On a summer evening in 1977, astronomers intercepted a signal from the cosmos.
The unusual signal astonished scientists at the Ohio State University Radio Observatory (aka Big Ear). It wasn't the usual background noise they had come across. With red ink, Jerry Ehman, who was monitoring radio signals at the station then, circled the code of letters and numbers on the signal printout and penned "Wow!" next to it.
Researchers ran tests and made sure the signal wasn't coming from Earth or other active places in our solar system. They never heard the signal again, despite a whole lot of searching for it.
Were aliens making an effort to communicate with our planet? Was the burst in radio waves a message from other life inhabiting the billions of systems in our universe?
The Wow! signal questioned the idea that we're alone in the cosmos. Yet the fact that we never heard it again is even more puzzling.
In this article, we'll revisit cornerstone moments in our search for extraterrestrial life. Keep in mind that our 10-step journey includes finding tiny, microscopic bacteria, as well as chasing the idea of otherworldly beings capable of intelligent thought.
So where did it begin?
10: Copernicus and Heliocentrism
Nicolaus Copernicus might not be the first person to come to mind when you go alien hunting, but this astronomer contributed more to the search for ET than you'd think.
Copernicus wanted to better describe what was going on in the heavens. In the 1540s, he came to a conclusion that was radically different from popular thought at the time — that Earth, along with the other planets in our solar system, orbits around the sun rather than the other way around.
It took nearly a decade for his heliocentric theory to gain ground with the public. Once it did, his view led to more people pondering whether we're really alone in the universe. If Earth wasn't special enough to be the center of everything, it would also mean there could be other planets out there, with foreign worlds potentially mirroring our own.
But connecting the dots between having a sun-centered solar system and aliens isn't that easy.
9: 16th Century Planet-Gazing
Italian intellectual Giordano Bruno lost his life for his unpopular stances. One was proposing the possibility of ET.
His ideas followed Copernicus' discovery that the Earth was neither the center of the universe nor the solar system. But Bruno didn't just side with Copernicus. He also wrote about the possibility of there being other worlds with people — much like us — on them.
Bruno surmised that Earth wasn't as special as previously thought.
In his 1584 book "On the Infinite Universe and Worlds," he wrote: "Innumerable suns exist; innumerable earths revolve around these suns in a manner similar to the way the seven planets revolve around our sun. Living beings inhabit these worlds."
It's unlikely Bruno's image of aliens matched some of the leathery, green extraterrestrials that grace movie screens today. Rather, he put forth the idea that other planets could have other humans or dimensions of reality.
Unfortunately for Bruno, his beliefs weren't tolerated. He was burned at the stake in 1600 by the Italian Inquisition.
8: Grabbing an Extraterrestrial's Attention
A couple hundred years later, alien enthusiasts dreamt of using land and fire to connect with potential foreign beings.
In the 19th century, German mathematician Karl Friedrich Gauss sought to make extraterrestrial contact in a few ways. He thought up a device called a "heliotrope" that reflected sunlight toward other planets to craft messages. He also tried to catch an alien eye when he suggested creating a large wheat triangle in the Siberian forest. Gauss figured the shape would be visible to extraterrestrials at a distance and show there was intelligent life on Earth.
Another gentleman by the name of Joseph von Littrow had a similar idea — but this time with fire. He wanted to dig gigantic trenches in the Sahara Desert and fill them with oil to set on fire. Ablaze shapes would spell out messages. Using symbols to reach out to ET was the norm, considering that math and physics were common ground across the universe.
7: UFOs and the Race to the Cosmos
Once humans actually began exploring space, it became easier to imagine running into an extraterrestrial neighbor. Science fiction became more popular, and so did the idea of intelligent aliens.
You've probably heard of the conspiracy theories about a UFO of unearthly origin crashing in Roswell, N.M., in 1947. Scrap metal wasn't the only thing found at the crash site, some say. Reports suggest that nonhuman bodies were found, too. The U.S. government, however, denies finding aliens, saying test dummies and even human casualties from top-secret missions were confused with the Roswell incident. Conspiracies about what really happened lingered into the 1970s, when the event popularized iconic images of light-skinned aliens with slits for mouths and eyes.
The space race between the United States and the Soviet Union likely stirred up people's imaginations, too. Both countries' space programs gained public support and even encouraged thinking about life beyond Earth's atmosphere in the '50s and '60s.
6: SETI, Ozma and Cyclops, Oh My!
By the mid-20th century, researchers were serious about scanning the cosmos for other life, but they weren't exactly sure how to do it. With billions of light-years to explore, where do you even start?
