What Caused the Wow! Signal?

In search of an answer, scientists have used radio telescopes to broadcast signals into space and listen for responses since the 1960s. Unlike the optical telescopes we are more familiar with, which collect visible light, the waves that come into radio telescopes sound a lot like the static we hear when tuning between stations. The goal is to find something that stands out from all that background noise. 

It finally happened on August 15, 1977. That evening, while eavesdropping on deep space, we heard a response from the cosmos.

The birth of a mystery

A giant radio telescope in Ohio, affectionately known as the Big Ear, recorded an unusual 72-second signal: “6EQUJ5.” The code is pretty simple to crack. Low-power transmissions were recorded with numbers zero to nine. As the power increased, letters corresponded to numbers based on their placement in the alphabet. So, in this case, Q corresponded to 17, and U corresponded to 21. Numbers that high meant that the signal had registered 30 times louder than the background noise in space. The signal was so remarkably powerful that Big Ear team member Jerry Ehman circled the sequence and jotted down his reaction: “Wow!” 

Thus, the mystery of the Wow! signal was born.

Celebrations were almost in order for those involved in SETI, the search for extraterrestrial intelligence. The Big Ear team traced it back to the Sagittarius constellation, near a cluster of stars known as Messier-55, about 17,600 light-years away. That’s really far — about 30 million times the distance between Earth and Pluto. 

However, when they looked closer, there was nothing there — no planet and no star. The Wow! signal never returned, and nothing like it has been observed in any other part of the sky. Naturally, scientists did what they do best. They began to contemplate possible explanations, and there were a lot of them.

More than 40 years (and a lot of scientific detective work) later, there are still several competing theories that try to explain the seemingly unexplainable. Did humanity make its first contact with aliens? Could the signal have come from a comet? Or could it have been a star? 

Let’s break down some notable explanations for the Wow! signal, starting with — arguably — the most intriguing one.

Humanity’s first message from aliens?

Searching for messages from space would send even our best scientists on a wild goose chase. That’s precisely why 18 years before the Wow! signal was detected, physicists Philip Morrison and Giuseppe Cocconi imagined exactly how an intelligent alien civilization might try to signal Earth and published their findings in Nature. 

Morrison and Cocconi decided extraterrestrial life would likely contact Earth using radio waves, and they honed in on one particular frequency: 1,420 megahertz (MHz). 1,420 MHz is the absorption and emission frequency of hydrogen, the most common element in the universe. If any intergalactic communication was headed toward Earth, the scientists thought it would come in loud at 1,420 MHz. Coincidentally, the Wow! signal was 1,420.4556 MHz — just a smidge from where it was expected — and it was really powerful.

Unfortunately, the signal never repeated. If it was on purpose and aliens were trying to send a message into the cosmos, it would make sense for them to send it a few times instead of ghosting us all together. Perhaps it was an accident? After all, we have been accidentally sending radio signals into space since the invention of the radio. As electromagnetic waves are difficult to control, echoes from our television, news broadcasts, and phone conversations have been leaking into space for upwards of a century.

There’s always the chance, however slight, that alien transmissions could be bouncing off of us every day without us knowing for the simple reason that they might communicate beyond the limits of our understanding. However, until we find hard, empirical evidence, we can’t draw vast conclusions from our limited data. The sad reality of this explanation is that we may never know for sure, but it will always remain tantalizingly on the table.

A pair of passing comets

Just when hope was somewhat diminished (if not totally and completely lost), Antonio Paris, a former analyst for the US Department of Defense and an astrophysicist, took on the case and brought his own unique take. Paris told The Guardian, “I have this investigative background, so I approached the ‘Wow!’ signal as I’m going back to the crime scene.” He returned to the astronomical databases to find any “suspects” at the scene at the time.

Paris never found alien civilizations. However, he did find two comets that just so happened to be in the same part of the sky that Big Ear was monitoring on that un-fateful day: P/2008 Y2(Gibbs) and 266/P Christensen (phew! That’s a mouthful).

Comets were natural suspects because they are surrounded by massive clouds of hydrogen gas millions of miles in diameter. All that hydrogen would be detected at 1,420 MHz, so when the comets appeared again from November 2016 until February 2017, Paris was able to test his hypothesis. 

The radio signals from 266/P Christensen reportedly matched those from the Wow! signal. When he tested readings from three other comets, they all reportedly emitted radio signals at 1,420-MHz. The conclusion? There’s no way to say for certain that the Wow! signal was generated specifically by 266/P Christensen, but the researchers could say with relative certainty that it was generated by a comet.

However, Ehman — the signal’s discoverer I mentioned earlier — disagrees. He argues that scientists should have seen the source come through both receivers within three minutes if it was a comet. Big Ear never detected a second signal. 

Ehman wasn’t alone. In an interview with Discover Magazine, Chris Lintott, an astrophysics professor at Oxford University, pointed out that if the Wow! signal had been caused by comets, we would have seen similar signals all the time — not just once in 1977 and 2017. He also added that the comet's signal is not bright enough or rapid enough to match the Wow! signal. Scientists, including Lintott, have also voiced concerns about Paris’ methodology. Key details were allegedly missing from his research, making it impossible to recreate and substantiate.

Alas, the mystery of the Wow! signal remained very much unsolved. 

Sun-like star

Remember when I said the Big Ear couldn’t find a planet or star when it looked up at the source of the signal? All that changed shortly after the European Space Agency launched the Gaia space observatory to map out the sky. 

‍Gaia has mapped nearly 1.7 billion stars, determining their position, distance, and motion with an accuracy we’ve never seen before. The abundance of data has allowed astronomers to create the most detailed 3D map of our galaxy to date. This gave astronomer Alberto Caballero an idea: What if we could find the source of the Wow! signal in the new database?

‍Caballero looked for Sun-like stars — or stars that share the same radius, temperature, and luminosity as our Sun — among the thousands identified in the Wow! signal region of the sky. He found 2MASS 19281982-2640123 (OK, now this is a mouthful). It’s like our Sun, except that star sits in the Sagittarius constellation, 1,800 light-years away. 

Currently, 2MASS and its possible exoplanets are our best chance at finally cracking the Wow! signal mystery once and for all. However, that is still a ways off, literally.

None of the theories have received unanimous support from the scientific community — or even come close. In the end, the Wow! signal mystery may be just an astronomical ”cold case.” Perhaps it was, dare I say, caused by a glitch in the Big Ear telescope.

 I think I can speak for all of us when I say… I really, really hope not.