Nerd post of the day:
So this is pretty cool, if true: British and Brazilian astronomers have hypothesized that there may be at least two more Earth-sized planets in our solar system. Based on calculations of the behavior of icy particles called "extreme trans-Neptune objects" floating around 200-250 AU from the Sun, there should be at least two objects with enough gravity to cause this unexplained behavior. (1 AU is the average distance from the Earth to the Sun, about 150 Million kilometers)
[As a spatial reference, Voyager 1, launched in 1977, has just left the Kuiper Belt which orbits at about 30-50 AU, near the orbit of Pluto and won't get to those possible planets for another 100 or so years.
Our solar system, or the sphere of space that is influenced by our Sun's light and gravity, ends in the neighborhood of the Oort Cloud which is at about 100,000 AU from the Sun. So these two "planets" if they exist are still fairly close in.]