The Search For Extraterrestrial Life: A Brief History | Popular Science

The Search For Extraterrestrial Life: A Brief History

If (or, as some would say,_ when_) humans make contact with alien intelligence, the scientists who devote their careers to the search will be our first point of contact. Here, we look at the history of one of humankind's most persistent fascinations

For as long as humans have looked to the night sky to divine meaning and a place in the universe, we have let our minds wander to thoughts of distant worlds populated by beings unlike ourselves. The ancient Greeks were the first Western thinkers to consider formally the possibility of an infinite universe housing an infinite number of civilizations. Much later, in the 16th century, the Copernican model of a heliocentric solar system opened the door to all sorts of extraterrestrial musings (once the Earth was no longer at the center of creation and was merely one body in a vast cloud of celestial objects, who was to say God hadn't set other life-sustaining worlds into motion?) While that line of thinking never sat well with the church, speculation about alien life kept pace with scientific inquiry up through the Enlightenment and on into the twentieth century.

But it wasn't until the close of the 1950s that anyone proposed a credible way to look for these distant, hypothetical neighbors. The space age had dawned, and science was anxious to know what lay in wait beyond the confines of our thin, insulating atmosphere. The Russians had, in 1957 and 1958, launched the first three Sputnik satellites into Earth orbit; the United States was poised to launch in 1960 the successful Pioneer 5 interplanetary probe out toward Venus. We were readying machines to travel farther than most of us could imagine, but in the context of the vast reaches of outer space, we would come no closer to unknown planetary systems than if we'd never left Earth at all.

Our only strategy was to hope intelligent life had taken root elsewhere and evolved well beyond our technological capabilities—to the point at which they could call us across the empty plains of space. Our challenge was to figure out which phone might be ringing and how exactly to pick it up. And so it was in mid-September of 1959 that two young physicists at Cornell University authored a two-page article in Nature magazine entitled "Searching for Interstellar Communications." With that, the modern search for extraterrestrial life was born, and life on Earth would never again be the same.

_Launch the gallery to see how the search began and where it will take us next._

The Birth of SETI

Giuseppe Cocconi and Philip Morrison—two physicists at Cornell—began their 1959 article in Nature magazine quite frankly: we can't reliably estimate the probability of intelligent life out in the universe, but we can't dismiss the possibility of it either. We evolved and we're intelligent, so wouldn't it stand to reason that alien civilizations could arise on planets around other sun-like stars? In all likelihood, some of those civilizations would be older and more advanced than ours and would recognize our Sun as a star which could be host to life, with whom they would want to make contact. The central question of the paper was then: how would the beings send out their message?

Electromagnetic waves were the most logical choice. They travel at the speed of light and would not disperse over the tremendous distances between stars. But at which frequency? The electromagnetic spectrum is far too wide to scan in its entirety, so they made an assumption that has remained central to SETI research ever since. They would listen in at 1420 MHz, which is the emission frequency of hydrogen, the most abundant element in the universe. They reasoned it was the one obvious astronomical commonality we would share with an unknown civilization and that they would recognize it too.

The Drake Equation

Only a few years later, in 1961, the nebulous assumptions Cocconi and Morrison parlayed in their article got a bonafide mathematical equation. Frank Drake [with equation, at left], along with a handful of other astronomers and scientists (including Carl Sagan) met in Green Bank, West Virginia to hash out the formula and variables necessary to make an educated guess at just how many intelligent civilizations might be living in our galaxy. As it turns out, assigning numbers to nebulous assumptions nets you an answer with enough variance to make you wonder if you were really clarifying those assumptions in the first place. The group came up with a range from less than a thousand to nearly a billion.

You might think the formula would have been refined over the years, but that is not the case. It has held up surprisingly well (though, for such a nebulous equation "held up" is a relative phrase). Data collected since the 1960s, which can be used to support the original estimates of measurable quantities like how often sun-like stars form and how many of those stars have planets, has proven those estimates to have been relatively accurate. The rest of the variables will never be quantified, such as what fraction of life evolves to become intelligent and what the average lifetime of an intelligent civilization is. Still, the equation has served as a focal point for SETI investigations over the years and continues to be valuable framework, however controversial.


