Is there intelligence elsewhere in the universe? It sounds like a simple question, and for most people there's an obvious answer: Of course. After all, the universe is so big. As the iconic scientist Carl Sagan put it, "there are 100 billion galaxies, each of which contains something like 100 billion stars." That means there's an awful lot of real estate out there.
In addition to the staggering numbers, the history of science has shown that every time we think we are special in some way, we turn out to be wrong. The sun does not revolve around the Earth, the sun is not the only star, we are not the center of our galaxy, and our galaxy is not the center of the universe. There may even be multiple universes. Like so many important questions for which there are little data but different ways to examine the question, finding an answer can get complicated very quickly.
THE SETI PROJECT
Aliens have played a part in books and sci-fi movies ever since H.G. Wells' 1897 classic The War of the Worlds, but scientific interest in the topic of extraterrestrial intelligence, or ET, truly soared in the 1960s when space exploration took off. At the time Seth Shostak was a graduate student doing radio astronomy at the Owens Valley Radio Observatory in California, and while there he got hooked on a new book, Intelligent Life in the Universe, by the astronomers I.S. Shklovskii and Carl Sagan. "Reading that late at night, and using the antennas, made me think, gosh, this same technology could be used to hunt for signals," Shostak says.
"We've been so slow to realize these things. It's kind of been a failure of imagination."
The idea was that, just as we were broadcasting signals into outer space, the ETs might be doing the same thing. By tuning into the correct frequency, we could eventually hear from someone. Carl Sagan, along with astronomer Frank Drake, began the search for extraterrestrial intelligence, which came to be known as SETI, in the 1960s, and in 1984 the SETI Institute was formed. The organization's mission was to find evidence for alien intelligence using radio astronomy.
THE DRAKE EQUATION
To help frame the question and estimate the number of ETs that might be out there, Frank Drake devised an equation that came to be known, perhaps not surprisingly, as the Drake Equation. The equation included a factor for everything that seemed relevant to the question. Thus N—the number of civilizations in our galaxy with which communication might be possible—would be equal to the average rate of star formation per year in our galaxy, times the fraction of those stars that have planets, times the average number of planets that can support life per star that has planets, times the fraction of the above that go on to actually develop life, times the fraction of those that evolve intelligence, times the percentage of those that invent interstellar communication capabilities, times the average length of time that such civilizations release detectable signals into space. (Note that if any one of these factors is zero, then there are no ETs out there.)
Drake plugged in what he thought were reasonable values and came up with an estimate: 10,000 civilizations in the Milky Way. But at the time, the values he could assign were at best educated guesses, and in some cases little more than speculation.
Despite how little data existed, excitement about the possibility of finding ET grew during the 1970s as solar system exploration expanded. In 1977, the twin Voyager spacecraft 1 and 2 launched carrying gold-plated copper records inscribed with messages from Earth, including songs, images of our planet, and spoken greetings in 55 languages. The two spacecraft left the solar system in 1990 and are now travelling through interstellar space. It's expected that more than 40,000 years will elapse before they begin to pass other stars.
Flash forward more than three decades, and today there is greater confidence in the values we can assign to several factors in the Drake Equation. NASA's Kepler spacecraft and telescopes here on Earth have yielded so many discoveries of planets around other stars that some scientists admit they have trouble keeping track. In addition, the estimated value for another factor—the number of planets that might be habitable—keeps going up. Discoveries here on Earth of so-called extremophiles, organisms that can live in conditions previously thought to be inhospitable to life, coupled with discoveries in the solar system of environments not so dissimilar, has fueled optimism that we may find simple extraterrestrial life in our solar system in the not-too-distant future.
Carolyn Porco, a planetary scientist with the Space Science Institute and leader of the Cassini Mission Science Team, confirms that the new findings suggest there are more habitable zones out there than we'd previously thought. "We've been so slow to realize these things," she says. "It's kind of been a failure of imagination. Scientists don't go off and think completely wild and crazy things unless they have some evidence that leads them to do that. It's almost like we have a contract that says we're not supposed to do that." Today, suggesting that life might have taken hold elsewhere in the solar system, even on distant moons, is no longer considered "wild and crazy."
"Everything you say is slightly inconclusive."
Mars, for example, is a barren desert. But life survives in the driest spot of another desert, the Mojave, beneath the shelter of translucent white rocks. Says NASA astrobiologist Chris McKay of NASA's Ames Research Center, "These organisms are photosynthesizing here in the desert where nothing else will grow. They're living in a miniature greenhouse." On Titan, one of Saturn's moons, we now know that there are bodies of liquid as large as one of the Great Lakes that are filled with methane and ethane. While these chemicals might not seem conducive to living things, life may be more flexible than we give it credit for: Icy-cold Europa, one of Jupiter's moons, has cracks on its surface, which has led scientists to suspect that a salty ocean lies beneath the moon's frozen exterior—an ocean that contains its own forms of life.
