Someday astronomers will likely create a long list of Sun-like stars with Earth-like planets around them. But technology has yet to reveal such worlds, instead allowing the detection only of much larger planets. Most of the roughly 200 known extrasolar planets are larger than Jupiter. Many complete their orbital years in just a few days. This proximity to their stars creates noticeable wobbles in the stars that make the planets detectable. But astronomers figure the giants probably formed farther out, in a disk of material swirling around a newborn star, and migrated inward. In doing so they would have destroyed...