The planets in our solar system grew out of a disk of material that swirled around our Sun. Inner rocky planets formed as tiny grains stuck together, becoming pebbles, then boulders, and, ultimately, full-grown planets. The outer gas and ice giant planets also accumulated cores of rocky material, which then attracted halos of cooler gas and ice around them. Planets around other stars are thought to form in a similar fashion, called core accretion, but astronomers have wondered if this still