Planet 9

Even our greatest instruments cannot scan the region that lies beyond Pluto's orbit to the edge of the Oort cloud because it is so huge and poorly illuminated. That raises the question of whether there are additional huge bodies—possibly even planets—hiding in the shadows that we haven't yet discovered.

The answer is a resounding "yes," if a 2016 study that was published in The Astronomy Journal is to be believed. Six of the known Kuiper belt objects were discovered by researchers to be orbiting the Sun in peculiar, atypical ways. All six instances of the distortion pointed in the same direction, indicating that they are all being dragged by the same substantial object.

Even though we have never personally viewed or even photographed this body, their estimates indicate that it is around ten times larger than Earth. We have since discovered at least thirteen other Kuiper belt objects with comparable orbital distortions.