Astronomers generally agree that planets form out of the dusty debris disks that surround most newborn stars. When one of these so-called protoplanetary disks rotates around a nascent star, globs of material clump together. Over the course of a few million years, these clumps (called planetesimals) grow larger and larger, forming a protoplanet that eventually clears out its orbital path within the disk. And when a protoplanet gets massive enough, gravity forces it into a spherical shape, fin