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Diameter2274 km, 0.18 x Earth's
Mass0.0025 ME
Volume0.01 VE
Mean Density1.1 g/cm3
Mean Distance from Sun5900 x 106km = 39.44 AU
Eccentricity of Orbit0.25
Inclination of Equator to Orbit123°
Inclination of Orbit to Ecliptic17°
ColourUnknown. Possibly grey.
TemperatureProbably between -228°C and -238°C
AtmosphereProbably nitrogen, carbon monoxide and methane
Length of Day6.39 days
Length of Year247.7 years
No. of MoonsOne
Escape Velocity1.2 km/s

Pluto is the smallest and most distant planet in the Solar System. It is so small that many astronomers argue that it is not a planet at all but part of a huge band of rocky objects at the edge of the Solar System called the Kuiper Belt. The opposite sides of Pluto imaged by the Hubble Space Telescope

Pluto is the only planet not to have been visited by a spacecraft. This means that very little is known about it, especially its surface conditions, colour and structure.

Pluto’s moon, Charon, is very unusual because it is about half the size of the planet, making it closer in size to its planet than any other moon in the Solar System.

Pluto is so far away from the Sun that it gets no proper daylight like we do here on Earth. Instead the Sun just looks like a very bright star in Pluto’s sky. This also means that the planet gets very little heat. It is so cold that it’s atmosphere is probably frozen, and only when the planet gets closest to the Sun does the atmosphere turn into a gas.

Pluto spends most of its time as the furthest planet in the Solar System, but its orbit is so elliptical that it sometimes passes inside that of Neptune. Between 1979 and February 1999 Neptune was the furthest planet from the Sun. Pluto’s orbit is also very unusual because it is at an angle of about 17 degrees to the plane of orbit of the other planets, called the “ecliptic plane.” These abnormalities in Pluto’s orbit reinforce the case that Pluto is a Kuiper Belt object rather than a planet.

Planets Introduction




The Moon









Extra-Solar Planets

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Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001