When we aren't looking for beacons from intelligent life forms in deep space, our studies in the realm of extraterrestrial life turn inward. How did life on Earth originate? How did intelligent life on Earth originate? These are two of the key questions at the heart of the interdisciplinary field known as astrobiology. While much of the work of astrobiologists can be speculative—extrapolating what may be elsewhere from what we know to be on Earth—that speculation must first come from solid research on what we see around us.

From what we know of life, it's generally assumed that extraterrestrials will be carbon-based, will need the presence of liquid water, and will exist on a planet around a sun-like star. Astrobiologists use those guidelines as the starting point for looking outward. Of course, the discipline includes traditional astronomy and geology as well. These are necessary fields for understanding where we should be looking for life outside of Earth and which properties we should seek when studying stars and their planets. While astrobiologists are looking deep into space for evidence of all these things, the largest single object of study is currently right in our literal backyard: Mars.


Life on Mars

We can safely assume we won't find any little green men on Mars. Likely, too, that we won't come upon any grey humanoid beings with almond-shaped, black onyx eyes and elongated skulls. But the chances are good that we could find alien life in the form of bacteria or extremophiles, which are bacteria-like organisms that can live in seemingly inhospitable environments. We have sent a variety of probes, landers, and orbiters to Mars, from the Mariner 4 in 1965 to the Phoenix mission, which landed in the planet's polar region this past May and continues to send back a tremendous amount of data. What we're looking for first and foremost is water, whether liquid or ice, one of the three keys to extraterrestrial life.

"I think it's probably the best bet for life nearby," says Dr. Seth Shostak, Senior Astronomer at the SETI Institute. "You could argue that some of the Jovian moons—Europa, Ganymede, Callisto—or Titan and Enceladus, these moons of Saturn, might have life. Even Venus might have life in the upper atmosphere. All those are possible because all those are worlds that might have liquid water. Mars you can see things on the ground, you can go dig around in the dirt, so we have a lot of people who worry about Mars. They're looking for life and we hope it's one of the right places."

Even without visiting the red planet, scientists have been poring over meteorites from Mars, tracing fine lines in the rocks which they have theorized were left by bacteria. The trails contain no DNA, however, so the theory remains unproven.

NASA/JPL-Calech/University of Arizona

Project Cyclops

Cocconi and Morrison's 1959 article about a systematic search for intelligent life took over a decade to filter through the various arteries of the burgeoning exploratory programs at NASA before it took the shape of a formalized research team. Known as Project Cyclops, the team and its resulting report document were the first large-scale investigation into practical SETI. It outlined many of the same conclusions Cocconi and Morrison reached: that SETI was a legitimate scientific undertaking and that it should be done in the low frequency end of the microwave spectrum.

What was not advantageous to the endeavor was the report's scope of cost, scale, and timeline. It called for a budget of 6 to 10 billion dollars to build and maintain a large radio telescope array over 10 to 15 years. It also made note of the fact that the search would likely take decades to be successful, requiring "a long term funding commitment." Certainly that was the project's death knell, and indeed, funding for Project Cyclops was terminated shortly after the report was issued. It would be 21 years before NASA finally implemented a working SETI program, called the High Resolution Microwave Survey Targeted Search (HRMS). But, like its predecessor, it would be exceptionally short-lived, losing operational funding nearly a year to the day later in October of 1993.

Pioneer Plaques (Pioneers 10 and 11)

As the search for signals from intelligent life was gaining credibility in the late 60s and early 70s, plans were at the same time underway to send out messages of our own. The mission of the Pioneer 10 and 11 spacecrafts in 1973 was to explore the Asteroid Belt, Jupiter, and Saturn; after that point, they would continue their trajectories past Pluto and on into the interstellar medium. With that distant course in mind, Carl Sagan was approached to design a message that an alien race might decipher should either craft be one day intercepted.

Together with Frank Drake, Sagan designed a plaque [left] which shows the figures of a man and woman to scale with an image of the spacecraft, a diagram of the wavelength and frequency of hydrogen, and a series of maps detailing the location of our Sun, solar system, and the path the Pioneer took on its way out. It was a pictogram designed to cram the most information possible into the smallest space while still being readable, but was criticized for being too difficult to decode. While the Pioneer 10 became the first man-made object to leave the solar system in 1983, it will be at least two million years before either reaches another star.


