There seems to be no Planet X. There has been an ongoing idea of a planet outside the orbit of Pluto. Surveys by the Wide-Field Infrared Survey Explorer (WISE) turned up thousands of “new to us” stars and brown dwarfs within 500 light-years, but no Planets.
The outer solar system probably does not contain a large gas giant planet, or a small, companion star
– Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Penn State University, University Park, Pa., author of a paper in the Astrophysical Journal describing the results.
The recent study, looking at WISE data found no objects Saturn sized or larger to a distance of 10,000 A.U. and no Jupiter sized or larger out to 26,000 A.U. In rough terms 1 A.U. is about 150 million km / 93 million miles.