Apologies for a long preamble. There is a simple question at the end of this post. Wikipedia and the PF archive were not forthcoming on this subject. Saturn's rings seem sharply bounded in the vertical dimension. If I got it right, the initial orbits of debris were centered on the orbits of the originating protoplasmic clouds and/or moons. But the orbits may have been scattered among many axes. However collisions between particles is the negative feedback that eliminates vertical components of momentum. In a ring, the relative velocities of particles is nearly zero. A gravitational close encounter with a large object, might disturb the orbits of particles, but the feedback mechanism would return most of them to a ring. In the early solar system, similar processes could apply. However, after planet formation and formation of Earth's moon, the system's population is sparse and planetary collisions don't happen. If the planetary orbits were disturbed by a gravitational close encounter, then I see no feedback mechanism to return them to approximately coaxial planes (right?). Yet, Wikipedia says that the orbits of the planets are coaxial to within 6 degrees. Can the small 6 degree spread in planetary orbit axes infer that no gravitational close encounter strong enough to disturb planetary orbits, has happened since planetary formation?