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.
I’m no astrophysicist, but if it’s in the Kuiper belt doesn’t that disqualify it for one of the criteria of being a planet, since it hasn’t cleared its neighborhood?
The Kuiper belt is way too big to really be considered a “neighborhood”. We’re talking about an unfathomably huge area full of many orders of magnitude more objects than we have so far been able to identify.
A planet-like body could easily be clearing its orbit out there without us being able to definitely detect the absence of material in that orbital area.
Moreover, various “Planet Nine” theories I have seen estimate this theoretical object’s size as 1.5-5x the size of the earth. If an object that big isn’t qualifying as a planet by definition, it’s the definition that’s wrong.
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.