- Is it neccesary that our planets rotate in more or less a plane? 2. If not, why do they?
There is no medium to pull them all along in the same direction (think of water being stirred in a glass).
You mean, for them to all orbit in the same plane (more or less)? It’s because of their common origin. Originally, the entire Solar System, Sun included, was just a big cloud of gas and a little dust. It had some intrinsic angular momentum to it, as all such clouds will, and in time, due to friction and gravity, it collapsed. Most of it became the Sun, and what was left became the planets and everything else in the System. Since angular momentum is conserved, the new planets all got orbits that carried some of the angular momentum, and since they were all getting it from the same source, it’s all in the same direction, leading to coplanar orbits.
Would this also apply to comets and astroids of other origins?
As I understand it, the planets mostly rotate in the same direction because they formed from a cloud of gas and dust that rotated in the direction. If an object has an origin within the solar system, it will likely rotate the same direction as the planets do. If the object is a rock from outside the solar system, it could rotate in any direction, depending on the circumstances where it was created.
So basically: rocks from inside the solar system spin pretty much like anything else in our system. Objects from outside could spin any old way they like.
In other words, asteroids do orbit in more or less the same plane and same direction as planets. There are probably some exceptions whose orbits were twisted by close encounters with planets.
Looking at a list of a dozen comets (from a handbook which I had lying around) their orbital inclination tends to be more varied, but very few are inclined more than 90 degrees (i.e. orbit the other direction). They start out at the very outer reaches of the solar system, and have very little orbital speed to begin with, so their orbits tend to be more scattered.