1. It's in the Zone
Life that depends on water can survive only in a narrow band of distance from a star: Too close, and the water evaporates; too far, and it freezes. The best place to look for habitable planets is in this "Goldilocks zone," a roughly 74-million-mile-wide area generally defined as no closer to a star than Venus is to our sun and no farther away than Mars. Three and a half billion years ago, Mars had an atmosphere that kept it at about the same temperature as Earth, but with little atmosphere left to trap heat, now it's a brisk –81°F. Venus, at 847°, sits just outside the edge of tolerable heat. For stars larger or smaller than the sun, scientists adjust the width and distance of the sweet spot accordingly.