Ceres was also reasonably considered a planet, until evidence was discovered that showed it was just one of many similar bodies.
Including Pluto in a classification with the 8 major planets, but not including Eris, is simply not a useful way to describe the bodies in the solar system.
Personally, I use the word “planet” for anything even vaguely planet-like, but there's no logical way to count “planets”, under any definition of the word, that leads to Pluto being the 9th.
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).
I was really hoping this was a reprint of an article from 1930 and not something written this week.
I have two young daughters. They like space as much as any not-yet-in-gradeschool kids might. We only buy planet books in our family that include Pluto as a planet.
Including Pluto in a classification with the 8 major planets, but not including Eris, is simply not a useful way to describe the bodies in the solar system.
Personally, I use the word “planet” for anything even vaguely planet-like, but there's no logical way to count “planets”, under any definition of the word, that leads to Pluto being the 9th.