Arecibo Message

Since the advent of powerful radio and television broadcasting antennas, the Earth has been a relatively noisy place. News and entertainment signals have for decades been bounced off the upper reaches of our atmosphere, with plenty leaking out every which way into space. Those not pulled in by our TVs could one day reach distant stars, in a kind of scatter-shot bulletin announcing our presence through I Love Lucy and Seinfeld. (An unintended consequence of satellite and cable transmissions is the gradual end of high-powered radio signals, making the Earth a much more difficult place to "hear" for anyone listening in.)

In 1974, however, a formalized message was beamed out from the newly renovated Arecibo telescope in Puerto Rico. Again designed by Drake and Sagan, the binary radio signal [left] held within it information about the makeup of our DNA and pictographs of a man, the solar system, and the Arecibo telescope. The broadcast was ultimately more a symbolic demonstration of the power of the new Arecibo equipment than a systematic attempt at making contact with ET. The star cluster to which the signal was sent was chosen largely because it would be in the sky during the remodeling ceremony at which the broadcast was to take place. What's more, the cluster will have moved out of range of the beam during the 25,000 years it will take the message to get there.

It was an indication that we would likely not be in the business of sending messages, as it was much cheaper and easier to use radio telescopes to listen, rather than talk. But Sagan and Drake would have one more shot at deep space communications in 1977 with the launch of the Voyager probes.

Voyager Golden Records (Voyagers 1 and 2)

While the Pioneer Plaques were devised during a compressed timeline of three weeks and the Arecibo Message was sent according to the timetable of a cocktail party, the Voyager Golden Records were meant to be a brief compendium of the human experience on Earth and so were given the time and NASA committee resources to make them exceptional. The golden records contain 115 video images, greetings spoken in 55 languages, 90 minutes of music from around the world, as well as a selection of natural sounds like birdsongs, surf, and thunder.

Again, hydrogen is the key to unlocking the messages; the same lowest states diagram which appeared on the Pioneer Plaques is here describing the map locating the sun in the Milky Way. It informs the discoverer how to play the record, at what speed, and what to expect when looking for the video images. It's even electroplated with a sample of Uranium so that it might be half-life dated far in the future. Since the Voyager probes are moving much more slowly than radio waves, it will take them nearly twice as long as the Arecibo Message to reach their target stars. Even then, after 40,000 years, they'll only come to within a light-year and a half away. That's equivalent to about 130 times the distance Pluto is from our sun.

It's an understatement to say that any of these beacons we've sent have a very long shot of reaching an intelligent civilization, if one exists and happens to exist in the general direction in which they're traveling. It's a reminder of just how inhuman the scales become when we measure the distances in outer space and try to find ways to best them in our search for others like us.



As astrobiologists contemplate the origin of life on our planet, they often look to external sources for the ingredients. Asteroids, comets, and meteorites are the ancient relics of the birth of our solar system. They're the icy and rocky bits zipping around, crashing into each other and into moons and planets, delivering minerals, water, and, as it turns out, amino acids. It's amino acids—twenty in particular—that are the basis for protein formation, which in turn are the basis for life. So far, we have only discovered eight of those twenty in meteorites.

Where the others formed may be one of the secrets to life on Earth and possibly life on other planets. In the historic 1953 Miller-Urey experiment, a concoction of water and the elements of a primordial atmosphere were mixed and electrified to simulate the soup of early Earth. At the end of a week, amino acids had been formed. Of course, there are myriad other unknown processes which need to occur to take us from amino acids to life. As Dr. Seth Shostak of the SETI Institute put it, "just because you have a brickyard in your backyard doesn't mean you're going to see a skyscraper appear one day."


Studying extremophiles may be as close as we get to studying aliens before we actually find extraterrestrial life. Extremophiles are organisms which live in environments inhospitable to all other life as we know it. Some may even physically require these extremes of temperature, pressure, and acidity to survive. They have been found miles under the ocean's surface and at the tops of the Himalayas, from the poles to the equator, in temperatures ranging from nearly absolute zero to over 300 degrees Fahrenheit. Most extremophiles are single-celled microorganisms, like the domain Archea, whose members may account for 20 percent of the Earth's biomass. These are the kind of creatures we would expect to find on Mars.

