Has this happened before?

You might not know that almost the exact same thing happened about two hundred years ago. Scientists discovered a little world named Ceres. At first they thought Ceres was a new planet. They called Ceres a planet and even listed it with the big planets for almost fifty years. But then they found more and more little worlds around Ceres. They realized that Ceres wasn’t a planet like Earth. They decided to call Ceres and all of the little worlds around it asteroids instead of planets.