The question now is how many of those 100 billion potential Earths can we reasonably expect to have harbored H2O and served as a cradle of life, intelligent or not? Enter Kepler, an ambitious new NASA mission. Launched via satellite in March, Kepler's $600 million space telescope uses a sophisticated photometer to stare at all 100,000 stars located in a particularly promising region of the Milky Way while measuring the size and orbit of every planet that passes in front of them. The larger the shadow, the larger the planet; the more often it appears, the closer the orbit. The point is to isolate for the very first time alien worlds orbiting alien suns at distances where temperatures are right for liquid water and possible life. "This mission is like Columbus," says principal investigator Bill Borucki. "We will get Earth-sized planets, terrestrial planets, in the habitable zone. It won't be 'close.' We will know."
