Planets in the hundreds of billions are likely caught up in the vast whirlpool of the Milky Way galaxy. From Earth, a lonely outpost on one of its spiral arms, we’ve begun to peer across the void. We can already make out, dimly, the light from planets orbiting distant stars. We’ve even tasted a few of their atmospheres by dissecting those faint traces of light. But the ultimate goal of NASA's exoplanet program is to find unmistakable signs of current life. How soon that can happen depends on two unknowns: the prevalence of life in the galaxy and how lucky we get as we take those first, tentative, exploratory steps.
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