Pluto is not officially a planet according to the International Astronomical Union (IAU) as of last year.

Why? For one, it's too small. I think there are a half-dozen larger moons in the solar system, including our own, that are larger. There's also another "Pluto like" object out in the Kuiper Belt that is larger, so if Pluto is a planet, then Eris is too (and normal people have never heard of Eris). That's a pretty good argument right there.

Still, the real distinction here (and it's very teachable) is that the word "planet" was invented by humans (and defined now by scientists) to classify some objects in the Universe. And while we humans like nice, crisp, black and white distinctions, the Universe doesn't always (or even usually) oblige.

http://www.newscientist.com/article/mg20327181.600-is-pluto-a-planet-after-all.html

Here's a little example:

Clearly, we have a chair on the left and a stool on the right. What's the difference? Can you make a rule to define "chair" and "stool"? What about a third example that was halfway between the two in height? At what point do we need the International Stool Association to step in and decide that that thing we've been sitting on all these years is really not a stool, but is instead only a chair?
Source(s):
http://www.newscientist.com/article/mg20327181.600-is-pluto-a-planet-after-...


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