Our Solar System is in some respects an orderly place. There are eight planets, in two well-defined groups. The four inner planets, Mercury to Mars, are rocky and comparatively small; beyond them comes the main belt of asteroids, of which only one Ceres is over 500 miles in diameter, and the only one, Vesta is ever visible with the naked eye. Next come the four giants, Jupiter to Neptune, and then the Kuiper Belt objects, of which the best-known, though not the largest, is Pluto. But things have not always been as straightforward as this.