Pluto was once included in the list of planets, but is now considered a “dwarf
” planet—an object in orbit around the Sun that is massive enough to have its own gravity to pull itself into a round (or nearly round) shape. Dwarf planets
are generally smaller than Mercury. Pluto’s composition is similar to that of comets, and its orbit is quite different from that of the other planets. Pluto also has one moon, Charon. It is located within a part of our solar system called the Kuiper Belt. Scientists have discovered many objects in our solar system recently, but none of them have been classified as planets. Most are called either asteroids or minor planets, also called small solar system bodies. Some are called trans- Neptunian objects (TNO), because they are objects located past Neptune. One of the most recent discoveries of a TNO was in October 2000, when astronomers discovered a TNO and called it 2000 EB 173. It is between 186 and 435 miles (300 and 700 kilometers) in size and, as far as non-planets go in the solar system, it is second in size to the asteroid Ceres.