Building a solar system pebble by pebble

Broadly speaking, planets form in a gaseous nebula around a star as dusty matter accumulates into ever larger associations. But implementing that evolution in a detailed simulation has proved challenging. In many models, the nebula does not endure long enough for the cores of giant planets to agglomerate. Other models suggest that “pebbles,” perhaps a millimeter to a meter across, are slowed by frictional interactions with the nebular gas and rapidly coalesce into 100- to 1000-km-sized planetary embryos that grow to planet size by accreting remaining pebbles. That mechanism, however, seems too effective; simulations typically produce hundreds of Earth-sized planets in a solar system. Now Harold Levison of the Southwest Research Institute and his colleagues have added a twist to the simulations—a nonzero formation time for pebbles—and obtained a realistic number of rocky and gas-giant planets. As nebular dust coalesces into pebbles, they find, the largest of the massive planetary...