The solar system that we have learned about from a very early age was described as consisting of 9 planets which go around the sun in evenly spaced concentric orbits. These orbits are thought to have existed since the formation of this solar system, approximately 4.5 billion years ago, and this assumption has led to inferences about the formation of the planets and the processes, which have led to their present states.
There are several ways to try and explain the formation of the existing planetary orbits from the mass of gas and dust that gave rise to this system, often referred to as the solar nebula. One of the simplest and most accepted hypotheses suggests that the present locations of the planetary orbits are in fact identical to the position they occupied at the time of their formation. This assumption has become the foundation for our understanding of the solar system and many other theories depend on it.
There are other bodies within our solar system, which tend to intermingle, dangerously at times with the planets. These include comets, asteroids, moons, and cosmic dust. These objects also exist within orbits however it has been accepted that the orbits they exist in are not rigid and that migration is able to and has occurred. The earth's moon is a perfect example of this migration. Though the moon is thought to have been formed at approximately 30,000 kilometers from the earth, the present orbit, which it occupies, maintains the moon at a distance of 384,000 kilometers and this change in orbital position is the result of gravitational forces exerted by the earth over a period of a billion years.
The recent spectacular images that we were able to see of the destruction of the comet Shoemaker-Levy 9 as it crashed into the gaseous surface of Jupiter also serves as evidence that the orbits in which these smaller bodies find themselves in are under constant gravitational stresses and can be quite dynamic.
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