The Pluto, The Smallest

• Discovered: 1930
• Diameter: 2,301 km
• Temperature on surface: -235 °C to -210 °C
• Distance from the Sun: Varies between 4.3 and 7.4 billion km
• Satellites: 3

Pluto lies beyond Neptune and is so tiny – its diameter is smaller than the width of the Atlantic Ocean - that for years many astronomers questioned its status as a planet.

Finally, in 2006, the International Astronomical Union reclassified Pluto as a ‘dwarf planet’.

Pluto was discovered by the American astronomer Clyde Tombaugh in 1930 after predictions in the late 1800s concluded that another planet’s gravity was affecting the orbits of Uranus and Neptune. In fact, Pluto was not the cause so its discovery was a fortunate accident.

The name of the dwarf planet’s largest moon, Charon, reflects its mythological association with Pluto (Pluto was the Roman god of the underworld and Charon was the ferryman who took the dead across the river Acheron to Hades). Charon was only discovered in 1978 and is almost as half the size of Pluto itself. The other moons are called Nix and Hydra.

Pluto takes 248 years to orbit the Sun and is a member of the Kuiper Belt. This consists of thousands of small icy worlds which orbit beyond Neptune and is also believed to be the source of short period comets. These are comets whose orbits take less than 200 years.

Our knowledge of Pluto is relatively recent – based on observations by the Infrared Astronomical Satellite on Earth and the Hubble Space Telescope. In 2006, NASA launched the New Horizons mission – the first to explore both Pluto and the Kuiper Belt region. New Horizons will reach Pluto and its moons in 2015.