The "fifth planet" is a hypothesized giant world that was flung out of our solar system 4 billion years ago.
Some recent simulations on how the Solar System was formed have indeed shown highly unlikely results when assuming 4 giant planets only (Jupiter, Saturn, Uranus and Neptune). But a much better outcome with a 5th planet.
Now, where is it? Somewhere wandering in the dark cold space within the Galaxy...
Via Duncan Kaiser, Randi Springer, Guillaume Decugis