Pluto

Pluto

Questions About Pluto

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  • Pluto was first seen by use of a telescope in 1930.  Like Uranus and Neptune, Pluto can not be seen by the naked eye and its existence was not known to the ancient world.

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  • In 1930 the discovery of Pluto by Clyde Tombaugh in the Lowell Observatory was heralded as the discovery of the “ninth planet”.
  • Even in 1930, however, many astronomers did not agree that a ninth planet had been discovered.

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  • In 2005 another possible planet, Eris, was found beyond Neptune, the eighth planet and since then 2 further small planet-like bodies have been discovered.

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  • These bodies are unlike the other planets in the Solar system.  The first eight planets orbit the Sun in a path which they have cleared of other objects.  Pluto, however, orbits the sun in a zone which is full of other objects that often pass between Pluto and the Sun.

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  • The first eight planets orbit the Sun while keeping the same distance from the Sun.  The Earth is always 93 million miles, one Astronomical Unit, from the Sun.  Pluto, however, orbits in an ellipse, an oval shape which means its distance from the Sun varies.  See figure 2.

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  • In 2006 the International Astronomical Union agreed that these “planets” should be called “dwarf planets”.

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  • Pluto was the Roman God of the Underworld.

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  • Of the nine planets which most people think of as being in the Solar system, Pluto is the 2nd smallest, only just bigger than Mercury.

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  • We know very little about Pluto.  The US National Aeronautics and Space Administration  (NASA) has sent the unmanned spacecraft New Horizons to Pluto.   New Horizons left the Earth in 2005 but will not reach Pluto until 2015.

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  • Pluto is probably even smaller than Mercury, with a diameter of around 1400 miles (about 2240 kilometres).

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  • It takes Pluto 248½ years to complete its orbit round the Sun.  The Earth takes one year.

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  • Like all the planets Pluto turns on its own axis as it orbits round the sun.  Pluto takes about 6½ days to turn on its axis.  The Earth takes 24 hours, one day.

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Figure 1: A NASA photograph of Pluto and its four moons.  Charon is the closest.

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  • Since Pluto was the Roman God of the Underworld, the planet’s main moon, Charon, is named after the ferryman who carries the dead souls across the River Styx into the Underworld.  Pluto is known to have four moons, shown in Figure 2.

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  • Pluto’s distance from the Sun varies.  Since the planet was only discovered in 1930 and it takes 249 years to orbit the Sun, a full orbit has not been observed.

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  • From calculations astronomers have worked out that Pluto’s orbit round the sun is not regular.

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  • The orbit is tilted when compared to the orbits of the other eight planets, as shown in Figure 3.

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Figure 2: The Planets’ Orbits round the Sun, showing Pluto’s unusual orbit.

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  • In this diagram, produced by the BBC, you can see that at some points in Pluto’s orbit the planet will be closer to the Sun than Uranus and Neptune, the seventh and eighth planets.

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  • Between 1979 and 1999 Pluto was closer to the Sun than the planet Neptune, moving inside Neptune’s orbit.

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  • Distances in astronomy are measured in Astronomical Units, with the Earth’s distance from the Sun being taken as one Astronomical Unit  (1AU).

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  • The photograph in Figure 2 was taken in 1994 by the Hubble Space Telescope.  Pluto was about 30 AUs from the Sun, approximately 2796 million miles, or 4500 million kilometres.

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  • However, during its long 248 year orbit round the Sun, Pluto will move as far as 49 AUs from the Sun – the position which is shown in Figure 2.

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