A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.
The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Apart from Earth itself, five planets in the Solar System are often visible to the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit.
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