One new theory about the formation of the Kuiper belt that appears to explain its architecture pretty well is the possiblitiy of a "rogue" planet. This study was presented by Patryk Lykawka and makes a big prediction: we'll discover a new planet soon. This planet would be about the size of Mars and would currently be on an orbit where it would have been pretty hard to find, but future surveys (like PanSTARRS) are likely to find this planet, if it exists. The planet originally formed between the giant planets, but got thrown out when they got big. It stayed in the Kuiper belt region for a while, stirring things up (for instance, creating the high-inclination objects) and then got thrown out to a relatively distant orbit (somewhere between Eris and Sedna). So, keep your eyes open in the next few years for the discovery of another "planet". (Planet X? Planet IX? The Large Dwarf Planet?) Note that there are other theories of the formation of the outer solar system that do an excellent job of explaining the facts, but do not require another planet (such as the "Nice" Model). This planet would be extremely cold all the time, in a great contrast to the vaporizingly-hot HD 149026b that I mentioned at the beginning.