Anyhow, the point is that the search for Planet X only continues because we know there isn't a chance in hell for Neptune to have an orbital
wobble for no good reason. There is a mass at least three-quarters the size of Neptune, if I remember correctly, orbiting in those cold, cold reaches
of space.
The discrepancies in the orbital motions of Uranus and Neptune were cleared up when Voyager 2 provided much more accurate figures for the masses of
both planets when it made close approaches in January 1986 and August 1989 respectively. However, it is the curious orbits of many scattered disk
objects (the outermost members of the Kuiper Belt) that leads some astronomers to believe that there is a relatively large planet well beyond Neptune.
Three quarters the mass of Earth is a good ballpark figure, and would mean that the diameter of this theoretical planet is greater than that of Earth
(probably around 15000 kms), due to the lower density that an outer planet would have.
For what it's worth, I suspect that there really is a planet lurking beyond the outer edge of the Kuiper Belt, and there may be more even further
out. However, they have nothing whatsoever to do with Nibiru, which is entirely mythical.