Is Pluto the ninth planet?
Historically it was indeed the ninth planet to be discovered, but now Pluto is also known as the first pluton, with its moon Charon being the second pluton. The classical planets can be numbered by their distance from the Sun, and there is no change in their order. Plutons, on the other hand, may due to their high eccentricity change their relative distances from the Sun with time (and hence their order).
