Since Pluto was demoted to "dwarf planet" status in 2005, the same grim pedants who insisted it wasn't a real planet have also insisted that our solar system has eight planets circling its sun. Recently, though, researchers have found some unexplained motions that suggest a large planet far on the edges of the system, and we may be back to nine planets sometime soon.
Most stars with planets have at least one planet called a "super-Earth," because it's rocky like our world but several times its size. Our solar system doesn't, as far as we've discovered, which makes us weird. But the hints provided from other observations suggest where it might be if it exists, and computer simulations of how the solar system formed offer some reasons why it might be there.
Of the currently known and named planets, Neptune was first predicted by its gravitational effects on Uranus. Those clues allowed Johan Galle to find it while observing on September 23, 1846. The possible Planet Nine is quite a ways farther out, but we have much better instruments today.
Locating a ninth planet will not necessarily put an end to the list of planetary bodies that revolve around our sun, but history teaches us that traveling to Planet Ten is not as easy as it sounds, and could bring the danger of invasion.