Discovered in 1930 by Clyde Tombaugh, Pluto was long regarded as the ninth planet of our solar system. However, after astronomers deepened their exploration of the intriguing worlds in the Kuiper belt, the icy Pluto was reclassified by IAU as a dwarf planet in 2006. Because additional objects in Kuiper belt has been discovered including Eris, which is 27% more massive than Pluto.
Then more accurate description of Pluto should be: the second largest known dwarf planet and tenth largest objects orbiting the Sun.
Pluto is only half as wide as the United States and its diameter is only about two-thirds as wide as Earth’s moon. Pluto is about 40 times farther from the sun than Earth is. Thus, it takes 248 Earth years to go around the sun and its surface is mostly covered with methane and nitrogen ice. Pluto is about 230 degrees below zero Celsius, much colder than Antarctica.
Pluto has five moons: Charon, Styx, Nix, Kerberos and Hydra, with Charon being the closet to Pluto and Hydra the most distant. Since Charon and Pluto are similar in size, Pluto and Charon orbit a point in space that lies between them and they continuously face each other as they travel through space.
With the successful flyby of the New Horizons mission launched by NASA in 2015, for the first time we get to know Pluto and its moons through clear pictures. And information about Pluto and its moons provide astronomers insights into the history of the solar system and how dwarf planets are formed.