@oscar23 and @eviemae - I agree with both of you in that we aren’t really sure what is out there. But, there probably is not a planet x. (Sorry, guys.)
It just isn’t in the mathematics. The math that we had used to calculate the possibility of another planet was skewed, and we were wrong. Hubble got better numbers, and now we know that we were wrong. Plain and simple.
Besides, we’ve known about the other planets for thousands or more years. Isn’t it likely that we could find it by now? (Now watch; we'll find planet x in 2012 just because I opened my big mouth on wisegeek.)