The Kuiper Belt suddenly terminates at 48 AU. FYI, Neptune is at 30 AU from the Sun. The number of mini planets like Pluto and Quaoar quickly decrease. As you can see, something is really going wrong here. Computer modelling by Patryk Lykawka of Kobe University has claimed that the gravitational attraction of an unseen major planet, perhaps the size of Earth or Mars, might be responsible. To have any real affect on the Kuiper belt, they believe this putative planet to lie between 50 and 72 AU and be between the mass of Earth and Mars. Another guess is that this putative planet is a frozen world between the sizes of Uranus and Earth, but less massive and having an highly elliptical orbit that takes it out to 5 billion miles from the Sun, hardly next door but still within the Sun's gravitational influence. This planet may also should be dim, possibly between 14-18 magnitude. There is a very high probability of this, a passing star, as pur forward by Alan Boss and the late Tom van Flanders, isn't able to leave a gap on the Kuiper Belt, and scientists are so sure of it that they think that there is nothing left but to name it. The New Horizons spacecraft, which will explore Pluto and MakeMake, should eventually provide some forensic evidence for Planet X's existence. One of the more amusing pieces for the existence of Planet X is a picture from the 1987 New Science and Invention Encyclopedia. In a section on space probes, the encyclopedia shows the paths of the 2 Pioneer probes and illustrates how the probes were used in the search for more planets. It shows the Earth, the Sun, a dead star (at 50 billion miles or 538 AU) and a tenth (errr, ninth since Pluto god demoted 19 years after this book got published) planet (at 4.7 billion miles or 50 AU). As for the dead star, the WISE mission (an infrared sky survey that covered most of our solar neighborhood in movement-verifying parallax measurements) was expected to be able to find it. WISE can detect 150 kelvin brown dwarfs out to 10 light-years. But the closer a brown dwarf is the easier it is to detect. Preliminary results of the WISE survey were released on April 14, 2011. On March 14, 2012, the entire catalog of the WISE mission was released and the post cryo data will be released later this year. The dead star might to be found by the end of 2013 or early 2014.