Pluto - Charon system

Pluto - Charon system

The largest satellite of Pluto is Charon.



Pluto - Charon system, Pluto, Charon, moons of Pluto, Kuiper Belt, Solar System, dwarf planet, Sun, Hydra, Nix, Kerberos, Styx, astronomy, geography

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Pluto was regarded the smallest planet of the Solar System, until 2006. Its diameter is smaller than that of the Moon (2,372 km or 1,474 mi). However, it has been recategorized as the largest dwarf planet. It is 39.5 times farther from the Sun than Earth.

Its largest natural satellite is Charon. Relative to Pluto, its size is considerable (diameter: 1,208 km or 751 mi), their size ratio is 2 to 1, thus the Pluto-Charon system was earlier considered a binary planet system.

Their mass ratio is 8 to 1, their common center of mass is outside Pluto, thus they revolve about each other.
Mean density of Pluto is 1.87 g/cm³ (0.0676 lb/in³). It is composed of materials compressed by high temperature. Its surface is mainly composed of frozen nitrogen and methane, which melt during the day and create a thin atmosphere.

Average temperature: -230°C (-382°F)
Orbital period: around 248 Earth years
Rotation period: 6.39 days
It has 5 known moons


Dwarf planet: A celestial body orbiting the Sun massive enough to be spherical but has not cleared its neighboring region. The size of dwarf planets is between that of asteroids and regular planets. Trans-Neptunian dwarf planets are called plutoids.


Diameter: 2,372 km ( 0.19 Earths or 1,474 mi)

Mass: 1.305·10²² kg (0.0021 Earths or 2.877·10²² lb)

Average density: 1.87 g/cm³ (0.0676 lb/in³)

Surface gravity: 0.063 g

Number of moons: 5

Average distance from the Sun:
5,900,000,000 km (3,666,000,000 mi) = 39.5 AU =
5.5 light hours

Orbit length: 36,530,000,000 km (22,700,000,000 mi) =
244,18 AU

Orbital eccentricity: 0.24880766

Orbital period: about 248 years

Charon (moon):

Average distance from Pluto: 19,600 km (12,180 mi)

Diameter: 1,208 km (751 mi)

  • common center of mass - Due to the mass ratio of Pluto and Charon (8:1) and their large distance their common center of mass is outside Pluto, and both celestial bodies revolve about it.

  • average distance from the Sun: 5,900,000,000 km (3,666,000,000 mi)

  • common center of mass - Due to the mass ratio of Pluto and Charon (8:1) and their large distance their common center of mass is outside Pluto, and both celestial bodies revolve about it.
  • average distance from the Sun: 5,900,000,000 km (3,666,000,000 mi)


Until 2006, Pluto was classified as the smallest planet in the Solar System, the ninth and the farthest planet from the Sun. In that year, however, the International Astronomical Union reformulated the definition of the term ‘planet.’ In the light of the new definition, the icy celestial bodies similar in size to Pluto and found at the edge of the Solar System in the Kuiper Belt have been categorized as dwarf planets.

Discovered in 1930, Pluto was named after the Roman mythological ruler of the underworld. Pluto has a diameter less than one-fifth that of the Earth, and a mass which is approximately 0.2% that of the Earth. Its surface is composed of frozen nitrogen and methane, with a mean surface temperature of almost -230°C (-382°F).

Pluto is nearly 30 times further from the Sun than the Earth at its closest point to the Sun and almost 50 times further away than the Earth at its furthest point. It takes 6.39 Earth days to rotate around its axis, and 248 Earth years to revolve around the Sun. Pluto has not yet completed half of its orbit since its discovery. It was closest to the Sun in 1989, and is now moving away from it.

Pluto has 5 known moons, the largest of them being Charon, with a diameter nearly half that of Pluto and a mass 8 times smaller relative to Pluto. Charon is the ferryman of Hades, according to Greek mythology. For many decades, Charon was thought to be Pluto’s only moon. Since 2005, however, 4 additional moons have been discovered, that is Hydra, Nix, Kerberos and Styx, listed in order of size.

Pluto and Charon were long considered to be a binary planet system, due to their unusual mass ratio and the fact that their common center of mass is outside Pluto.
Charon is tidally locked, which means that it takes just as long to rotate around its axis as it does to revolve around Pluto. Furthermore Pluto and Charon are locked in a synchronous rotation, which means that they always show the same face to each other.

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