The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. Ancient Greeks called these lights πλάνητες ἀστέρες ( planētes asteres, "wandering stars") or simply πλανῆται ( planētai, "wanderers"), from which today's word "planet" was derived. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the " fixed stars", which maintained a constant relative position in the sky. The five classical planets of the Solar System, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. other objects orbiting the Sun has changed several times, previously encompassing asteroids, moons, and dwarf planets like Pluto, and there continues to be some disagreement today. The consensus definition as to what counts as a planet vs. The concept has expanded to include worlds not only in the Solar System, but in multitudes of other extrasolar systems. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. Multiple exoplanets have been found to orbit in the habitable zones of their stars, but Earth remains the only planet known to support life.ġ660 illustration of Claudius Ptolemy's geocentric model These include hot Jupiters-giant planets that orbit close to their parent stars-like 51 Pegasi b, super-Earths like Gliese 581c that have masses in between that of Earth and Neptune and planets smaller than Earth, like Kepler-20e. 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 16th and 17th 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 planets of the Solar System other than Mercury possess a considerable atmosphere, 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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