Pluto Neptune also showed unexpected perturbations. Calculations and searching led to the discovery of Pluto in 1930. Pluto is very small and was not causing significant perturbations. But this was a lucky accident. There were some errors for planet masses.
Recent measurements indicate there are no current unexplained perturbations.
No more large planets seem likely. This distance r would be the semimajor axis length. The textbook goes through this derivation in detail, but that's not crucial. The strength of gravity depends on the masses and distances. So the size and speed of orbits should be related to the amount of gravitational force.