Planetary Formation

Interesting two part article on the formation of solar systems by Steven Soter at Astrobiology magazine. The gist is that solar systems tend to be relatively unstable with gravity creating a delicate balancing act between the planets. Much of the material that doesn't accrete into planets is ejected by the formation of large gas giants, and quite a bit of what remains gets locked into relatively stable resonance points in front and behind of the larger planetary bodies.

"Despite its orderly appearance, the solar system actually includes many elements of what mathematicians call chaos. A defining feature of chaos is the extreme sensitivity of a system to its initial conditions. The most trivial disturbance in such a system can profoundly change its large-scale configuration at a later time. A pool table provides a familiar example: Microscopic variations in the trajectory of a billiard ball, especially one involved in multiple collisions, can completely alter the outcome of the game. Chaotic systems are deterministic, in that they follow precisely the laws of classical physics, but they are fundamentally unpredictable. The nature of chaos was not well understood until recently, when increasing computer power allowed mathematicians to explore it in sufficient detail. No one in Laplace’s day imagined that the solar system, then taken as the paradigm of clockwork stability, is actually vulnerable to chaos."