Hidden planets in the solar system

Beyond the confines of Pluto, there may lurk many worlds with masses as big as 70% that of earth. One, Sedna, has been found already but there may be others that are just as massive, if not bigger. Since most of these objects are in an irregular orbit around the sun, they cannot be considered planets, but instead would be considered planetoids, dwarf planets, or the newly coined term, plutoids. All these objects are part of a system called the Kuiper Belt which orbits the sun at a distance somewhere between 100 AU(100 times the distance from Earth to Sun) to 200AU.

At this distance from the sun, all water would be frozen on the surface but if the planet had a hot core, there could exist liquid oceans miles beneath the surface — a potential for life. Which means that not only are we not alone in the Universe, but we are not even alone in the solar system. I would be extremely excited at the prospect of finding complex aquatic life on what was thought to be frozen and lifeless chunks of rock. Heck, maybe someone evil squid-men from Planet-X might even invade us. Right now data on these objects is sparse so not much is known about them and anything we conjecture would be in the realm of science fiction.