We have yet another new word to describe some of the things out there beyond Neptune: Plutoids. As you may recall, a couple of years ago, the International Astronomical Union finally defined the word “planet.” It was something of a shock to many people in the public, but astronomers had never actually come up with a definition of “planet” in all these years. Originally, planet meant a “wandering star.” That would be a star that appeared to move from constellation to...