The planets of our solar system orbit around the sun in 'almost' the same plane as the earth.
For one planet I can see why: given that the suns gravity acts as a central force, there is little in the way of other forces to lift the planet out of its plane of orbit.
But why do all planets lie in the same plane of orbit? If an incoming asteroid passed by the sun in a direction perpendicular to the usual plane of orbit as seen by the planets, would it align itself into that plane if it were to orbit the sun? Has it something to do also with how a lot of galaxies, with a centre of gravity, seem almost (but less so) 2-dimensional/flat?