INGREDIENTS FOR LIFE
But if you ask Carolyn Porco, Enceladus ("en-CELL-uh-duss"), another of Saturn's moons, is "the go-to place" in the solar system for examining a potential habitable zone, because it has the three ingredients thought necessary to support it. "It's got liquid water, as far as we know," says Porco. "It definitely has organic compounds. And it has excess heat up the wazoo, more heat than we know what to do with." Porco believes the evidence suggests that liquid water on Enceladus may have persisted indefinitely, a finding that's relevant to one of the factors in the Drake Equation: the number of planets or moons with habitable zones that go on to develop life.
Here on Earth, life took hold more than half a billion years after our planet formed, but because we only have one example of life arising, there is no way to know if the development of life will always take that length of time. It could take longer, which would reduce the number of planets where life has enough time to evolve, or it could happen more quickly, which would increase that number. "It's so hard," says Shostak. "Everything you say is slightly inconclusive."
EVOLUTION AND EXTINCTION
Likewise, the evolution of intelligent, communicating civilizations took well over another three billion years. But would it always take that amount of time, and was it even inevitable? Some scientists point out that human intelligence was contingent on numerous things going just right, such as an asteroid hitting the Earth 65 million years ago, simultaneously wiping out the dinosaurs and making our evolution possible. The late evolutionary biologist Stephen Jay Gould questioned whether, if you rewound the tape of life, a creature like us would evolve again. Notes Michael Shermer, author of The Believing Brain and the founding publisher of Skeptic Magazine, "ET is the same question. It's rewinding the tape on another planet and seeing if we end up with something with intelligence."
"Personally, I don't think they'll ever find a signal anywhere."
Peter Ward, who studies mass extinctions, has long been doubtful that SETI will ever succeed. He found a kindred spirit in astronomer Donald Brownlee. Over lunch one day at the University of Washington he mentioned to Brownlee, "You know, I'm just sick and tired of all the damn alien shows. [Finding aliens is] hardly likely at all." Brownlee responded, "Really? I think the same thing," and they were off. In 2000, they published a book called Rare Earth: Why Complex Life Is Uncommon in the Universe. While Ward agrees odds are that ET must exist somewhere, he concludes that the universe is so large that, for all intents and purposes, we are alone.
To drive home the point, Ward and Brownlee came up with a new version of the Drake Equation, to which they added factors they believed had been overlooked, such as the fraction of planets with a large moon, the fraction of planetary systems with Jupiter-sized planets, and the fraction of planets with a critically low number of mass extinctions. They called it the Rare Earth Equation.
Ward and Brownlee also assigned a very low number to how often intelligence would develop and to the length of time communicating intelligences would last. "There is going to be some fraction of planets with worms, where the worms finally evolve into human being-like equivalents," says Ward. "But the odds against even getting worms is so high." And if you can make it past worms to high-tech life, Ward believes that such intelligent life will be relatively short-lived. "We're going to come to that test here pretty quickly in the next century or the next millennium if we continue to increase carbon dioxide, producing what I think will be a runaway sea-level rise. When you have too many people and no food, and civilization falls into chaos, how much civilization is retained? How long will an intelligent species survive?"
Just what we mean by intelligence is a matter of much debate, but SETI has a simple definition. "If you can build a radio transmitter," says Shostak, "then you're intelligent. That's it. It's real simple. It doesn't mean that you're very good at composing rock 'n' roll, for example. But it does mean that we can hear you, and then, suddenly, we define you as intelligent."
"We're never going to be able to look at every nook and cranny of the universe."
Despite the doubts of people like Ward and Brownlee, the search goes on. The SETI Institute temporarily lost funding for its Allen Telescope Array—the most advanced instrument for detecting electromagnetic signals from ET—but private funding is bringing it back online. Of the search for ET, Shostak says, "It's a question that people were asking a thousand years ago, but there was nothing they could do about it. We can do something about it. We can actually mount an experiment that looks good on paper. How crazy would it be not to at least try?"
Even skeptics like Peter Ward don't argue with that. "Personally, I don't think they'll ever find a signal anywhere," he says. "But as long as there are people willing to fund it and it doesn't cost the public anything, why not?"
When asked whether the search for ET comes down to a question of beliefs, Michael Shermer had this to say: "It's a provisional belief, given that it could change in a heartbeat if it's confirmed. But it can never be disconfirmed. We're never going to be able to look at every nook and cranny of the universe." Shermer also pointed out that even though we're unlikely to hear from ET, asking the question has value. "Just thinking about how life might start on another planet and evolve into intelligence forces us to think about how that happened here," he says. "And that is legitimate science."
In the meantime, many scientists believe that finding simple life beyond Earth could well happen in our lifetimes. Says Carolyn Porco of the Cassini mission to Saturn, "If we ever discover that genesis has occurred independently twice in our solar system, that means that no matter where we find it, the spell has been broken. We could infer that life is not a bug but a feature of the universe in which we live, and that means that life has occurred a staggering number of times throughout the 13.7-billion-year history of the universe."