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The Solar System | Mercury | Venus | Earth | Mars | Ceres | Jupiter
Saturn | Chiron | Uranus | Neptune | Pluto
The Kuiper Belt and the Oort Cloud | Quaoar | Eris | Sedna

The dwarf planet Pluto.

Pluto was, until 2006, normally classed as the furthest (known) planet out from the Sun. Tidally locked with its satellite Charon, meaning they always have the same sides facing each other, there was room to argue that Pluto-Charon was in fact a double planet. However, Pluto's status as a planet was called into question for many years before scientists came to an official conclusion....

Pluto's orbit is very eccentric: it is inclined at an angle of 17 degrees to the plane which the other planets orbit on. For part of its orbit, Pluto is actually closer to the Sun than Neptune. If the Earth's orbit was as elliptically shaped as Pluto's, it would approach within 12 million miles of Mars at its farthest point from the Sun and as near as 2 million miles from Venus at its nearest approach to the Sun.

The Facts

  • Mass: 0.0125×1024kg
  • Equatorial Radius: 1,195km
  • Mean Density: 1,750kg/m3
  • Length of Day: 153.3 hours
  • Period of Revolution about Sun: 90,465 days
  • Acceleration due to Gravity on surface: 0.58m/s2
  • Mean Orbital Velocity: 4.72km/s
  • Inclination of Axis): 122.5°
  • Mean Distance from the Sun: 39.48 AU

The Discovery of Pluto

Clyde Tombaugh discovered Pluto on 18 February, 1930. Pluto is named after the Roman god of the underworld, a name it got after the naming of the planet was opened up as a competition. Tombaugh discovered Pluto during a systematic search for a new planet, imaginatively called 'Planet X', which Percival Lowell thought was causing Neptune to stray off course. Tombaugh took slides of the region the new planet was thought to be in, in pairs. He then had the painstaking task of comparing the pairs of slides to see if anything had moved against the background of stars.

What Do We Know?

Pluto has a bright surface, as it is covered in various types of ice. It is cold enough to have methane frozen on its surface, a feature that was discovered in 1976 by looking at the absorption lines in Pluto's spectrum. The discovery of frozen methane showed that Pluto was a small but bright planet, rather than a larger darker one. If it was larger and had methane frozen on it, it should be brighter than it is. Light reflected from the surface of Pluto takes 5 hours 40 minutes to reach Earth.

Pluto's surface, colder than any planet in the solar system, is too frigid for an atmosphere. The estimated surface temperature of -230°C would freeze any atmosphere and keep it on the surface. The Sun is so far away that it appears no larger than Jupiter does from Earth at its closest point.

It is very hard to resolve any surface detail of Pluto. It's like trying to read the writing on a golf ball 33 miles away1. The Hubble Space telescope has taken pictures of Pluto, which show it to have a northern polar cap, and various brighter and darker spots. These are probably not permanent - as Pluto moves nearer and farther from the sun the ices will melt and refreeze on the surface.


Observations of Pluto in 1978 showed it to have a bulge, which appeared to rotate approximately every six days. At first people wondered whether the telescope had somehow been knocked to blur the images of Pluto, but all the other stars around were still round, which they wouldn't have been if the picture had been blurred. James Christy discovered Pluto's satellite Charon2, named after the mythological being that ferried dead souls across the river Styx to the underworld. This was confirmed when Charon began to eclipse Pluto every 6.4 days.

The eclipsing of Pluto and Charon meant that it was possible to determine their relative sizes. Charon is the largest satellite relative to its primary in the solar system. The next largest is the Earth's moon.

Pluto has four other moons: Nix, Hydra, Kerberos and Styx.

When Will We Know More?

Just before Clyde Tombaugh died, NASA asked his permission for a space probe to fly past Pluto to gather more information on his planet. This mission, called the New Horizons, launched in 2006.

A Planet No More

Although Pluto's status as a planet had been contested for many years, its status was finally confirmed in August 2006 when astronomers meeting in Prague voted to strip it of its planetary status. Although some experts wanted to keep Pluto as a planet, they were unhappy with the proposal to upgrade three other objects to planets - the asteroid Ceres, Pluto's 'moon' Charon and an object listed as 2003 UB313. Now, publishers of school text books face the sorry task of having to erase the 'planet' Pluto from history.

The h2g2 Tour of the Solar System

Take the h2g2 Shuttle for your whistle-stop tour of the Solar System.

  • Next Stop (Inward Journey):
  • Next Stop (Outward Journey):
1If you had someone hold your golf ball in Guildford, UK and then you stood looking at it from the top of Canary Wharf in London, that's about as hard as it is to see Pluto from Earth.2Not to be confused with the similarly named comet Chiron, which orbits the Sun between Jupiter and Uranus.

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