"A fairy tale of the early solar system." That's how planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colo., whimsically refers to his team's new computer simulation. In it, the four biggest planets start out bunched together but then break into a planetary version of bowling that violently rearranges the structure of the outer solar system.
Over the past quarter-century, several researchers have suggested that planetary migration shaped the solar system. But "the [new] model reproduces quantitatively many of the characteristics of the outer solar system to an accuracy that has never been achieved before," Levison says.