I was thinking the other day as to why the majority of the bodies in the Solar System orbit the sun on the same plane (or really close to it). You would think that if the planets & moons were cast offs from the Sun or some other catostrophic event the distribution would be more or less random in 3 dimensional space. Is there an answer to this or am I just missing the boat here?
Maybe I should just grab my towel and head off to Planet X for a while. 
Notice that they are also nearly all moving in the same direction? The formation of a solar system (and the same appears (to me) to be true of a galaxy) involves the mass of matter picking up “spin” at some point. The angular momentum imparted by the spin tends to flatten out the mass. (Not too much unlike a pizza being spun to increase its size.)
Objects that are not moving in the same direction within the same plane tend to me captured objects that were not part of the origin event of the system.
A very nice summary of how the solar system formed will help make it clear.
You will also note that the majority of the planets also spin in the same direction on their axis.
Whereas Uranus’ axis is almost level with the “plane” of the solar system so it just sorta “rolls”.
I believe Venus is one of the planets with opposite rotation and its posited that at some point in its history it spun like the rest of them do, except a massive impact caused the rotation to reverse.
A protoplanetary cloud in which the particles are revolving about the center of mass in all possible orbits will be unstable because the orbits of the particles intersect. The resulting collisions and ricochets will continue until the majority of the mass is in orbits that do not intersect anything else. The only way that can happen is if the particles are in a single plane about the central mass.
Objects need not even collide for this tendency to make itself felt. Gravitational attraction alone is enough to change an objects orbit and move the whole system towards a planar arrangement.
It might be interesting to try to model a more 3-dimensional solar system. Odds are a very large planet like Jupiter would be very disruptive to the orbit of a small, more eccentric planet and the system wouldn’t stay stable for very long.
Over a few billion years, natural selection comes into play and any planet or large asteroid that was really eccentric would either have been tossed out of the system, drawn into the Sun, or captured as a moon by a larger body. What’s left is a system that is simple, stable, and self-sustaining.
Pluto is not in the same plane
Both Pluto and Mercury move at orbits from 5-10 degrees off the “plane of the solar system” – the other seven are a few degrees off from each other, and the “plane” is merely an average.
Smart-alec answer: The solar system is flat because most of it is vacuum – like anything else, it goes flat when all the air leaks out! 
The Britannica article on the solar system says that the Oort Cloud is a spherical shell surrounding the rest of the solar system with things flying in helter-skelter orbits, many of them highly elliptical making them comets. This would imply that the Oort Cloud is relatively young and hasn’t had a chance to settle down yet, or else it keeps picking up new objects in random orbits as the solar system travels around the Milky Way center.
The timescale of the effect depends on the cloud density, as well as the orbital period of the objects involved. If objects in near-earth orbits have interacted with each other over the course of 4 billion orbits, or years, the thinly scattered objects out past Pluto will have had only had 10 million or so chances to affect each others trajectory. The Oort cloud can still be old and spherical.
I believe that Venus may have actually been “flipped over,” rather than that the direction of spin was reversed directly.
If Uranus was “flipped” 90 degrees by a collision, then Venus might have been flipped almost 180 degrees. This results in an apparent direction of spin opposite those of the other planets, even though Venus, with reference to itself, is still spinning in the same direction as it was originally.
Colibri - YOU believe that Venus may have actually been “flipped over”? Based upon what evidence? Or are you simply saying that you can see that as an alternative hypothesis to the one already proposed? If you believe it, do you have reason to? Do tell.
Tidal effects from the Sun on the thick Venusian atmosphere revesed it’s rotation. Article from Nature about it.
I would suggest that this would be an alternative hypothesis. I am not aware that there is any evidence one way or another. However, a retrograde rotation can be viewed as a “normal” one flipped 180 degrees. In fact, the inclination of Venus’ axis is sometime expressed as 177 degrees (instead of 3 degrees) to indicate this.
Uranus’ axial tilt is either about 98 degrees, if you consider its rotation to be direct, or 82 degrees, if you consider its rotation to be retrograde. If it is assumed that a collision knocked Uranus on its side, then it also knocked it into (apparent) retrograde rotation.
If a collision could flip Uranus more than halfway over, which is often assumed to be what happened, then I don’t see why a collision might not also have flipped the much smaller Venus over another 80 degrees. But this is just speculation on my part.
Well, they say that this is a possibility, not that it is definitelyabsolutely proven.
So, if the angular momentum present in the solar system during is formation has lead to seven of the nine planets having prograde rotation, would it be reasonable to assume that the same ratio might exist on a larger scale within the galaxy?
That is, of the systems orbiting the center of the galaxy, those stars and their satellites rotate (and maybe even orbit?) in the same general direction and on the same plane?
Each star and its planets will have its own plane of rotation. But all these planes will be independently oriented and not be close to that of the galaxy. In fact, the galaxy rotates some 60 degrees retrograde compared to our solar system.
Because God made it that way! What’s next? Why does the sun revolve around the Earth?
Quoth tomndebb:
It’s not so much that it picked up a spin, it’s that it had that spin from the beginning. Originally, it might have had some less obvious form, in a big jumbled cloud of gas, but you’re almost always bound to have some angular momentum, and it’s almost impossible for a solar system to lose any of it.
Similar effects cause the disk shape of a spiral galaxy, but there’s not the internal correlation in a galaxy that you see in a solar system. I suspect that this is due to scale and a difference in method of formation, but I’m a bit hazy on the details.
Quick question: does the sun itself rotate in the same direction that the planets revolve about it? That would seem to lend even stronger credence to the (well-nigh universally accepted) cloud -> protodisk -> planets theory.