Hi Lorien,

If there were more than 9 planets orbiting our Sun I think we would have discovered them a long time ago.

However, you can get a little picky by worrying about the definition of 'planet'. The recently discovered object, Sedna, orbiting our Sun could be called a planet in that it has some similarities to Pluto. However, it is much smaller, much farther away and unlikely to be classified as a planet (in fact some people argue that Pluto isn't really a planet either).

Here's a link to more information about Sedna along with some discussion about the definition of planets: http://www.gps.caltech.edu/~mbrown/sedna/

-Christina