| MadSci Network: Astronomy |
Well, in general, the orbits of the planets are all in the same plane (in technical terms, they are ``coplanar'')! The differences are actually pretty small; the largest is Pluto which has about a 17% tilt with respect to the Earth's. We usually use the Earth's orbit as the reference. We might as well, we live here!
The reason the orbits are roughly coplanar is due to the way the solar system formed. We started out as a gas cloud in space, and it was probably roughly spherical (though it need not have been). Something made the cloud start to collapse. Perhaps it was a shock wave from a nearby supernova, or it collided with another cloud. Whatever it was, when the cloud started to collapse, any tiny bit of circular motion it had inside would get hugely amplified, causing the cloud to rotate. [Think of an spinning ice skater---as she pulls her arms closer to her body or becomes smaller, she spins faster.] Something that rotates wants to flatten out, and so after a time the cloud became more disklike. So when the planets formed, they were all in about the same plane: the plane of the disk.
Incidentally, we have detected disks of gas around other stars, like Beta Pictoris. There is even evidence that planets are forming there too!
Also incidentally, the Mad Science site has a search engine, and I found some other answers there that relate to yours. Give it a try!
Try the links in the MadSci Library for more information on Astronomy.