Two astronomers, Philip Morrison and Giuseppe Cocconi, wanted to get the ball rolling. In 1959, they published a journal article in Nature titled, "Searching for Interstellar Communications." This is where the idea to use electromagnetic waves to scan the skies took root.
The result was the foundation for SETI, also known as the search for extraterrestrial intelligence. At around the same time, scientist Frank Drake was experimenting with radio observatories that could intercept signals from the skies. He named his efforts Project Ozma. It would later become an inspiration for larger projects.
In the 1970s, the Cyclops Report raised questions for discussion among the scientific community. Much talk from that moment forward focused on building the SETI community. A researcher named Bruce Murray, who was involved in the efforts, argued it was useless to focus on star systems where humans figured aliens might reside. Rather, he reasoned, it would make more sense to scan the entire sky for any possible signal at all, writes Amir Alexander in the Planetary Society's short history on SETI.
Soon SETI became backed by NASA. And later, it became the focus of an institution.
5: The Drake Equation
Following the burst of interest in extraterrestrial life in the early 1960s, astronomer Frank Drake came up with a math equation to predict the formation of stars and planets surrounding them.
His calculation, called the Drake Equation, would help pioneer the search for ET in future years. He looked at the statistical chance of other Earth-like planets that can host life. At the Green Bank conference, where several researchers met and the equation was first discussed, it was suggested that anywhere between 1,000 and one billion planets may be home to intelligent extraterrestrial life, according to the Planetary Society's short history of SETI.
Though Drake used his equation as more of an organizational tool, it ended up boosting scientists' eagerness to monitor the atmosphere for possible signals from outer space.
4: Exobiology Rises Up
At around the same time as Drake was messing with mathematics, NASA funded its first exobiology project in 1959 as a part of its space program.
Exobiology examines the possibility of life outside of our planet. Think about it this way: In what conditions would other life be able to survive? What chemicals and temperatures would be good or bad for extraterrestrials throughout the universe?
At this point, NASA wanted to look at conditions beyond the Earth and moon. Learning about the foreign terrains and atmospheres of other planets in our solar system — like our neighbors Venus and Mars — became a priority.
The space agency funded exobiology projects during the Viking Mars mission in 1959, where the team tried to detect possible life on the red planet. The field no longer limits itself to other worlds, though. Scientists have also focused on organisms on Earth that get by in extreme environments (hence naming them "extremophiles").
3: SETI Finds a Home
Before the mid-1980s, SETI was made up of a broad framework of scientists searching for alien life.
NASA helped fund the search for ET into the early '90s at centers in Puerto Rico and the Mohave Desert. But U.S. Congress soon cut both programs from its budget. At the time, SETI appeared dead.
But the privately funded SETI Institute stepped in to create a new home for the alien search. The move united astronomers and gave them a place to continue working. The SETI Institute still leads efforts to find ET to this day.
The Planetary Society, founded by famous astronomer Carl Sagan, also vigorously supported the search for extraterrestrial life. Different from the SETI Institute, the society concentrated on more specific projects with unique approaches to hunting down other cosmic life.
2: Exoplanets Galore
If humans can't get to planets in person, they can at least study the spheres from afar. Fortunately, scientists began discovering planets outside of our solar system in the mid-1990s.
NASA's Exoplanet Exploration Program (ExEP) seeks to discover and catalog exoplanets and systems with the goal of finding Earth-like planets that could sustain life. Ideally, such planets would be located at the perfect distance from the host star. This habitable zone — where planets aren't too close to stars for water resources to boil, or too far to keep water frozen — often gives places in other solar systems the moniker "Goldilocks" planet.
The space agency's Kepler Observatory has also given scientists a better look at where extraterrestrial life might reside.
1: Allen Telescope Array
Despite a long history of searching for aliens, there's plenty of reason to believe we'll keep on looking.
For instance, the Allen Telescope Array in the Cascade Mountains has taken the reins in finding ET. As a joint project of the SETI Institute and the University of California- Berkeley, the telescopes amplify transmissions to look at celestial signals at a closer level. The project will use more than 350 radio dishes in its scans. But what makes the array unique is the fact that it can situate equipment in several different directions to make a bigger radio ear than ever before.
Hopeful to find another hint like the Big Ear Radio Observatory's Wow! signal in 1977, scientists continue their search for aliens. With advances in technology and a better idea of what it might take for other life to exist, Earth will persist in scanning the skies for ET — likely for years to come.