Are we alone? For a moment on August 15, 1977, it certainly looked like the answer might be no. That night the Big Ear radio observatory at the Ohio State University was blasted by a remarkably intense transmission from the sky. Lasting at least 72 seconds and coming in on an extremely specific frequency, it didn’t appear to have any of the hallmarks of a natural astrophysical phenomenon. Instead it resembled what we’d expect from an artificial source.
The radio signal vanished as quickly as it had appeared, and neither it nor anything quite like it has ever been detected since in the long, unrequited search for extraterrestrial intelligence (SETI). Named the Wow! signal, after an exclamatory note that a SETI researcher scrawled on a printout of the recording, various ideas have arisen that attempt to explain it. Maybe it was strange radiation from a comet. Many researchers argue it was most likely some form of human-made radio interference. Or, just maybe, it was a message from some staggeringly advanced cosmic civilization—a possibility that, even now, has not yet been definitively ruled out—not for scientists’ lack of trying, however.
The latest explanation emerged last week from a trio of astronomers in a preprint that has not yet been subjected to peer review. And sorry, once again, it’s not aliens. The researchers suspect that the Wow! signal was created when a flare from a hypermagnetized, hyperdense star called a magnetar struck a cold interstellar cloud of hydrogen gas. The flare caused the cloud to incandesce in the radio wavelength, and this fast-and-furious outburst was detected by Big Ear.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Lead author Abel Méndez, director of the Planetary Habitability Laboratory at the University of Puerto Rico, for many years dismissed the Wow! signal as a mere instrumental glitch. But after scrutinizing several somewhat Wow!-like signals found unexpectedly in archival data from the late, great Arecibo Observatory, he and his colleagues now suspect that the famous yawp from 1977 was caused by a very rare kind of astrophysical anarchy.
“I would say, wow—I never thought of that. I never thought of the Wow! signal as being real and being produced by some weird astrophysical phenomenon,” Méndez says.
Other astronomers applaud the fresh attempt to solve this longstanding puzzle. Exactly how this deep-space fracas would produce the Wow! signal is not yet fully understood, however. “It has promise, but the details are still pretty scant,” says Yvette Cendes, a radio astronomer at the University of Oregon, who was not involved with the new work.
The researchers’ flare-meets-hydrogen-cloud story is “definitely a bit speculative,” says study co-author Kevin Ortiz Ceballos, a graduate student of astrophysics at the Center for Astrophysics | Harvard & Smithsonian. “We’re not saying that this is definitely the case. We’re saying that it’s a very exciting hypothesis.”
Looking for suspicious radio waves is a popular method of questing for aliens. The trouble is and always has been that almost any strange emission can in principle be explained as some rare-but-all-natural astrophysical phenomenon, from burping black holes to convulsing planetary atmospheres. The key thing that distinguishes “natural” from “artificial” in this context—think of our own terrestrial transmissions, for instance—is that the latter tend to be narrowband, focused in a very small swath of radio frequencies rather than smeared across a huge range. They also often have a structure to them that intentionally encodes information.
Scientists searching the stars for such artificial transmissions can be rather tightly focused themselves, often preferring to scan a specific frequency: 1,420 megahertz. Neutral hydrogen, the simplest element there is, naturally emits radio waves at this frequency, which is something that presumably all astronomers—human or otherwise—would know about. (Additionally and more fortuitously, 1,420 MHz lies in a thin band of electromagnetic spectrum that, on Earth, is held off-limits from human transmissions by international agreement.) This confluence of factors makes the frequency a SETI favorite.
The Wow! signal was not only a narrowband signal—it was also right in the 1,420 MHz sweet spot, and it was 30 times more intense than any observed background noise. Its celestial provenance couldn’t be precisely located. It appeared to come from near M55, a dense cluster of stars in the constellation Sagittarius, however. It had the look of something like an intentional transmission that might be expected to repeat. But with just a single 72-second snippet in our data banks, followed by no further detections from that region of the sky, nobody could say for sure.
