The Hundredth Monkey Phenomenon

Kudos for pointing this out. From a story over at Centauri Dreams:

There is precedent for planetary demotion, incidentally. After its discovery in 1801, Ceres was generally thought to be a planet, but by the early 1850s so many of what we now call ‘asteroids’ were being discovered that planetary status for all was ruled out.

Ceres, for those who aren’t familiar with everything that’s flying around our solar system, is, by far, the largest object in the asteroid belt, which includes (roughly) all objects in orbit around the sun between Mars and Jupiter.