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 embryos present at the beginning of the simulation gravitationally scatter their smaller siblings out of the protoplanetary disk. Thus most embryos are starved of the material needed for further growth; only a few become large enough to form rocky planets or gas-giant cores during the 1 million- to 10 million-year life of the disk. The figure summarizes a representative simulation. The angle θ denotes the inclination of an embryo orbit with respect to the protoplanetary disk, whose angular width is indicated by the blue region. After just 3000 years (purple triangles), a substantial fraction of the embryos initially present (black dots) has already been ejected. By the time the simulation terminates (red hexagons), less than a handful of embryos have grown to Earth size. (H. F. Levison, K. A. Kretke, M. J. Duncan, Nature 524, 322, 2015, doi:10.1038/nature14675 .)