Why do most of the planets orbit the sun fairly on the same plane?
The solar system originally formed from a huge cloud of gas and dust. This cloud hung in space with each molecule attracted to every other molecule by gravity. Also the cloud started with some net angular momentum, or spin. That is as the molecules moved about, there were a few more which orbited around the center of the mass of the cloud in one direction than in the opposite direction.
Over time, the gravitational attraction between the particles caused the cloud to condense to a smaller volume. Keep in mind that as the radius of the cloud decreased, the angular momentum didn't decrease, and so the speed of the spin increased. This is the same effect you see when an ice skater starts spinning slowly with arms outstrtched, and then picks up speed the tighter she pulls in her arms.
The cloud continued to spin and contract. However not all parts of the cloud pulled in to the center equally. Those particles around the plane perpendicular to the axis of rotation felt centrifugal "force" - actuall just their linear momentum - keeping them from moving towards the center and fighting against gravity. Particles closer to the axis of rotation felt this less, and were pulled in more. Thus the cloud became a disk, just as a blob of pizza dough thrown in the air with a spin flattens out into a pie shape.
The dust particles is areas of slightly higher density within this disk coalesced to form the sun, planets, asteroids, etc. and this is why most of these object are located in the same plane today.
Rob Landolfi, None, Science Teacher, Washington, DC
'The important thing is not to stop questioning. Curiosity has its own reason for existing. Never lose a holy curiosity.'