Today we’re going to talk about something that really shouldn’t be controversial but is anyway.
Let me tell you a story. A long time ago, way back when people were first starting to think about astronomy, they looked up at the night sky and said “Hey! Look at all these shiny things! Some of them are always in basically the same place, and we’ll call those stars. Some of them move differently from the stars! They go all over the sky in like a month! Whoa, cool! We’ll call those planets. Because ‘planet’ means ‘wandering star’. We’re so clever.”
This was an okay definition for the data at the time, but it counted not only Jupiter, Saturn, Mercury, Venus, and Mars (the only planets they could see) as planets, but also the sun and the moon. Notable exceptions to this list of planets: The Earth. You can see how this definition is going to give us trouble later on.
When people got telescopes and realized that the solar system moved differently than they thought it did (i.e. not revolving around the Earth), scientists decided to call a planet anything that orbits around the sun. Huzzah! At last a functional, not at all problematic, brand new definition!
…Except it did have problems. Even later on scientists found these awesome objects in between Mars and Jupiter, and since they orbited the sun, they were planets!
“But wait,” you say, “there aren’t any planets in between Mars and Jupiter!”
Right you are, and good job. There aren’t any planets there. As scientists at the time found more and more of these tiny planets that weren’t really like any other planet in the solar system, they realized that they might be better suited to a different classification. Like, say, an “asteroid belt,” which is how we know these objects today.
You might be able to guess where I’m going with this. Yes, the same thing happened to Pluto. We found more and more objects that were similar to Pluto, and, in fact, more similar to each other than to any of the other planets. So now Pluto is part of the Kuiper belt, where it is assuredly much happier and fits in much better.
“But wait!” The pluto defenders cry. “Pluto has always been a planet! And now it won’t be! And now my kids won’t have Pluto as a planet!”
That’s sweet, but nostalgia really doesn’t have a place in science. Plus, now you get to present to others a much more accurate map of our lovely solar system.
And you can still teach your kids about how when you were young, they called Pluto a planet for a while, there were only 11 Doctors on Doctor Who, and Twilight was a popular book series.
If you’re up for a little more in depth planet talk, you can watch CGPGrey’s awesome video on the topic. And all his other videos. Because they are excellent.