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Although your preconceived idea may be that a planet is something quite easy to define, that is not the case, as the recent and ongoing dispute has shown. Throughout the centuries, the definition of planet has meant many things to different people. The Greeks thought of them as stars that moved, which was where the term planet came from, and at some point in time, the term has referred to the Sun, Moon, satellites, and asteroids. If the definition of a planet still included asteroids, there would be over 100,000 planets. Over time, the Sun, Moon, and asteroids lost the title of planets. When Uranus when discovered in the 1700’s, Neptune was found in the 1800’s, and Pluto was sighted in the 20th century, they were all added to the class of planets.
Although dictionaries have defined planets, those definitions vary and the International Astronomical Union (IAU) – which takes responsibility for mediating these kinds of debates – never had. Instead, they – and many astronomers – avoided the issue.
Avoidance became extremely difficult in 2005 when a celestial object larger than Pluto is was discovered in the scattered disk, which is a region of space beyond the Kuiper Belt. This object, which later was termed Eris, was called a planet by some and simply a large asteroid by others. The IAU stepped in to settle the issue, but it took them until a conference in summer of 2006 to decide on a classification system. Their official IAU decision was that a planet was an object that was large enough to force itself into a roughly spherical shape, that orbits the Sun, and that cleared its neighborhood. An object that has cleared its neighborhood has enough gravity to force other objects of similar size, and those that are not under its gravitational influence, out of its orbit.
In addition to the now eight planets in our Solar System, there are at least hundreds of other planets in other Solar Systems, also called extrasolar planets or exoplanets. In addition to planets, there are also numerous other celestial bodies, such as minor planets. Because many celestial objects fall into more than one of these categories, taxonomy becomes even more difficult. Many people, including famous astronomers, are not happy with the IAU’s definition of a planet and continue to demand that the definition changes. While the IAU might still modify the definition of a planet, it is not going to be a simple decision.
Astronomy Cast has an episode on Pluto’s planetary identity crisis.