The Kuiper belt terminates suddenly at a distance of 48 astronomical units (AU) from the Sun (by comparison, Neptune lies 30 AU from the Sun), and there has been some speculation that this sudden drop-off, known as the "Kuiper cliff", may be attributed to the presence of an object with a mass between that of Mars and Earth located beyond 48 AU. The presence of a Mars-like planet in a circular orbit at 60 AU leads to a TNO population incompatible with observations. For instance, it would severely deplete the plutino population. Astronomers have not excluded the possibility of a more massive Earth-like planet located further than 100 AU with an eccentric and inclined orbit. Computer simulations by Patryk Lykawka of Kobe University have suggested that a body with a mass between 0.3 and 0.7 that of the Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 101 and 200 AU from the Sun, could explain the Kuiper cliff and the peculiar detached objects such as Sedna. While some astronomers have cautiously supported these claims, others have dismissed them as "contrived".
In 2012, Rodney Gomes of the National Observatory of Brazil modelled the orbits of 92 Kuiper belt objects and found that six of those orbits were far more elongated than the model predicted. He concluded that the simplest explanation was the gravitational pull of a distant planetary companion, such as a Neptune-sized object at 1500 AU or a Mars-sized object at around 53 AU.
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 2 years before we get any concrete answers.