radio blips of unknown origin, nearly all of which could be swiftly classified as coming from radio interference on Earth, from sources such as mobile-phone towers or aircraft radar.

The 2019 signal was different. It was detected by the 64-meter Parkes Murriyang radio telescope in southeastern Australia and came from the direction of Proxima Centauri—the nearest star to the sun, just 1.3 parsecs (4.2 light-years) away. Proxima Centauri is of intense interest to SETI researchers, not just because it is nearby. The star has at least two planets, one of which orbits at the right distance for liquid water to be present on its surface—a prerequisite for life as it exists on Earth. A sibling initiative to Breakthrough Listen, known as Breakthrough Starshot, aims to send a tiny spacecraft to this planet in the future to look for life there.

The mysterious signal was first spotted in 2020 by Shane Smith, an undergraduate at Hillsdale College, who was working as a research intern with Breakthrough Listen. Smith was combing through data that Parkes collected over six days in April and May 2019. The telescope had been making observations in the direction of Proxima Centauri for 26 hours. It was not searchng specifically for aliens at the time but was instead monitoring flares on the star's surface, which could hurt the chances for life to arise on nearby planets.

The data included more than four million signals from the vicinity of the star, but Smith noted one signal near 982 megahertz that seemed to originate from the star itself and lasted about five hours. “I was excited to find a signal that matched all the criteria I was looking for, but I immediately remained skeptical of it and thought there had to be some simple explanation,” Smith says. “I did not ever think the signal would cause such excitement.”

Smith shared the information with his supervisor Danny Price, who posted it on a Breakthrough Listen Slack channel, and the team started investigating in earnest. “My first thought was that it must be interference, which I guess is a healthy attitude, to be skeptical,” says Price, an astronomer at the University of California, Berkeley, and the Breakthrough Listen project scientist in Australia. “But after a while I started thinking, this is exactly the kind of signal we're looking for.”

The signal, named BLC1 for “Breakthrough Listen candidate 1,” was the first to pass all the program's initial screening tests to rule out obvious sources of interference. “It definitely had me wondering ‘what if?' for a bit,” Sheikh says.

Sheikh, Price and a large group of colleagues began working through possible explanations, from uncatalogued satellites to transmissions from planetary spacecraft. In Australia, the radio-frequency band around 982 megahertz is primarily reserved for aircraft, but the scientists could not identify any airplanes that had been in the area and could account for the signal—and certainly not one present for five hours.

In November 2020 and in January and April 2021, the researchers pointed the Parkes telescope at Proxima Centauri to see whether they could pick up the signal again. They could not.

Eventually the team spotted other signals in the original data that looked a lot like the 982-megahertz signal but were at different frequencies. The team's automated analysis had tossed these signals out as earthly interference. Further analysis showed that BLC1 and these “lookalike” signals were all interference from an unknown source. The signals had modulated and muddied one another, much as a guitar amplifier modulates and distorts a guitar note, which is what made it so difficult to identify BLC1 as interference.

Earthly Origins

Because the signal didn't reappear in the 2020 and 2021 observations, it might have been coming from malfunctioning electronic equipment that got shut down or fixed, Sheikh says. The team suspects the equipment was relatively close to Parkes, perhaps within a few hundred kilometers. The frequency of the signal drifts in a way that is consistent with inexpensive crystal oscillators such as those commonly used in computers, phones and radios, says Dan Werthimer, a SETI astronomer at U.C. Berkeley who specializes in signal processing.

Working with another student, Sheikh is now using machine-learning algorithms to tease out what frequency the interfering equipment was transmitting at, which might help them track down its source. One lingering mystery is why the signal seemed to appear only when the telescope was pointed at Proxima Centauri. That might just be an unfortunate coincidence if the cadence of the interference mimicked the cadence with which the telescope was looking at the star.

Radio interference has bedeviled other astronomical searches before, such as when flickering signals picked up at Parkes turned out to be the result of people microwaving their lunches. The famous “Wow!” signal, detected in 1977 by a radio telescope in Ohio, was a powerful blip so intriguing that the observing scientist scribbled “Wow!” in the margins of the computer printout—but its origin could never be traced.

Alien searches have become much more sophisticated since then, Sheikh notes. “Many groups assumed that if you had a detection that only showed up when you were pointed at the source, that was it, break out the champagne, you're done,” she says. “As technology changes, the way we vet signals also has to change—and that hadn't come together until BLC1.” One of the Nature Astronomy papers features a detailed checklist to help astronomers determine whether their signal is truly from aliens.

“The universe gives us a haystack,” says Ravi Kopparapu, a planetary scientist at the nasa Goddard Space Flight Center. “It is our need to find the needle in it and make sure that it is actually a needle that we found.”