Questions relating to the discovery of a new ninth planet (not Pluto) hypothesized in the Solar System.

On Jan 20, 2016, an announcement was made that using the trajectories of Kuiper Belt Objects, a 9th planet was expected to be found in the far reaches of the Solar System. Such a planet has been hypothesized several times, but this tag is specifically intended for questions relating to a hypothetical planet sized object which dominates the Outer Kuiper Belt or inner Oort Cloud region.

The planet is theorized to have a mass of 5-10 Earth masses, orbiting 400-800 AU away from the Sun.