Pluto was once the ninth planet of the Solar System but was reclassified as a dwarf planet in 2006. It was discovered in 1930 by Clyde Tombaugh.

Diameter: 2,306 km
Mass: 1.31 x 1022 kg
Composition: Ice and rock
Orbit: 4.447 billion to 7.38 billion km from the Sun

Located in the Kuiper belt, a collection of icy objects beyond the orbit of Neptune, Pluto has a moon called Charon which is unusually large for a satellite. Charon is nearly half the size of Pluto and was discovered in 1978 by astronomers James Christy and Robert Harrington. Charon orbits in 6.4 days at a distance of 19,640 km from Pluto, which is also the same time it takes for Pluto to complete a rotation. Pluto rotates backwards and from east to west, which is known as a retrograde rotation. Charon is tidally locked to Pluto, meaning that it always remains in the same place over it. The same side of the satellite always faces the planet.

Pluto was named by a young British girl, Venetia Burney, whose grandfather submitted the suggested name on her behalf. In Roman mythology, Pluto is the god of the underworld. The theme of the underworld continues with Pluto’s moons. Charon is named after the boatman who ferries souls to the underworld, Nix is Charon’s mother and Hydra is a serpent who guards the underworld and has nine heads.

When the dwarf planet Eris was discovered, it was found to be larger than Pluto. Because of this, clarification was sought from the International Astronomical Union on the true definition of a planet. As a result, Eris was classified as a dwarf planet and because it is bigger than Pluto, Pluto was downgraded. Other dwarf planets are Ceres, Haumea and Makemake.

The composition of Pluto consists of a rocky core, an ice-water mantle and a surface of frozen gases that include methane and nitrogen. Pluto appears red on observation, but its moon Charon is grey in colour and therefore they must have different compositions to each other.

Pluto orbits the Sun in 248 years in an elliptical pattern, so is closer to it at certain times during its orbit. When it is nearer to the Sun, the heat partially melts Pluto’s icy surface, which yields a thin atmosphere. As Pluto moves away from the Sun to complete its orbit, much of the atmosphere freezes again.

Both Pluto and the satellite Charon are very small and are difficult to observe from Earth. Images were gathered of both objects by the Hubble Space Telescope in 2005 which allowed experts to map out features. Further observations gathered from Hubble alerted astronomers to the existence of two more moons, Nix and Hydra, in 2005.

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