For years after the discovery of the planet Neptune, perturbances in its orbit led astronomers to believe that there was another planet tugging at the eighth one. This belief was largely what led astronomer Clyde Tombaugh to search for Planet X which resulted in the discovery of Pluto in 1930. However, Pluto was much too small to have the observed effect upon Neptune. Many astronomers have continued to consider that a larger Planet X was out there.
Now new computer models are indicating that indeed, a larger world is out there, not only perturbing the orbits of other planets, but causing some of the activity in the Kuiper Belt. If such a world exists, it is so far out that the sun would appear as just a bright star. (Reminds me of the line in Three Dog Night's song, Out in the Country, that says "Before the sun is just a bright spot in the nighttime.") What a fascinating place the outer reaches of the Solar System must be! Although one bad thing about it, is that it could be the holding ground for an extinction-level asteroid that could smash into Earth, ending humanity's existence. Have a nice day.