Could there really be a ‘Planet X’ ?

Have we discovered all the planets in our solar system or could there still be more out there to find ?
While the hunt for new planets is now mostly focused on other solar systems, the idea that there could be one or more new planets lurking within our own solar system’s Kuiper Belt has not been forgotten. The idea of a ‘Planet X’ has even been associated with doomsday predictions and 2012 with some people believing that the end of the world will be brought about by this enigmatic world colliding with the Earth. There is however no evidence that such an event is likely to happen.
One particularly compelling phenomenon that might indicate the presence of a planet in the outer solar system is the ‘Kuiper Cliff’, a point at which the frozen debris in the Kuiper Belt suddenly stops. Could there be an unknown planet out there influencing these objects or is there another explanation that science has yet to discover ?
The announcement of the recent discovery of exoplanet Alpha Centauri Bb is a testimony to how far planetary detection techniques have come over the last few decades.
The Kuiper cliff
It would be reasonable to expect the millions of frozen lumps of rock would gradually decrease with distance from the sun, but at a distance of 48 Astronomical Units (beyond the orbit of Pluto) they seem to drop off suddenly, at the so called “Kuiper Cliff.”
Maybe Planet X is responsible for this strange unexpected feature in the outer edge of our solar system … or maybe not.
The Voyager and Pioneer spacecraft heading out of our solar system haven’t detected any substantial planets that might cause the ‘cliff,’ but space is vast; the chances of a spacecraft happening to fly past a previously undiscovered world would be highly unlikely.
Also, ground-based observatories and space telescopes (like NASA’s Wide-Field Infrared Survey Explorer) have turned up little evidence.
But the jury is still out. Until an answer is found for the Kuiper Cliff, the ghost of Planet X will remain as a tantalising, but unlikely, explanation.