Our solar system is often depicted, as it is on the poster on my wall, as the Sun and all of its satellites in coplanar orbits around it. I thought, however, that this "flat" representation was just a creative liberty taken to make is possible to put on a poster, but now I come to find out that the orbits of the planets are coplanar. Why is this? There is no bias for a particular plane of orbit present in Newton's theory of universal gravitation, right? At first I thought it could be possible that it's just by chance, but then I learned that it's not just our solar system. Apparently, of the 300 or so planets we know of, only about 50 have oblique orbits. It also happens that the plane in which planets tend to orbit is the plane of the equator of their star. Can someone tell me where this trend toward coplanar orbits comes from?