Because odds are within a couple decades we'd have 50+ or 100+ planets if you count stuff like Pluto and Eris.
We already went through this two centuries ago with the discovery of Ceres, closely followed by a “series” of other asteroids. Initially, Ceres was hailed as the ninth planet. Note that in the TV series, “The Ascent of Man”, Jacob Bronowski invokes Ceres’ status as a planet to ridicule Hegel, somewhat unjustly IMHO.
Bronowski states that Ceres’ discovery confounded Hegel’s putative “proof” that there could only be 8 planets. I took the trouble to obtain a copy of Hegel’s thesis and found that he made a more limited claim. He criticized “Bode’s Law” on the basis that it allowed any number of planets, and said that any such law should accommodate only the known planets. Well, it’s much of a muchness.