Besides the eight planets we recognize today, objects previously considered local planets include:
- One star: The sun.
- Three dwarf planets: Ceres, Eris, and Pluto.
- 10 moons: Jupiter's moons Callisto, Europa, Ganymede, and Io; Saturn's moons Dione, Iapetus, Rhea, Tethys, and Titan; and Earth's moon.
- 14 asteroids: Astrea, Egeria, Flora, Eumonia, Hebe, Hygeia, Irene, Iris, Juno, Metis, Pallas, Parthenope, Vesta, and Victoria.
Now, no one would seriously argue that the sun should be considered a planet. It's clearly a star. Most wouldn't argue that moons should be considered planets, as they orbit other planets more directly than they orbit the sun. Where the distinction gets fuzzy is the border between asteroids and planets. You can't go by size alone; eight non-planetary objects orbiting our sun are larger than Pluto, and two of them -- Neptune's moon Triton and the trans-Neptunian object Xena -- have never been classified as planets.
That's why the IAU had to define a planet with objective criteria, which to date requires 1) that the object directly orbit the sun, 2) that the object be massive enough to have collapsed into a generally spheroid shape, and 3) that the object has "cleared the neighborhood" by absorbing and deflecting any other major objects in its orbital path.
Pluto fails point #3, but meets the other two. Thus, it's a dwarf planet, and no amount of historical precedent can make it otherwise.
That's more than just an otherworldly orbital reorganization; it's an empirically interplanetary instance of Geek Trivia.
Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger -- amusing and abusing IT pros. Read his full profile. You can also follow him on his personal blog.