Ok, so if you throw a ball up in the air, it comes down in an (almost, slight deviation due to jerk) parabolic trajectory. Now, if you look at the dance that two (let's keep it simple) planets make when effected by eachother's gravity, you get certain conic sections. You get ellipses, circles (in very exact cases) and hyperbolas. Why not any parabolas? Or are there parabolas, but they're just as rare as exactly circular orbits?