National Geographic : 2013 Jul
dIsturbIng the gIants The Late Heavy Bombardment of Earth may have resulted from a dramatic disturbance of planetary orbits. That led neptune (foreground) and uranus to disrupt a belt of comets, and jupiter the asteroid belt. According to the nice model (named for the french town where it was conceived), jupiter, Saturn, uranus, and neptune had been born close together in the solar nebula, a disk-shaped cloud littered with rocky and icy debris. As the four giants’ strong gravity sucked in or slung away such debris, their own orbits slowly shifted—until they hit a tipping point.