Astronomers searching for signs of life beyond Earth have spotted something strange. An as-yet unexplained radio signal appears to be coming from the direction of the star closest to the sun—a small red star roughly 4.2 light-years away called Proxima Centauri. Adding to the excitement, at least two planets orbit this star, one of which might be temperate and rocky like Earth.
Breakthrough Listen, a decade-long search for alien broadcasts from the nearest million stars, was using Australia’s Parkes Observatory to study Proxima Centauri when the team detected the conspicuous signal, which they dubbed BLC-1. The radio waves were picked up in observations made between April and May 2019.
“It’s pretty expected that every now and then you’ll see something weird, but this is interesting because it’s something that’s weird that we’re having to think about the next steps,” says Sofia Sheikh, a graduate student at Pennsylvania State University and the Breakthrough team member leading the signal analysis.
Though Sheikh and others strongly suspect that the signal is really human in origin, BLC-1 is the most tantalizing detection Breakthrough has made so far in its search for extraterrestrial intelligence, or SETI. The team is preparing two papers describing the signal and a follow-up analysis, which isn’t yet complete. (The detection was leaked to The Guardian before the research was ready for publication.)
While researchers continue to analyze the signal—and experts caution that there is almost certainly an ordinary, terrestrial explanation—even a remote hint of life beyond Earth has people excited.
“There’s a lot of talk about sensationalism in SETI,” says Andrew Siemion, Breakthrough Listen’s principal investigator. “The reason we’re so excited about SETI, and why we dedicate our careers to it, is the same reason why the public gets so excited about it. It’s aliens! It’s awesome!”
Six decades of searching for extraterrestrials
Scientists have been scanning the skies for radio signals that could be artificial in origin for 60 years—starting with Project Ozma, a search conducted in 1960 by my dad, Frank Drake.
Unlike radio waves the cosmos produces naturally, these whispers from extraterrestrials are expected to look a lot like the transmissions humans use to communicate. Such signals would cover a very narrow range of radio frequencies. They would also have a characteristic “drift” indicating that the source is moving toward or away from Earth—a clue that the radio source is coming from a distant cosmic object, such as a planet orbiting a star.
“Only human technology seems to produce signals like that,” Sheikh says. “Our WiFi, our cell towers, our GPS, our satellite radio—all of this looks exactly like the signals that we’re searching for, which makes it very hard to tell if something is from space or from human-generated technology.”
Over the decades, astronomers have detected numerous candidate signals. Some turned out to come from previously unknown astronomical sources such as pulsars, the rapidly rotating corpses of dead stars that beam radio waves into the cosmos. The first known fast radio bursts—brief blasts of radio waves that are still somewhat mysterious—initially seemed as though they could be artificial signals. Signals called perytons, which are less energetic bursts of radio emission, also raised eyebrows until scientists determined their origin: a microwave oven.
BLC-1 could be beaming from an object that isn’t transmitting as expected: a satellite that hasn’t been identified yet, a plane traveling overhead, a transmitter on the ground near the telescope’s line of sight, or perhaps something even more mundane, like faulty electronics in a nearby building or a passing car.
“All of our SETI experiments are conducted in an absolute sea of interference. There are tons of signals,” Siemion says. ”It comes down to being able to tell the difference between a very distant technosignature and our own technology.”
And then there are the signals that astronomers haven’t been able to definitively pin to a natural source, such as the famous “WOW!” signal picked up by the Ohio State University Radio Observatory, colloquially known as Big Ear, in 1977. This extremely bright barrage of radio waves initially looked like a real SETI detection, but no one has been able to verify it or find it again.
A strange signal
In 2015, Breakthrough Listen kicked off a decade-long search funded by Silicon Valley investor Yuri Milner, and so far, the team hasn’t found anything definitive in their scans of the skies.
Starting in April 2019, Breakthrough aimed the Parkes telescope at Proxima Centauri—not necessarily because scientists were searching for aliens, but because they were hoping to better understand the gargantuan flares that small red dwarf stars like Proxima frequently emit. While processing those observations this summer, Shane Smith, an undergraduate from Hillsdale College in Michigan working with Breakthrough, spotted BLC-1 apparently radiating from the star.
Although the signal is faint, BLC-1 passed all the tests the Breakthrough team uses to filter out the millions of signals generated by humans: It was narrow in bandwidth, appeared to drift in frequency, and disappeared when the telescope shifted its gaze from Proxima to a different object. Over the following days, four similar signals appeared, although some have been ruled out as radio interference.
“Our algorithm is very optimistic about what might be alien technology,” Sheikh says. “But this is super exciting because we’ve never gotten to the stage where the algorithm found something that’s actually interesting.”
If BLC-1 is, against all odds, a postcard from the star system next door, then statistically speaking the Milky Way must be absolutely stuffed with communicating civilizations, says the SETI Institute’s Seth Shostak. “In this case, there would be more than a half-billion societies out there in our own galaxy—that seems like a lot.”
Since the detection, the team has again observed Proxima Centauri—and found nothing. Scientists are working on developing new tests that could pinpoint the signal’s origin, including continuing to aim the Parkes telescope at Proxima.
“If you want to make any scientific statements, you’re going to need to be able to reobserve and replicate the phenomenon,” Sheikh says. ”That’s just how the scientific method works.”
Earlier this year, Jill Tarter of the SETI Institute told me that the process of creating new tests and diligently working to confirm a signal’s origin is a natural part of the SETI endeavor, and one that everyone can learn and benefit from.
“We’re looking for something else out there, someone else out there,” Tarter said at the time. “To suddenly see interference, and think it might be what we’re looking for, and then figure out what we have to do to be able to discriminate and to have confidence in any result that we might get—that’s a good lesson.”
Already, Siemion says that evaluating BLC-1 has taught the team a lot about testing their data. Follow-up observations of Proxima Centauri will be valuable for understanding how such stars behave—as well as for achieving a comprehensive SETI search of a nearby star system with known planets, even if it isn’t populated by technologically savvy aliens.
“Ultimately, I think we’ll be able to convince ourselves that [BLC-1] is interference,” Siemion says. “But the end result will certainly be that it will make our experiments more powerful in the future.”