What makes a planet a planet?
Its not as easy a question as you might think.
Here’s a UC Berkeley online article which helps illustrate the variance in opinion. Heck, not everyone thinks poor little Pluto is a planet. At the American Museum of Natural History in NY…Pluto’s been demoted, and does NOT have a model in the new Planetarium as the other planets do. Instead, it is lumped with the Kuiper Belt objects. Those are, for the uninitiated, a sort of “asteroid belt” beyond Neptune, mainly made of ice-dominated bodies. Some of them are quite large, Astronomers recently found one, named Quaoar, which is about 800 miles in diameter. That’s larger than any of the traditional asteroids between Mars and Jupiter, and about 4/7th the size of Pluto. So if Pluto is a planet, is Quaoar? And if not, why not? If you consider planets to have a “rocky” core, Pluto gets demoted. But Pluto has a moon, shouldn’t that count for something? And maybe we should consider a “Grandfather” clause for Pluto, and exclude bodies like it from planet-hood?
So does the solar system have 9 planets? Or 8 (if you exclude Pluto)? Or more if you count objects like Quaoar?
Perhaps we can have “major planets” and “minor planets”…but where do you draw that line (one possibility I think are between Major Gas Giants and Minor “Ice/rock planets”. Of course, that means Earth is a “minor” planet under that definition.
Yes, this is the sort of thing which does float my boat. I’ve liked thinking about this stuff ever since good old Carl Sagan put together a little series called Cosmos.