Most of us were heartbroken when Pluto was demoted from its planet status by the International Astronomical Union (IAU) in 2006. But don't worry about it too much. Dwarf planets are still planets, just of a different kind. Plus, it's possible that the planets of the solar system will have a new 9th sibling to replace Pluto.
Last March, astronomers Chad Trujillo and Scott Sheppard announced that they discovered a 450-kilometer-wide dwarf planet (designated 2012 VP113) just outside the Kuiper Belt. Finding an object of this size near the Kuiper Belt is not astonishing in itself, as the Kuiper Belt is full of objects like VP113 (currently, there are more than a thousand known Kuiper Belt Objects like this). But the thing that caught the astronomers' attention is VP113's orbit.
Although it was originally seen by the Kuiper Belt, VP113 swings wildly about our solar system. This icy body has a vastly elongated orbit—one that is a bit different from the orbits of anything else that we know in the solar system. Well, everything except Sedna, another dwarf planet that passes near the Kuiper Belt. The thing that is so astonishing about these orbits is how they came to be like this, and how oddly similar they are in their approach to the Sun. Ultimately, it is believed that such revolutions can only happen if there is something massive that impacts their orbits.
But as far as astronomers know, there are no massive objects close enough to the two dwarf planets to do that. The key here is, "as far as they know," because there could be quite a bit that they don't know.
To confirm this, the astronomers that discovered 2012 VP113 observed the Kuiper Belt more intently, and they saw that 10 more objects followed the same orbit. Even more surprising, Sedna, 2012 VP113, and the 10 other objects all come closest to the Sun (this is known as the "perihelion") at almost the same time that they cross the plane of the solar system. For the astronomers, this is more than a mere coincidence. They suspect that there is something that causes the planets to be pulled out of the plane in this very similar manner, so they say that this find only strengthens the case for an undiscovered planet.
Ultimately, Trujillo and Sheppard estimated that there is an undiscovered planet very far out of the solar system (250 astronomical units, or eight times as far from the Sun as Neptune). However, this is not the only possibility. Trujillo and Sheppard also note that a closer, and less massive planet, could also impact these objects in this way.
Of course, this discovery is met with speculation and doubt. Physicist Lorenzo Iorio says that this undiscovered planet, if ever it exists, must be at least twice as far out as the original prediction indicates (which makes its existence rather unlikely). Some physicists think that the errant orbit of these Kuiper Belt Objects might be caused by the planet Neptune. While still others think that the observed "clustering" of the perihelions is not really a "cluster" but just an illusion, as 12 objects (out of the thousands of objects in the Kuiper Belt) is a very small sample.
Unfortunately, he solar system beyond Neptune is still a mysterious place for us, as it is very difficult to observe the objects at that distant place with a telescope. So there might be an undiscovered planet in this place without us knowing it. But whether there really is this unknown planet out there or not, the one thing that we learned from this discovery really is that the solar system is a very big place, and there may still be surprises lurking out there, waiting to be discovered.