In 2004, four leading dynamicists met in the seaside city of Nice, France, to try to address unsolved problems in the early evolution of the solar system. Several weeks of intensive work and computer programming produced a model that has been widely cited in the planetary science literature, and which may turn out to be a breakthrough in our understanding of how the solar system arrived at its current configuration. The model simulates conditions in the solar system about 600 million years after the planets formed.
The two movie below, provided to us courtesy of Hal Levison of the Southwest Research Institute in Boulder, Colorado, combines two different animations, showing two possible scenarios for the Nice Model. Each starts off with the four giant planets packed more closely together than they are now. Outside the planets’ orbits lies the Kuiper Belt, which was much more densely populated than it is now, with perhaps 100 times more mass. In the first animation, Neptune and Uranus end up swapping positions. In the second, Neptune begins outside Uranus's orbit, as it is today. In both animations, time is slowed down dramatically at the critical moment; otherwise most of the changes would be over in the blink of an eye.
The giant planets gravitationally perturb the Kuiper Belt, sending some of the objects (KBOs) inward into the giant planet’s realm, where the KBO’s orbits are extremely unstable. Most of these KBOs end up being ejected to the Oort Cloud or interstellar space. For every action there is an equal and opposite reaction, and in this case, the energy spent tossing KBOs out of the solar system comes at the expense of planetary orbits. The movie shows the orbits starting to migrate inward or outward, and orbits that started off circular become elongated. At a critical moment, Jupiter and Saturn hit a gravitational “sweet spot” — a 2:1 orbital resonance in which Jupiter goes around the Sun twice every time Saturn goes around once. That resonance pumps up the eccentricity (elongation) of Saturn’s orbit, so Saturn’s gravitational influence forces Uranus and Neptune to migrate outward. As Uranus and Neptune plow into the Kuiper Belt, they scatter huge numbers of KBOs, some of which end up in highly elongated trajectories that intersect those of the inner and outer planets. Some of these objects pummel the planets and their moons (including Earth and its Moon), causing the Late Heavy Bombardment.
Besides Levison, the other three members of the Nice team are Rodney Gomes, Alessandro Morbidelli, and Kleomenis Tsiganis. The Nice Model has not yet been universally accepted by the planetary science community, and Levison and his colleagues continue to refine their work. But the Nice Model provides a plausible scenario for how an intense pummeling of the planets could have taken place hundreds of millions of years after the solar system formed.