![]() įurther advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termed exoplanets. The International Astronomical Union (IAU) adopted a standard by which the four terrestrials and four giants qualify, placing Ceres, Pluto and Eris in the category of dwarf planet, although many planetary scientists have continued to apply the term planet more broadly. The discovery of other large objects in the Kuiper belt, particularly Eris, spurred debate about how exactly to define a planet. With the development of the telescope, the meaning of planet broadened to include objects only visible with assistance: the ice giants Uranus and Neptune Ceres and other bodies later recognized to be part of the asteroid belt and Pluto, later found to be the largest member of the collection of icy bodies known as the Kuiper belt. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during the sixteenth and seventeenth centuries. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies. In antiquity, this word referred to the Sun, Moon, and five points of light visible by the naked eye that moved across the background of the stars-namely, Mercury, Venus, Mars, Jupiter and Saturn. ![]() The word planet probably comes from the Greek planḗtai, meaning "wanderers". The giant planets bear planetary rings, the most prominent being those of Saturn. Apart from Venus and Mars, the Solar System planets generate magnetic fields, and all except Venus and Mercury have natural satellites. All of them possess an atmosphere, although that of Mercury is tenuous, and some share such features as ice caps, seasons, volcanism, hurricanes, tectonics, and even hydrology. These planets each rotate around an axis tilted with respect to its orbital pole. The Solar System has at least eight planets: the terrestrial planets Mercury, Venus, Earth and Mars, and the giant planets Jupiter, Saturn, Uranus and Neptune. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Sizes are not to scale.Ī planet is a large, rounded astronomical body that is neither a star nor its remnant. Shown in order from the Sun and in true color. Jupiter and Saturn ( gas giants) Uranus and Neptune ( ice giants) The eight known planets of the Solar System, according to the IAU definition: For other uses, see Planet (disambiguation). This article is about the astronomical object.
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