But in 2007, NASA will launch the Kepler Mission, a satellite probe able to detect smaller planets the size of Mercury, Mars, or the Earth. The mission is specifically designed to look for planets in what scientists consider the habitable zone: the distance from a star where liquid water can exist on the planet's surface.
Projects like the Kepler Mission and the new Allen Telescope Array, located near Mount Lassen, California, which will enable astronomers to survey 100,000 stars by 2015, should increase the odds of finding a radio signal broadcast by alien life, say the astronomers.
"The bottom line is that there is an enormous amount of real estate, and there doesn't seem to be anything particularly special about our neighborhood. The star that's our sun is nothing special. The Earth is just a rock," said Shostak. "To think anything else is to once again put ourselves at the center of the universe, and scientists are very [wary] of doing that. We've done it before and been proven wrong."
Searching for Intelligent Life
The building blocks of life on Earthcomplex organic compounds and amino acidsare abundant in the universe and can be found in meteorites, comets, and interstellar gas and dust.
"There's a growing realization that there may be some other biology in our solar system," said Shostak. "There are deep oceans on the moons of Jupiter, and some evidence that Mars in its early days really should have had some life. So if there are two or maybe even three instances in this solar system alone, where life could have emerged, it's not unreasonable to consider that similar situations arose in other solar systems."
Of course, there is a difference between life and intelligent life, and scientists disagree about the likelihood of intelligence evolving on another planet.
Some believe that it takes very special conditions for intelligence to evolve. The late Stephen Jay Gould, the preeminent Harvard University evolutionary biologist and paleontologist, wrote that the creation of intelligence was a freak occurrence, requiring a number of specific events to occur that could never be replicated again.
Shostak and Barnett believe differently. They argue that there are evolutionary mechanisms that encourage intelligence, particularly among social beings.
"For instance, if you can intuit the actions of the males next to youthey're about to steal your food or your mate, saythen you're going to have increased breeding success, so the next generation is going to have more ability to live in a social environment," said Shostak. "This is very general behaviorit's not miraculousyou see it in the great apes, of course, but you also see it in dolphins, whales."
As to what an intelligent alien life-form might look like should such a thing exist, it's anyone's guess. Shostak and Barnett created "JO Alien," an animated character for a planetarium show at the National Space Centre in Leicester, England, to explore the question.
"It's the poster child of aliens," said Shostak. "A very conventional kind of aliengrey and smooth and humanoidvery anthropomorphic. Not what we really expect."
However, the genderless JO does demonstrate some of the basic principles of physics and engineering that might dictate what another life-form might look like.
"An alien would probably have to be bigger than a rat because rats have quite small brains, and an intelligent life-form would need a bigger brain," said Barnett. "So bigger than a cat, but not bigger than an elephant because there are limitations on how much weight a body can support."
JO Alien has two eyes, based on the assumption that an alien life-form would be found on a planet circling a star. Everything on Earth that lives in light has developed eyes. Why two instead of one? Two gives you the evolutionary advantage of being better able to catch your next meal, said Barnett. Why not ten eyes? It would take an enormous amount of brain-power to process all the signals, she said, with little or no extra benefit.
As far as limbs are concerned, the pair speculate that an extraterrestrial would have more than one, particularly if it's building radio telescopes, but a score would be a stretch. Again, it would require a great deal of brain-power to coordinate all 20, Barnett said.
Given the enormous distances between the stars, measured in terms of trillions of miles, Shostak doesn't expect a visit.
"There's a tremendous amount of interest in alien life, mostly from the point of view that we've been visited," he said. "I don't believe that, and I don't think most scientists do. What we'd most like to convey is that there's a possibility they exist even if they haven't visited, and we're searching for them."
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