The Kuiper Cliff and Planet X

There have been ideas floating around the scientific community about the possibility of a 10th planet existing in the outer region of our solar system. Although this idea has received much criticism, there are also many who acknowledge the possibility as a realistic one. At the furthest edges of our solar system, far beyond Pluto (not a planet :(...) is what is called the Kuiper Cliff. This is an area in the Kuiper Belt extending from 30-50 AU from the sun, composed of thousands of "ice rocks." The Cliff is a region where, suddenly, there is nothing; the density of "ice rocks" in the region decreases dramatically. In the above picture, the green dots represent the Kuiper Belt. It is obvious that there is something going on in the area at the bottom of the screen, where suddenly, there are no more "ice rocks." This is the area where some people believe our solar system's 10th planet (for now, termed "Planet X") is located. This planet is supposed to be between the masses of Earth and Mars, located beyond 48 AU.

A planet would explain the sudden decrease in the amount of "ice rocks" in the Kuiper Belt because it would create a major gravitational force, sweeping up any nearby objects. There have been theories of a 10th planet somewhere outside Neptune's orbit ever since the discovery of Neptune, but when Pluto was discovered, people thought that this is what they were looking for. However, with the recent declassification of Pluto as a planet, other theories are starting to become more prevalent.
NASA has launched a probe (New Horizons) to check out Pluto, but it will not arrive until 2015, and since we do not yet have the technology to view this possible planet from earth, we'll have to wait another 3 years before we get any concrete answers.