The six outermost objects in our Solar System (like Sedna) all have similar orbital properties, much more similar than would be expected under the scenario of "random chance". Astronomers Mike Brown (of Pluto-killing infamy) and Konstantin Batygin have come up with a theoretical explanation that a ninth planet, perhaps 10 times as massive as Earth and well beyond the orbit of Pluto, could have caused this.
Video credit: Caltech/Robert Hurt.
More information here: http://aasnova.org/2016/01/20/a-ninth-planet-in-our-solar-system/
