Although Pluto lost its status as "Planet Nine" when it was downgraded to dwarf planet, there is ample evidence that our solar system either had or currently has a large planet far beyond Pluto that may one day claim Pluto's former mantle and become the rightful ninth planet. Unusually regular orbital patterns observed in the Kuiper belt hint that some celestial body more massive than Pluto lurks beyond the distant band of icy debris at the edge of the solar system where Pluto, Eris and other dwarf planets live.
The hypothetical existence of a distant Planet Nine or "Planet X" remains contentious, but evidence continues to mount in its favor. Certainly, it would not be the first time a hypothetical planet was found. Neptune was the first planet found through studying orbits of other bodies in the solar system; intriguingly, its location was discovered with predictions derived from pen-and-paper calculations about telescope observations.
