Explaining Pluto's Status to a Three-Year-Old

I received a solar system kit from Nana for Christmas.



Now Pluto did come with the kit, but I didn't add him. Why? Because he's not a planet anymore, based on the new definition by the International Astronomical Union (IAU).

Luca asked the same question, and I had to think for a minute how I could explain this to him. There are a few main components of what makes up a planet (simplified):

1. Orbit: All planets orbit a star.
2. Shape: Each planet is nearly round.
3. Neighborhood: Each planet has cleared its orbital neighborhood of other stuff.

The last criteria excludes Pluto from planethood, since there's still a lot of junk in its orbital path.

So I told Luca, "Pluto isn't a planet because he didn't clean up his room."

By the way, here are some new, spectacular images of Earth:

The New Blue Marble West
The New Blue Marble East