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A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.
The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and shared such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. Since 1992, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to observe similar features throughout the Milky Way Galaxy.
Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and 270 extrasolar ones. The Solar System also contains at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.