A pair of astrophysicists, one with Kindai University, the other the National Astronomical Observatory of Japan, both in Japan, have found possible evidence of an Earth-like planet residing in the Kuiper Belt. In their paper published in The Astronomical Journal, Patryk Sofia Lykawka and Takashi Ito describe properties of the Kuiper Belt that they believe are consistent with the existence of a planet not much bigger than Earth.
The kuiper belt is huge, sparse, and poorly mapped. It’s possible that it has cleared its orbit, (or still in the process of) but we haven’t noticed yet.