A planet (from Ancient Greek ἀστήρ πλανήτης astēr planētēs, or πλάνης ἀστήρ plánēs astēr, meaning "wandering star")[1] is an astronomical object orbiting a star or stellar remnant that
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 first telescopic 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 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 2 May 2016, 2111 known extrasolar planets in 1354 planetary systems (including 510 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, the Kepler 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.
- 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 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 first telescopic 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 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 2 May 2016, 2111 known extrasolar planets in 1354 planetary systems (including 510 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, the Kepler 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.