This movie shows the forming system of planets. We rocket through a molecular cloud, penetrating the cold cloud core where the Sun is being born. As we approach, we see the disk of material orbiting the protostar, the end point of the animation just above. It begins to glow bright red as energy is released by its contraction under gravity. Gas clouds come and go above the disk and then a wind starts from the young Sun and clears excess gas from the disk, leaving the young planetary system. Not shown, eventually the star blows away the excess gas and some of the dust to become visible. (adapted by G. Rieke, from JPL, M. Roessler, http://cougar.jpl.nasa.gov/HR4796/anim.html

(reload to restart lecture animations)

At an early stage (less than a million years old), when the young Sun was still surrounded by the dense disk of both gas and dust, the gas giant planets Jupiter and Saturn formed. The simulation below illustrates the growth of instabilities in a disk and the eventual formation of planets. (from Ken Rice, (http://faculty.ucr.edu/~krice/)

However, the excitement was not over for the Solar System. Around 700 million years after the formation of the system, Jupiter and Saturn got into orbits where the period of Saturn was exactly twice that of Jupiter. This "orbital resonance" caused strong interactions because of the continuously repeating gravitational effects from these two massive planets.* The outer planets were shifted into their current orbits as a result. At the same time, the overall gravitational field of the system kept changing in ways that resulted in the ejection of most of the smaller bodies that had not yet been captured into planets. As these bodies shifted orbits, there was a period of high impact rates, called the Late Heavy Bombardment. In this computer simulation of the process, the Sun is in the center, the circles are the orbits of Jupiter, Saturn, Uranus, and Neptune, and the green dots are the smaller bodies. The system appears to be stable as Saturn and Jupiter migrate slowly toward the 2:1 resonance, but when they reach it Uranus and Neptune are scattered into their current orbits and most of the smaller bodies are very quickly thrown out of the system - it is a cataclysm! (The event occurs a little late compared with the solar system in this particular simulation.)

(from http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html)

* Now that they are no longer in resonance, their gravitational tugs tend to cancel out.