PLANETS

A planet (from Ancient Greek ἀστήρ πλανήτης astēr planētēs, or πλάνης ἀστήρ plánēs astēr, meaning "wandering star")^[1] is anastronomical object orbiting a star 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.^[a][2][3]

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 andVesta (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 firsttelescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Keplerfound 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 two thousand planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way: as of 20 January 2016, 2049 known extrasolar planets in 1297 planetary systems (including 507 multiple planetary systems), ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter.^[4] On December 20, 2011, theKepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e^[5] and Kepler-20f,^[6]orbiting a Sun-like star, Kepler-20.^[7][8][9] A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.^[10] Around one in five Sun-like^[b] stars is thought to have an Earth-sized^[c] planet in its habitable^[d] zone.