This is not the first time this sort of thing has happened. When Pluto was found by Clyde Tombaugh he was looking for Planet Nine, which Percival Lowell had calculated must be present based on the orbits of the outer planets. But it was quickly realized that Pluto was too small and in the wrong orbit for it be Lowell's deduced planet. (And even then they worked with a too high estimate of Pluto's mass, it wasn't until the 1978 discovery of Charon that we got a good estimate of Pluto's mass. It is hard to get a good mass estimate without something else in orbit around it.)

The Pioneer and Voyager missions gave us much better estimates of the masses of the gas giants, and my understanding is that if you go back and redo Lowell's calculations with those correct masses, his planet disappears. That's my best guess as to Planet X, that our constants are wrong in some way, but we'll see.

https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2017%...