Why do all the planets orbit the sun on what seems to be almost the same plane?
After Newton explained the orbits of planets by universal gravitation, it occurred to Laplace that they might have formed by gradually clumping together from a disk-shaped cloud of gas and dust rotating around a newly formed star. (The other thinker who is remembered for proposing this nebular hypothesis is the philosopher Immanuel Kant.) The nebular hypothesis is generally accepted today: it not only explains the similar orientation of the orbital planes, and the common sense of revolution of all the planets around the sun, but also the direction of rotation about their axes of all but two planets. (The exceptions are thought to be related to collisions of some kind.) If there were a planet with a totally different orbital plane, it would have to be the result of capture by the sun or an exceptionally violent collision. The technical name for the original disk/cloud is a protoplanetary disk. The Hubble Space Telescope has taken pitcures showing such disks around newly formed stars, e.
