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The eight planets of the Solar System
Mercury, Venus, Earth, and Mars
Jupiter and Saturn (gas giants)
Uranus and Neptune (ice giants)

Shown in order from the Sun and in true color. Sizes are not to scale.

A planet (from grc ἀστήρ πλανήτης (astēr planētēs), or πλάνης ἀστήρ (plánēs astēr), meaning "wandering star")<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is an astronomical object orbiting a star, brown dwarf, or stellar remnant that

  • is massive enough to be rounded by its own gravity,
  • is not massive enough to cause thermonuclear fusion, and
  • has cleared its neighbouring region of planetesimals.<ref group="lower-alpha" name="footnoteA">This definition is drawn from two separate IAU declarations; a formal definition agreed by the IAU in 2006, and an informal working definition established by the IAU in 2001/2003 for objects outside of the Solar System. The official 2006 definition applies only to the Solar System, whereas the 2003 definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference.</ref><ref name="IAU">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref><ref name="WSGESP">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The term planet is ancient, with ties to history, science, mythology, and religion. Several planets in the Solar System can be seen with 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 (IAU) 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. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno, Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community are no longer viewed as such.

The planets were thought by Ptolemy to orbit Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.

Planets are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites.

More than a thousand planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way: as of , known extrasolar planets in planetary systems (including multiple planetary systems), ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter.<ref name="Encyclopaedia">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e<ref name="Kepler20e-20111220">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and Kepler-20f,<ref name="Kepler20f-20111220">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> orbiting a Sun-like star, Kepler-20.<ref name="NASA-20111220">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Nature-20111220">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name="NYT-20111220">{{#invoke:citation/CS1|citation |CitationClass=news }}</ref> A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.<ref name="">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Around one in five Sun-like<ref group=lower-alpha name=1in5sunlike/> stars is thought to have an Earth-sized<ref group=lower-alpha name=1in5earthsized/> planet in its habitable<ref group=lower-alpha name=1in5habitable/> zone.

Planet sections
Intro   History    Mythology and naming    Formation    Solar System    Exoplanets    Planetary-mass objects    See also    Notes    References    External links   

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