[InclinedPlane]

The first four detected asteroids, in fact. They were all discovered in a 6 year period during the early 19th century. And all were labelled planets.

That's how things sat for years, decades even. And then in a span of 5 years in the late 1840s the next 6 asteroids were discovered. By the end of the 1850s they were up to 57 asteroids, in the 1860s they found more than 50 more, in the 1870s they found over a hundred more than that. It had become rapidly apparent that Ceres, Pallas, Juno, and Vesta weren't just little quirky oddball planets, they were merely members of a much larger population of other bodies (that would come to be known as asteroids) separate from the planets.

The situation is identical with Pluto, the only difference being that Pluto was alone in its new classification of "little quirky oddball planet" in the 20th century for about 80 years. But now it has become apparent that Pluto isn't a weird little planet, it's a member of a different family of objects (Trans-Neptunians, of which Triton was probably also a member before being captured by Neptune).