Planet
This article is about the astronomical object. For other uses, see Planet (disambiguation).
A planet (from Ancient Greek ἀστὴρπλανήτης (astēr planētēs), meaning "wandering star") is an astronomical object orbiting a star or stellar remnantthat
- is massive enough to be rounded by its own gravity,
- is not massive enough to causethermonuclear fusion, and
- has cleared its neighbouring region ofplanetesimals.[a][1][2]
The term planet is ancient, with ties tohistory, science, mythology, andreligion. The planets were originally seen 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 planetswithin 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-discovered trans-Neptunian object), 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 epicyclemotions. Although the idea that theplanets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the firsttelescopic 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 capsand seasons. Since the dawn of theSpace 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 gas giants, 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 gas giants, Jupiter, Saturn, Uranus, andNeptune. 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 2 November 2014, 1849 known extrasolar planets in 1160planetary systems (including 471multiple planetary systems), ranging in size from Earth to gas giants larger than Jupiter.[3] On December 20, 2011, theKepler Space Telescope team reported the discovery of the first Earth-sizedextrasolar planets, Kepler-20e[4] andKepler-20f,[5] orbiting a Sun-like star,Kepler-20.[6][7][8] A 2012 study, analyzing gravitational microlensingdata, estimates an average of at least 1.6 bound planets for every star in the Milky Way.[9] Around one in five Sun-like[b] stars is thought to have an Earth-sized[c] planet in its habitable[d] zone.
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