How many planets???

18 PLANETS: Have you ever heard of the planet Hygea? It's listed in the 1850 Annual of Scientific Discovery along with 17 other planets:

Courtesy Joe Pollock, Appalachian State University. [full text]

In those days, large asteroids such as Hygea, Ceres and Vesta were widely deemed planets. They appeared so in textbooks and scientific journals. Adding asteroids to the other known planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, gave a grand total of 18. Imagine the pneumonic: "My Very Educated [insert ten adjectives here] Mother Just Served Us Noodles."
The asteroids were eventually demoted. It was a long, drawn-out affair, marked by decades of disagreement and confusion. (Sound familiar?) By 1900, however, order was restored to the Solar System: the planet count was down to eight.
And then came Pluto...