Four and a half billion years ago, the place we now call the solar system was a vast cloud of gas and dust enshrouding a newborn star. Gradually those dust grains cohered and formed pebbles, which then collided and coalesced into boulders. Over the course of about 100 million years, most of the material in that nebulous cloud accreted into the existing eight planets—four rocky (including Earth) and four gaseous. Or at least that’s how astronomers thought the story went.
Last November astronomer David Nesvorny of the Southwest Research Institute in Colorado added a new character to the tale. Nesvorny, who runs computer simulations to study how the solar system evolved over time, kept encountering the same problem: The four giant gas planets, whose orbits are comfortably far apart from each other today, kept violently jostling with each other in his models of the early solar system. Jupiter would end ...
