Planet

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Planet
From Wikipedia, the free encyclopedia
This article is about the astronomical object. For other uses, see Planet (disambiguation).
Planetary-sied objects to scale!
Top ro"! #ranus and $eptune%
second ro"! &arth, the "hite d"arf 'irius (, )enus%
bottom ro" (reproduced and enlarged in lo"er image)*abo+e! ,ars and ,ercury%
belo"! the ,oon, d"arf planets Pluto and -aumea
. planet (from .ncient /reek στὴ

ρ ἀ πλανήτὴς (astēr planētēs), meaning 0"andering
star0) is an astronomical object orbiting a star or stellar remnant that
• is massi+e enough to be rounded by its o"n gra+ity,
• is not massi+e enough to cause thermonuclear fusion, and
• has cleared its neighbouring region of planetesimals.
1a2132142
The term planet is ancient, "ith ties to history, science, mythology, and religion. The
planets "ere originally seen by many early cultures as di+ine, or as emissaries of deities.
.s scientific kno"ledge ad+anced, human perception of the planets changed,
incorporating a number of disparate objects. 5n 4667, the 5nternational .stronomical
#nion (5.#) officially adopted a resolution defining planets "ithin the 'olar 'ystem.
This definition is contro+ersial because it e8cludes many objects of planetary mass based
on "here or "hat they orbit. .lthough eight of the planetary bodies disco+ered before
39:6 remain 0planets0 under the modern definition, some celestial bodies, such as ;eres,
Pallas, <uno, )esta (each an object in the 'olar asteroid belt), and Pluto (the first-
disco+ered trans-$eptunian object), that "ere once considered planets by the scientific
community are no longer +ie"ed as such.

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