Why are scientists so optimistic?
The Kepler space telescope gets much of the credit. Before it was launched into orbit in 2009, astronomers couldn't be sure whether planets existed outside our solar system. The search for extraterrestrial life was mostly focused on our own solar system — on Mars and a number of moons around Jupiter and Saturn — and on an intergalactic eavesdropping project known as SETI (the Search for Extraterrestrial Intelligence). For 50 years, SETI has been using radio telescopes to listen for signals from an alien civilization somewhere out there in the cosmos, with no "Hello there!" detected thus far. But when Kepler began scrutinizing the stars from its perch beyond the atmosphere, its unprecedented resolution gave scientists a tool to detect the relatively dim planets circling them. Using the telescope's data, scientists now estimate there are more than 100 billion exoplanets (planets outside our solar system) in the Milky Way galaxy alone. One in every five stars, they've concluded, has a planet that is habitable, and Kepler can help pinpoint which ones to examine for signs of life. "It's within our grasp to pull off a discovery that will change the world forever," says telescope scientist Matt Mountain.

How does Kepler work?
Trained on a patch of sky containing about 100,000 stars, the telescope studies the light emitted by each star to look for telltale dips in brightness. The periodic dips are a sign of a partial eclipse, caused as a planet transits in front of its star's surface during orbit. From that one patch of universe, Kepler has used the "transit method" to confirm the existence of at least 2,000 exoplanets — from rocky spheres smaller than Earth to gaseous giants larger than Jupiter. The next challenge, says MIT astrophysicist Sara Seager, is to identify exoplanets that live within their star's so-called Goldilocks zone — neither too near nor too far from their star so that their environment is "not too hot, not too cold — just right for life." Astrobiologists believe they've already identified 86 Earth-like exoplanets that exist in this habitable zone and are now studying them for signs of life.

What kind of signs?
The traditional marker of life is liquid water. "Life needs a liquid; even the driest desert plant on Earth needs water for its metabolism to work," says MIT astrobiologist William Bains. So if an exoplanet contains liquid water, the planet might be home to an alien life-form, whether it's a simple bacterium or a complex and intelligent creature. A watery planet could be detected by the chemical signatures of water in its atmosphere. Some scientists think it's possible that aliens are more different than we can imagine and have evolved to exist on methane or other chemicals instead of water. "The things we can conceive of are probably a very small set of the possibilities that are out there," says biogeochemist Ariel Anbar. "We know we're going to be surprised."

What else would indicate life?
NASA also plans to use its telescopes to search for certain "biosignatures" in an exoplanet's atmosphere. A large amount of oxygen is one such biosignature: As a highly reactive element, oxygen needs to be consistently renewed by a biological process like photosynthesis in order to exist in large quantities. Another tip-off would be a carbon footprint that we've developed on our own planet: air pollution. (See below.) "You'd know that's an inhabited world, not just a habitable world," says Jill Tarter for the SETI Institute. "And then you can ask the question, Did they develop any technology we might detect?"

Could we contact any aliens we find?
Perhaps, but not easily. The biggest barrier for astronomers is the time lag involved in communicating with planets located trillions of miles away. Suppose NASA decided to try to communicate with Kepler-186f, an Earth-like planet about 500 light-years away (or 2,939,249,910,000,000 miles): Given the planet's distance, it would take half a millennium for a message to reach its inhabitants, and another 500 years for their response to be received. Either our civilization or theirs could perish during that time, rendering the communication moot. This assumes, of course, that the alien life is intelligent and can communicate; there will be no return signal from silent microbes in an ice-capped lake.

What if we do find life?
Legendary astrophysicist Stephen Hawking believes we're taking a big risk just by looking for it. He has warned that any signals we send out could invite the visit of a far superior alien civilization intent on our colonization or destruction. But even the discovery of friendly, intelligent aliens — or primitive life-forms — could have a mind-blowing effect on humanity. Traditional religious teachings about man's central role in creation would be overturned, and our species would be forced to redefine itself in the knowledge that we've got company — perhaps plenty of company — in the universe. "Soon, we're going to have an existential shock," says theoretical physicist Michio Kaku. "Even if we find a fossil, a DNA strand from another species, that would be absolutely staggering."

Finding a marker of civilization
As astrobiologists search for biosignatures in a planet's atmosphere, they'll now also be looking for a telltale indication of an advanced civilization: pollutants like those found in our own atmosphere. Astronomers will look for industrial pollutants such as chlorofluorocarbons (CFCs), the chemicals found in aerosols and refrigeration units, which have helped to eat a hole in Earth's ozone layer. These chemicals aren't known to occur in atmospheres naturally, so they would be a helpful indicator that an industrial civilization once existed on that planet — though because they take approximately 100,000 years to disappear, those aliens may long be extinct. The irony is that "aliens are often referred to as green little creatures," says theoretical astrophysicist Avi Loeb, but "detectable CFC-rich civilizations would not be 'green,' since they are environmentally unfriendly."