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Generally, planets are celestial bodies larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which they revolve. Earth is a part of the solar system that includes planets visible to the naked eye such as: Mercury; Venus; Earth; Mars; Jupiter; and Saturn.


Ancient astronomers used the term "planet" to refer to one of the seven celestial bodies visible to the naked eye thought to revolve in the heavens about a fixed Earth and among fixed stars. Under this definition, Mercury, Venus, the moon, the sun, Mars, Jupiter, and Saturn were thought to be planets.

Once it was discovered that earth revolves around the sun rather than the other way around, the definition of "planet" was changed. The term was redefined to refer to Earth and other large celestial bodies that revolve around the sun. When Clyde Tombaugh discovered Pluto it became widely accepted that Earth's solar system includes nine planets: Mercury; Venus; Earth; Mars; Jupiter; Saturn; Uranus; Neptune; and Pluto.

Recently, it has been proposed that planets are:

  • celestial bodies that orbit a sun of their solar system;
  • have sufficient mass to assume nearly a round shape;
  • clear out dust and debris from the neighborhood around its orbit; and
  • are not a satellite of another planet.

This new definition excludes Pluto as a planet.

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