“The Wow! signal has always been divisive in the SETI community,” says Jason Wright, an astronomer at Pennsylvania State University, who was not involved with the new work. “It’s very common to get one-off signals that one can’t completely explain.” And most of the time, Wright notes, such anomalies show hints that they’re instrument malfunctions or the interception of a weird-looking, human-made radio transmission.
Curious to whether the Wow! signal was genuine or not, Méndez and his team decided to see if they could find anything resembling it by digging through the archival data of the now-defunct Arecibo Observatory, which for decades was humanity’s largest and most sensitive single-dish radio telescope. To their surprise, they unearthed “signals that were very similar to what the Wow! signal looked like,” Ceballos says—several narrowband radio emissions thrumming close to the vaunted 1,420 MHz frequency. In total, the team’s analysis revealed eight Wow!-like signals that Arecibo had recorded in an about an hour’s worth of observing time during brief and intermittent scans of small, disparate patches of sky between February and May 2020. Each of these signals was at or extremely close to 1,420 MHz but at anywhere from 50 to 100 times less intensity than the one that blasted the Big Ear nearly a half century ago.
Seeing so many spatially scattered Wow!-like signals in such a short time suggests a natural origin—and the most obvious culprit would be the innumerable clouds of 1,420-MHz-emitting neutral hydrogen that astronomers have long known to be common in interstellar space. Perhaps, Méndez and his coauthors suggest, the Big Ear was coincidentally pointed at one such cloud on that fateful night in the summer of 1977. And because it had been bombarded by a powerful source of radiation, this normally frigid cloud was instead shining brightly in 1,420-MHz radio waves.
To create such an intense radio burst, the initial salvo of radiation would have been savage. Even a supernova explosion cascading into the cloud and compressing it wouldn’t have sufficed: that would have unleashed plenty of radio waves, but they would have occurred over a broad range of frequencies. Instead the team posits that a directed burst of x-rays and gamma rays—perhaps a flare from a magnetar—could have slammed into the hydrogen cloud with such staggering intensity that the result was a potent narrowband glow, which we registered as, well, “Wow!”
Such a bright, directed emission of radio waves is known as a maser (the radio equivalent of a laser). In this case, it’s a type of hydrogen maser. Hydrogen masers in space are rare, but they have been observed for several decades. None at 1,420 MHz have been definitively detected, though—and although such emissions have been made in the lab, and there is some theoretical work partly explaining how they may naturally appear, astrophysicists figured this process was extremely unlikely to occur in space. This study suggests that it can happen somehow in nature, yet the specific details of the physics remain murky.
“I like this creativity,” says Michael Garrett, chair of the International Academy of Astronautics’s SETI Permanent Committee, who was not involved with the new work. “But it feels a bit contrived to me.” Several improbable things needed to happen all at once: Big Ear just happened to have been looking at the exact snapshot of sky in which a magnetar flare smashed into a hydrogen cloud. And a hydrogen maser at that exact narrowband frequency—a celestial event which has otherwise never been observed before—was serendipitously produced so that Big Ear could detect it.
“It’s not impossible, for sure,” Cendes says, and the Wow!-like signals picked up by Arecibo make the study authors’ case an intriguing one. For now, however, their hypothesis needs more astrophysical theory work to underpin it and explain how nature can manufacture this specific type of maser. Ultimately, she says, the narrowband nature of the original—a key hallmark of putative technology—means that some sort of human-made radio interference is the more likely explanation.
Of course, its narrowband feature could still mean that the Wow! signal’s origin is nonhuman technology—an intentional transmission beamed from minds elsewhere in the Milky Way or beyond. So could it be aliens after all? “No, I don’t think so,” Méndez says. Nothing else going bump in the night has ever fulfilled SETI’s hopes for first contact, he notes, so why should this one be so different? Time after time, each tantalizing hint of alien whispers instead proves to be a misunderstood echo of esoteric astrophysics. What begins with a “wow!” ends as a “meh”—so far, anyway.