The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several of them in the Solar System can be seen with the naked eye.
What are planets?
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 them within the Solar System.There are eight planets in the Solar System, which are in increasing distance from the Sun: Mercury, Venus, Earth, Marsn Jupiter, Saturn, Uranus and Neptune.
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 Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such.
They 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.
Several thousands of planets around other stars (“extrasolar planets” or “exoplanets”) have been discovered in the Milky Way. As of 1 August 2017, 3,639 known extrasolar planets in 2,729 planetary systems (including 612 multiple planetary systems), ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the habitable zone.
It is not known with certainty how they are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimalsform, and these accelerate the accretion process by drawing in additional material by their gravitational attraction.
See also: Uranus, the seventh planet from the Sun
Image: By NASA/JPL-Caltech, Public Domain
Text: Wikipedia contributors. “Planet.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 31 Jul. 2017. Web. 9 Aug. 2017.