Facts and Theories about Planets

Twenty years ago, astronomers had only eight (or nine) planets to work with, and a detailed theory about how they formed. Now they have hundreds of planets, which seem, in their size, make-up, rotation, and orbital behavior, to refute all the old theories about planetary formation. If, as we thought, planets form from rotating disks of gas, they ought all to orbit in the same direction, and in approximately the same plane, and their orbits should be roughly circular. But not we have many examples of reverse orbits and extremely elliptical orbits, and "orbital inclinations are all over the map." As astrophysicist Geoffrey Marcy of UC Berkeley puts it, "theory has struck out."