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There are hundreds of thousands of objects orbiting the Sun, yet only nine of them are considered to be planets'. The nine (known) planets are extremely diverse in terms of their size, number of satellites, presence of an atmosphere, and other properties, so what distinguishes these bodies from all of the other objects orbiting the sun? A PLANET IS: any large, spherical, natural object which directly orbits a star, and does not generate heat by nuclear fusion. This definition is simple, is based on a physical definition of how large an object must be to be considered a planet, and without modification it will give the same results when applied to any planetary system. This definition distinguishes planets from asteroids and comets, which directly orbit our star, but are generally not large enough to be pulled into a spherical shape by their own gravity, and it distinguishes planets from stars, which frequently are in direct orbit around other stars, but generate heat internally by nuclear fusion. According to this definition, Pluto is clearly a planet. Some astronomers argue that Pluto is not a planet. In particular, the views of 3 astronomers been presented extensively by the news media and in popular literature in recent years. However, they have yet to offer a simple, concise, generally applicable definition of what a planet is - they simply offer a list of comparisons between some of Pluto's properties and some of the properties of some of the other objects in the solar system. Such a relativistic definition of what qualities define a planet is cumbersome, arbitrary, and scientifically unsatisfying.
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Do you think Pluto should stay as a planet? The controvery about what is and is not a planet in our solar system is in its home stretch. |
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