I've heard that there was potentially a 10th planet, but there was no conclusive evidence of any until recently.
http://www.cnn.com/2004/TECH/space/0...net.discovery/
The debate is whether this is something that could be considered a planet. At 2000 KM in diameter it's smaller than Pluto, and some would debate that given it's size that Pluto isn't a planet either.