This mysterious radio signal has puzzled astronomers for nearly 50 years

In 1977, radio astronomers at Ohio State University’s Big Ear telescope in western Ohio picked up a remarkably intense radio signal from deep space. It came from something exceptionally bright and fell in a frequency sweet spot that scientists thought intelligent alien life might use to send a radio transmission out into the universe.

When reviewing a printout of data from the observatory a few days later, Big Ear’s project scientist Jerry Ehman was so amazed at the signal that he circled it and scrawled a single word with red ink next to it: “Wow!” Today, it’s known as the “Wow!” signal.

That single whisper from the cosmos was never heard from again, but it has remained one of the most lingering mysteries in the history of astronomy. To this day, it has no definitive explanation. Now, a research team looking at the unusual signal—and others like it—are building a case for a more natural, less extraterrestrial explanation for the fleeting blip.

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A fleeting blip from the cosmos

In 1963, the Big Ear Observatory was built to scan overhead skies for radio signals from intelligent life on other planets. The survey fell into a category of astronomy projects called SETI, the Search for Extraterrestrial Intelligence. Big Ear was roughly the size of three football fields and focused on a small range of radio waves just covering a few frequencies. Scientists suspected that such “narrowband” signals would be more likely to come from extraterrestrials rather than stars or other phenomena that release radiation across broad ranges.

On a fateful summer day in August of 1977, the telescope detected a blast from space that lasted 72 seconds and came from somewhere in the vicinity of the constellation Sagittarius. It was 30 times more powerful than all background noise detected around it, and more importantly, it was a narrowband signal at 1,420 megahertz. This frequency almost exactly matched that of neutral hydrogen atoms called the hydrogen line. Since hydrogen is the most abundant element in the universe and is easily distinguishable from natural background noise, it's hypothesized to be a likely go-to frequency for aliens wanting to beam out any interstellar transmissions.

Over the years, the “Wow!” signal “has gained a reputation as one of the most intriguing phenomena logged by SETI searches,” according to Seth Shostak, a senior astronomer at the SETI Institute in California.

After 22 years in search of messages sent by aliens, the Big Ear was dismantled in 1998. It was the longest running SETI project in history. Still, its most iconic signal has only been observed once, astronomers can’t conclusively say whether it's a message beamed from another world or not. “In science, if you come up with an experimental result that cannot be replicated, then it’s really impossible to say that you know what caused it,” Shostak adds.

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Astronomers have thoroughly debated the signal’s origin. These theories run the gamut from passing comets, human-made radio interference from satellites, instrumental or data-processing artifacts, or some as--of-yet unknown astrophysical process.

Many years ago one of the faculty members at Ohio State told Shostak that the most likely explanation for this signal was terrestrial radio interference, and he Sagrees.

“In other words, it was a signal clearly caused by an intelligent species, but not an extraterrestrial one,” he says.

A natural phenomenon could be the missing puzzle piece

This hasn’t stopped researchers from investigating possible astrophysical sources for “Wow!” In the last few years, a team from the Arecibo Wow! Project at the University of Puerto Rico at Arecibo have been trying to get a clearer picture of the tantalizing signal by reanalyzing the signal picked up by Big Ear and hunting down similar signals.

“‘The Wow!’ signal has loomed large in the imagination of most astronomers,” Abel Méndez, an astrobiologist who leads the effort. “Most think it might have been caused by human-made radio interference, but we found a process that explains it pretty well.”

In a 2024 paper, Méndez and his colleagues suggested that the “Wow!” signal came from hydrogen in interstellar gas clouds. These clouds are known to emit narrowband radio signals, but they’d need an injection of radiation to produce a signal strong enough for Big Ear to pick up.

Méndez theorized a possible mechanism that could radiate the clouds: A super-magnetized, extremely dense star called a magnetar could have beamed concentrated microwave energy much like a laser. This magnetar flare could have eventually hit a cold hydrogen cloud and amplified it enough to appear as a blip from a distant civilization from the vantage point of our Earthly abode.

According to Méndez, these theoretical events would be uncommon, which could explain why the “Wow!” Signal has only been definitively detected once. If so, it would be the first recorded instance of this cosmic phenomena.

Back in 1977, magnetars hadn’t even been theorized, but astronomers wondered if some unknown phenomena could be at play. “Maybe the events we've found are caused by some new kind of astrophysical phenomenon, something that nobody has thought of yet,” Carl Sagan, famous astronomer, wrote in his 1994 book Pale Blue Dot.

Sagan considered it could be “not civilizations, but stars or gas clouds (or something)” that led to the baffling narrowband signal.

Digging through long-lost data

As part of the 2024 study, the Arecibo “Wow!” researchers also analyzed data collected by the Arecibo Observatory in Puerto Rico prior to its collapse in 2020. In Arecibo’s archived data, researchers found similar—albeit much less intense—signals.

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A year later, the team retrieved and reanalyzed signals from Big Ear’s archives using modern day digital tools that allow them to efficiently look at large sets of observational data.

“It was a surprise, and it allowed us to more precisely pin down its location of origin, intensity, and frequency,” Méndez says, adding that the team found the historic signal picked up in 1977 was four times greater than previously thought.

After the team’s two studies were uploaded to the preprint server arXiv.org, it caught the attention of some of the researchers and volunteers who’d originally worked at Big Ear while it was still surveying the sky for alien communications. Méndez says he’s received hundreds of documents and punch cards that have been kept away and gathered dust for decades in garages and attics.

The team is digging into the treasure trove of detections and presented some of this work in at astrobiology conference in May. (Ultimately, they hope to publish the data in a forthcoming scientific journal article, as well.) The effort is a bit like space archaeology—looking through old archives with modern science equipment and signal analysis techniques not available at the time of the initial detection.

So far, they’ve found 45,000 unpublished detections in Big Ear’s repository of signals. Méndez suspects some could be “Wow!”-type signals, meaning strong, narrowband radio signals right near the frequency naturally emitted by hydrogen.

In tandem with this deluge of observations, the project is revisiting 37 signals picked up between 1986 to 1991 from Harvard’s Project META, a SETI effort which searched for strong narrowband signals near the touted hydrogen line.

“This changes the panorama,” Méndez says. “The entire population of signals makes a strong, exciting case that the famous signal was some astrophysical phenomenon.”

Astronomers can’t deny an otherworldly missed connection

Méndez and his team aren't ready to rule out the possibility that it was a true signal from our interstellar neighbors attempting to do some cosmic outreach. “So far, two independent telescopes have detected [“Wow!”-type signals], and we have a good explanation.” Méndez says. “But we always have to bring in a certain amount of doubt.”

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While the origin of “Wow!” may be impossible to definitively pinpoint, the Arecibo Wow! project team does hope their research can narrow the hunt for similar blasts in follow-up radio surveys. “Throughout this process, we’ll be able to tease out which regions are more likely to emit these signals,” Méndez says. “That way we can dedicate telescope observing time to those spots where signals are more frequent, find any repeats and, then, confirm its origins.”

By understanding signals like “Wow!”, astronomers may be able to tell whether they’ve detected a strong flare that zapped a gas cloud with energy or a missive from another world—if or when E.T. gives Earth a call.