The discovery of Sedna a planet-like rock 75% of Pluto’s size but three times farther from the sun than Pluto is, has raised some interesting questions for astronomers. Chief among these: do you count Sedna as a planet, or stop counting Pluto as a planet?
The argument is along these lines: Pluto and Sedna are very similar. They are both about the same size, are much smaller than any of the other planets, have very strange orbits compared to the other planets, and are a long, long way from the sun. If you don’t count Sedna as a planet, then you shouldn’t count Pluto, which means our solar system only has eight planets. If you do count Pluto, then you should count Sedna and maybe some other objects in the Kuiper Belt. That would put our solar system at at least 10 planets, maybe as many as 12.
Personally, I don’t care what they are called. I just think it is pretty cool that we are still discovering new objects in our solar system.