So I've been playing some Kerbal Space Program, and it's taught me a couple things. Primarily, that circularizing an orbit (bringing its eccentricity down to near 0) is a matter of extremely precise engineering, and that matching planes with another satellite's orbit is also extremely nontrivial. How is it that an inordinate amount of the stuff orbiting Sol all shares the same plane of the elliptic? All of the planets, all the asteroids, most of the transneptunian junk... I scoured Wikipedia for an explanation, but couldn't find one. Is it normal for star systems to have a single plane of the elliptic for most of their satellites? What caused this? Secondly and more importantly, why do the orbits of all the planets and junk have such low eccentricity? In KSP, I can spend hours with a high-ISP engine and kilotons of rocket fuel trying to get my eccentricity below 0.10, and yet all of the planets in our system seem to have accomplished this feat without even any course corrections! How did this happen? I would expect Earth's orbit to be circular á la the anthropic principle, but why do the other planets have this property as well? I suspect the answer is simple and I'll feel foolish when I hear it, but I'm still quite eager to hear it.