Diamond Planets: Rich Possibilities for Other Worlds The solid planets in our solar system are made mostly of silicates. Rock, basically. A new study shows that planets around some other stars might be made mostly of carbon instead. Deep inside such worlds, where pressures are intense, the carbon would make layers of diamonds that could be miles thick. The rich-sounding worlds are modeled after a certain type of space rock, known as the carbonaceous chondrite, which are thought to be broken bits of asteroids. Many of them have been collected on Earth. "These meteorites contain large quantities of carbon compounds...