Why are the orbits of planets nearly circular yet the orbits on many comets and asteroids highly eccentric?
All of these objects probably formed from the same swirling disk of material. As such they initially had nearly circular orbits. However, the planets grew much larger than comets or asteroids did. Because of this, it takes a lot more force to move them out of their orbits. Comets and asteroids being much smaller and, therefore, it is much easier to change their orbits such that they get thrown into these more eccentric orbits. Collisions with other bodies or the gravitational effect planets can accomplish this. Note, that most comet nuclei and asteroids are in nearly circular orbit just as the planets are. It is only a relatively small portion of them that are not.
