Can there be more planets undiscovered in this solar system? Many astronomers believe so while others argue against it. The reason many believe that there may be an undiscovered planet is the fact that the orbits of Neptune and Uranus have small irregularities that must be affected by the gravitational field of an unknown body in the solar system that orbits the sun also. It was also observed that Halley's comet did not arrive at the time calculated and believe that Halley's comet must be influenced by another force.
The argument that astronomers make against this theory is that the current observations of the orbits of Neptune, Uranus and the Halley's comet are consistent and there have not been any irregularities. However, other astronomers point to the fact that the unknown planet could have a high orbital inclination that is currently too far away from those planets to exert a force to change their orbits. As a matter of fact some believe that the unknown planet could orbit perpendicularly to earth. This means the orbit is approximately 90 degrees away from the orbital inclination of earth.
Well, if there really is an undiscovered planet that orbits the sun in this solar system, keep your eyes open (actually, look carefully with a telecope) and if you discover it, it could be named after you!