Pluto was discovered in 1930 and considered to be the ninth planet in our solar system. Recently, its status has changed because it has less in common with the other eight planets than it does with similar objects in the Kuiper Belt, a band of cometary debris at the edge of our solar system. Pluto's a small piece of ice and rock, which makes it more similar to comets than to the other planets.

Image credits: main image, "Pluto: The Frozen Planet", R. Albrecht (ESA/ESO), Hubble Space Telescope, and NASA; Neil Tyson: AMNH.