7) It's a double planet
OK, whether you think Pluto is a planet or not, it's still a double
something.
Think of it this way: imagine two kids, each holding on to the end of a rope, and they start to swing around each other. If one kid is really big, and the other really small, then the little kid makes a big circle, and the big kid makes a small circle as they dance. Each kid is circling some point between them (the
center of mass, or, in fancy science-speak, the
barycenter), and the size of the circle each makes depends on how massive the kid is, and how far apart they are.
Same thing for bigger masses, too, except that instead of a rope, the tie that binds is gravity. For the Earth and Moon system, the center of mass is inside the Earth, about 3/4 of the way from the center to the surface (or about 1700 km down, if you like). That's a long way from the Earth's center, but still inside the Earth.
Pluto though is different. Its big moon Charon is massive enough (about 1/8th of Pluto's mass) and at such a distance that the barycenter of the system is
outside Pluto's surface! About 600 km above it, to be close enough. So really Pluto and Charon orbit each other -- which is really true for any two bodies, but really obvious for these two guys. Every time Charon orbits Pluto once, Pluto itself swings around this invisible point in space too.
If there's a takeaway lesson from this, it's that to us, Pluto may seem like a little kid, but to Charon it's a big kid. Just not a huge one.
OK, that's a stupid lesson. But this is a Top Ten list, not a Very Special Episode of Bad Astronomy. If you want a heart-warming lesson, stick with some place closer to the Sun.