Pluto

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.

Diameter: 1,400 miles
Location: orbits Sun just beyond Neptune
Average Distance from Sun: 3.7 billion miles
Average Surface Temperature: -370 degrees F
Orbital Period: 248 Earth years
Number of satellites: 1
Significance: largest object in the Kuiper Belt
Characteristics: a frozen ball of methane, nitrogen, and ice with a rocky core

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