Could Earth once have had a planetary ring like the ones around Saturn? Scientists from Monash University in Australia think so. In a recent paper they identify 450m-year-old craters that lie close to Earth’s historical equator. They propose these were caused by the collapse of a ring of debris (possibly the remnants of a large asteroid) that once circled the planet. The ring might have lasted tens of millions of years, cooling Earth with its shadow. ■