How many of these things are there?

Astronomers at California's Palomar Observatory said Friday they have discovered what they called the solar system's 10th planet. Unless, of course, you consider Sedna the 10th planet. Or Quaoar. Or 2004 DW. The latest discovery has the advantage of being bigger than Pluto, which is still the ninth planet, as third-graders gladly will tell you if you try to claim otherwise.