Pluto used to be characterized as the ninth planet but, in 2006, scientists revoked Pluto’s full-fledged planet status. Instead they classified it as a dwarf planet. True to this new name, Pluto’s diameter is 1,432 miles, which makes it more than 1,500 miles smaller in diameter than Mercury, the smallest planet.
Pluto was discovered in 1930, and its moon Charon was discovered in 1978. This dwarf planet orbits the sun in a bizarre fashion, which makes it the 8th planet from the sun at times and, at other times, the 9th. This occurs because its orbit is not on the same plane as the rest of the solar system.
Various scientists believe that Pluto is not an ordinary dwarf planet. Rather, they believe that it is an icy planetesimal. These are the precursors to planets, and they are made out of rock and ice. Most planetesimals form in the Oort cloud, which surrounds the Kuiper belt, a region beyond Neptune.
A year on Pluto is equivalent to 248 Earth years. Strangely, Pluto’s days and months are the exact same length, about 6.4 Earth days. This is because Charon revolves around Pluto at the same speed that Pluto rotates on its axis. Pluto is also unique because its average temperature is -396 degrees Fahrenheit. Scientists believe that Pluto has a crust of methane ice and nitrogen and, under that, a layer of ice water and a core of rock.
In 2006, a team of scientists launched the first probe to Pluto. The probe, called New Horizons, launched past Pluto and the Kuiper belt. It sent back data that scientists used to help us better understand this dwarf planet. Some think that Pluto might ultimately regain its status as a planet; either way, Pluto is an interesting part of the solar system.
[Sources: Children’s Atlas of the Universe; Cool Cosmos]