All manner of planets circling other stars have been popping up of late: big ones, little ones; gassy ones, rocky ones; hot ones, cold ones. But the freakish diversity of worlds starts much closer to home. From the 1960s to the 1980s, space probes returned the first close-up looks at eight of the then-nine planets. To researchers expecting a simple story that would explain what shaped our solar system, the observations sent a sobering message: in your dreams. Today, enigmas such as Mercury's makeup (mostly iron core, with a thin veneer of rock) and Uranus's skewed magnetic field continue to bedevil planetary scientists, and no tidy resolution is in sight. Part of Science's special section, The Mysteries of Astronomy.