Planet

A planet is a large astronomical body that is neither a star nor a stellar remnant. At least eight planets exist in the Solar System: the terrestrial planets Mercury, Venus, Earth and Mars, and the giant planets Jupiter, Saturn, Uranus and Neptune. The word probably comes from the Greek planḗtai, meaning "wanderers", which in antiquity referred to the Sun, Moon, and five bodies visible as points of light that moved across the background of the stars. These five planets were Mercury, Venus, Mars, Jupiter and Saturn. Earth was recognized to be a planet when heliocentrism supplanted geocentrism during the sixteenth and seventeenth centuries. With the development of the telescope, the meaning of "planet" broadened to include objects not visible to the naked eye: 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. The discovery of other large objects in the Kuiper belt, particularly Eris, spurred debate about how exactly to define "planet". The International Astronomical Union adopted a standard by which the four terrestrials and four giants qualify, placing Ceres, Pluto and Eris in the category of dwarf planet, though this standard has not been universally embraced.[1][2][3] Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, or exoplanets. 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. Multiple exoplanets have been found to orbit in the habitable zones of their respective stars, but Earth remains the only planet known to support life.

According to the best available theory, planets form when a nebula collapses to create a protostar and a surrounding protoplanetary disk, in which planets grow by the process of accretion.

The planets of the Solar System, including Earth, each rotate around an axis tilted with respect to its orbital pole, and some share such features as ice caps and seasons. Since the dawn of the Space Age, close observations by space probes have found that Earth and other planets share additional characteristics such as volcanism, hurricanes, tectonics and even hydrology. Apart from Venus and Mars, the Solar System planets generate magnetic fields, and all of them save Venus and Mercury possess natural satellites. In addition, the giant planets bear planetary rings, the most prominent being those of Saturn.

Historically, planets have had religious associations. Multiple cultures identified celestial bodies visible to the naked eye with gods, and these connections with mythology and folklore persist in the schemes for naming newly-discovered Solar System bodies.

The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The consensus definition as to what counts as a planet vs. other objects orbiting the sun has changed several times, previously encompassing asteroids and dwarf planets like Pluto.[4]

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. 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.[5] Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"),[6] from which today's word "planet" was derived.[7][8][9] In ancient Greece, China, Babylon, and indeed all pre-modern civilizations,[10][11] it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day[12] and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.[13]