But maybe the most alien-like of all extremophiles known to man are the millimeter-long tardigrades, or water bears [left], so called because they have the ability to undergo cryptobiosis. It's an extreme form of hibernation during which all metabolic activity comes to a near complete standstill and allows the animals to survive everything from massively fatal doses of radiation (to humans) to the vacuum of space. Some argue this suspended state doesn't technically qualify tardigrades as extremophiles because they aren't thriving in these environments, they are merely protecting themselves from death. Nevertheless, the more we understand about these organisms' ability to withstand environments thought to be inhospitable to life, the closer we may come to discovering them outside our planet.

The Wow! Signal

Though NASA killed Project Cyclops before it begin, that didn't mean no one was listening in on the cosmos during the 1970s. Several small-scale SETI projects existed around the country and around the world, many of them operating on university equipment. One of the most prominent—and longest running on SETI work—was the Big Ear radio telescope operated by Ohio State University. The Big Ear was the size of three football fields and looked like a giant silver parking lot with scaffolding for enormous drive-in movie screens at either end.

On August 15, 1977, the Big Ear received a signal for 72 seconds which went so far off the charts that the astronomer monitoring the signal print-outs circled the alphanumeric sequence and wrote "Wow!" in the margin. The pattern of signal rose and fell perfectly in sync with the way the telescope was moving through its beam of focus. As it came into view, it became progressively stronger. If the signal had been terrestrial, it would have come in at full strength. It was the best anyone had yet seen.

Unfortunately, two other attributes of the Wow! signal worked against it being a legitimate ET beacon. The first had to do with how the Big Ear collected radio waves. It used two collectors, spaced three minutes apart, side-by-side. Any signal caught by the first would have to be caught by the second three minutes later, but that wasn't the case with the Wow! signal. Only the first horn caught it. Even more discouraging, it hasn't been seen since. Many operations have tried, using more sensitive equipment and focusing for much longer on the alleged source to no avail.

Project Phoenix and the SETI Institute

NASA's High Resolution Microwave Survey Targeted Search really never stood a chance. Just as soon as it got underway in 1992, members of Congress began to hold it up as a waste of taxpayer money and deride it as frivolous (even though it accounted for less than 0.1 percent of NASA's annual operating budget). When it was cancelled in the fall of 1993, the SETI Institute moved in to save the core science and engineering team and continue the work under its auspices.

It was renamed Phoenix Project and ran for a decade from 1994 to 2004 entirely on funding from private donations. The project used a variety of large telescopes from around the world to conduct its research, observing nearly 800 stars in the neighborhood of up to 240 light years away. After sweeping through a billion frequency channels for each of the 800 stars over the course of 11,000 observation hours, the program ended without having detected a viable ET signal.

SETI@home at UC Berkeley

If you know anything about SETI and are of a certain age, chances are you know about it because of the SETI@home project at the University of California, Berkeley. SETI@home was one of the earliest successful distributed computing projects. The concept behind these projects works like this: researchers who have tremendous amounts of raw data and no possible way to process it all themselves split it into tiny chunks and subcontract it out. When you sign up for a distributed project, your computer gets one of these chunks and works on it when it's not busy, say when you leave your desk to get a coffee or take lunch. When your computer finishes, it sends that chunk back and asks for another. Taken as a whole, distributed computing projects are able to harness an otherwise impossible amount of processing power.

The SETI@home project currently gets all its data from the Arecibo radio telescope. It piggybacks on other astronomical research by collecting signals from wherever the telescope happens to be pointed during the brief moments when it is not being used. While the project has not yet detected an ET signal, it has been tremendously beneficial in proving that distributed computing solutions do work and work well, having logged over two million years of aggregate computing time.

Vatican Observatory

Galileo wasn't the only astronomer to have been accused by the Catholic Church of heresy for his beliefs in a heliocentric universe. Giordano Bruno was burned at the stake in the 16th century for arguing that every star had its own planetary system. How far the Church has come, then, with the announcement earlier this year from the Vatican Observatory that you can believe in God and in aliens and it isn't a contradiction in faith. The Reverend Joes Gabriel Funes, director of the Observatory, says the sheer size of the universe points to the possibility of extraterrestrial life. Because an ET would be part of creation, they would be considered God's creatures.

