Layman's question about orbital planes

[Electricblue2]

Hey all, I have a layman's question that's been bothering me for a while. As I understand it, the orbit of a given planetary body around a high-gravity object (like a sun) is determined by the balance between its tangential velocity and the force being applied on it by the high-gravity object (so it moves X distance tangentially for every Y distance it falls inward, creating a circular orbit with a radius proportional to the values of X and Y).

I know this is an over-simplified understanding of orbits, and it leaves me with this question: Why is it that all the planetary bodies in a solar system seem to follow orbits that lie on the same plane? (relative to the Cartesian coordinate system) My understanding of orbits doesn't account for this, so I have to assume that I'm missing some piece of the puzzle. If anyone could point me in the right direction on this, that would be great! :D

 

[D H]

The planets formed from a protoplanetary disk (google that term).

Stars form from interstellar gas clouds. Once a protostar has gained sufficient mass to start influencing the remaining material in the cloud, those little bits of that remaining material that have near-zero angular momentum with respect to the star will fall in toward the star. Those bits that have non-zero angular momentum will fall into orbit about the star.

Suppose a star forms from an interstellar cloud that has zero net angular momentum. The remaining stuff will still have pretty close to zero net angular momentum. Most of it will just fall into the star. The little bits that don't will go in orbit, but those orbits will have a more or less random orientation. Particles will collide, and the net result of these collisions will be more stuff with zero angular momentum falling into the star. End result: A star with zero planets.

Now instead suppose the gas cloud has some non-zero angular momentum. A somewhat poor analogy, but think of a hurricane. Air circulating near sea level will for the most part remain near sea level while air circulating miles above the surface will for the most part remain at altitude. (There are significant updrafts and downdrafts in a hurricane, but ignore these; this is a somewhat poor analogy after all.) The same phenomenon can happen in a gas cloud before the star forms. Those parallel tracks are no longer physically possible once the protostar forms and begins to act gravitationally on the left-behind stuff in the cloud.

Let's use the net angular momentum vector to denote a direction. For lack of better terms, call this the north-south axis of the gas cloud. After the star starts forming, particle collisions will do something quite different than they did in the zero net angular momentum case. The left-behind stuff will start to form a disk that is normal to this north-south vector and centered on the nascent star. Collisions are more likely in the disk than away from it. These collisions mean that stuff orbiting at an inclination to the disk will have their orbits flattened out. As a result, the disk becomes thinner in terms of north-south extent but thicker in terms of density.

These thin in extent but thick in density protoplanetary disks are the essential ingredient for the formation of planets.

 

[Electricblue2]

Thank you for your quick response! Having now done some research into protoplanetary disks, the formation of planets and orbits makes a lot more sense to me. Thanks again!