Chris Ware/Keystone Features/Getty Images

Extrasolar Planets

If it could be said a single discovery kick-started the search for extrasolar planets, it would be that of 51 Pegasi b [left], in 1995. It was the first extrasolar planet to be found orbiting a normal star and was discovered using the same Doppler effect we experience every day when a siren passes by us at high speed. It was a popular news story at the time—finally we had confirmation that just maybe our solar system was not unique. Since that day, we've learned how common, in fact, our system may be. As of early June 2008, the number of confirmed extrasolar planets is nearly 300; it climbs exponentially every year as our technologies for detection grow more sophisticated.

To be sure, the vast majority of these planets are gas giants in close, short orbits around their stars—not the kind of celestial bodies on which we expect to find life. That's not to say that Earth-like, terrestrial planets aren't out there as well. It's just that the gas giants are much easier to "see" when we go looking because they tend to zip around their parent stars in a matter of days. We watch those stars for variations in the way they give off light, but don't actually spot the planets themselves because they are so many magnitudes dimmer than their parent stars.

Gas giants are large enough and move quickly enough to produce a noticeable effect on their stars from here on Earth, but for a planet similar to Earth's size, that's not the case. In order to find an Earth-sized planet, we would need to watch a star nonstop for years on end and be able to detect the slightest change in brightness as the planet passed in front of it (known as a transit). Fortunately for SETI enthusiasts, NASA has just that mission on its schedule for launch next year.

The Kepler Mission

Looking for planets is necessarily hard work. In the astronomical scheme of things, most planets are very small and Earth-like planets are tremendously, even imperceptibly small. It is difficult enough for astronomers to detect planets on the scale of a Jupiter; nearly impossible to find an Earth, some 1,000 times smaller. NASA's Kepler Mission is the solution to that problem. It's a space telescope [left] designed to point itself at one field of stars in our galaxy for nearly four years, never wavering from that single point of focus, continuously monitoring the brightness of more than 100,000 stars.

The idea behind the mission is to use the transit method of discovery to find extrasolar planets like Earth. A transit occurs when a planet passes between its star and the observer (the Kepler telescope) during which time the star appears momentarily to dim, lasting anywhere from 2 to 16 hours. Of course, the orbit of the planet must be lined up to our plane of view, the chances of which are 0.5 percent for any given sun-like star. But with the tracking of 100,000 stars, NASA hopes at the very least to detect 50 Earth sized planets by the time the mission is complete; more if the observable planets prove to be up to twice as large as Earth.


Allen Telescope Array

Finally, we return to the SETI of Cocconi and Morrison's 1959 theory. While most astrobiologists study the origins of life and peer through telescopes at wobbling stars, there remains a dedicated core who continue to search the skies for the elusive ET beacon. Now, finally, they have a home in the Allen Telescope Array—more reliable than the piggybacking they used to cobble together. A joint project of the University of California, Berkeley and the SETI Institute, the Array is currently 42 20-foot diameter dishes (of an eventual 350 to be completed in the next three years). Upon completion, it will be unprecedented in its research capabilities, able to conduct complicated radio astronomy and SETI analysis simultaneously.

The project came out of a series of workshops held by the SETI Institute and UC Berkeley in the late 1990s as frustration mounted over having to use other institutions' antennas for research. What the team quickly discovered—and what made the project feasible—was that the cost of the receiver electronics has dropped by a factor of 100 over the previous twenty years. That put very sophisticated technology into an affordable price point and the Allen Array was born.

Whether the Allen Array will be the telescope to catch the first ET signal is, of course, anybody's guess. It will certainly be one of the most powerful when it is completed, able to capture much larger pieces of the sky at one time than previous technologies. It's an unfathomably large universe out there and we're only ever talking about searching our own galaxy, one of an estimated 100 billion. As the original 1971 Project Cyclops report suggested, the search for extraterrestrial intelligence is not one that can be completed overnight:

"The search will almost certainly take years, perhaps decades and possibly centuries. This . . . requires faith. Faith that the quest is worth the effort, faith that man will survive to reap the benefits of success, and faith that other races are, and have been, equally curious and determined to expand their horizons. We are almost certainly not the first intelligent species to undertake the search. The first races to do so undoubtedly followed their listening phase with long transmission epochs, and so have later races to enter the search. Their perseverance will be our greatest asset in our beginning listening phase."

So let us believe.



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