According to the International Astronomical Union (IAU), a planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity (but not massive enough to cause thermonuclear fusion in its inner core), and has cleared its neighbouring region of planetesimals.   There are eight bodies in the Solar System that meet this definition: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Bodies that orbit the Sun and are rounded by their own gravity, but have not cleared the neighbourhood, are called "dwarf planets." One of them is Pluto, which was formerly considered a planet. Other bodies orbiting the Sun are generically called "small bodies of the Solar System."
The planets of the Solar System
The eight planets orbiting around the sun can be subdivided into three main groups. The inner four, called terrestrial planets, have a rocky composition and are relatively small in size. The fifth and sixth planets (Jupiter and Saturn) are gas giants, and the outermost two (Uranus and Neptune) are ice giants, being considerably larger in mass but less dense than the terrestrial planets. At one time, the four outer planets were all considered "gas giants". The planets are listed here from nearest to farthest in distance from the sun.
|Name||Distance from the sun||Diameter||Mass||Date of Discovery|
|Mercury||57,910,000 km (0.38 AU)||4,880 km||3.30e23 kg||known in antiquity|
|Venus||108,200,000 km (0.72 AU)||12,103.6 km||4.869e24 kg||known in antiquity|
|Earth||149,600,000 km (1.00 AU)||12,756.3 km||5.972e24 kg|
|Mars||227,940,000 km (1.52 AU)||6,794 km||6.4219e23 kg||known in antiquity|
|Jupiter||778,330,000 km (5.20 AU)||142,984 km||1.900e27 kg||known in antiquity|
|Saturn||1,429,400,000 km (9.54 AU)||120,536 km (equatorial)||5.68e26 kg||known in antiquity|
|Uranus||2,870,990,000 km (19.218 AU)||51,118 km (equatorial)||8.683e25 kg||1781|
|Neptune||4,504,000,000 km (30.06 AU)||49,532 km||1.0247e26 kg||1846|
Etymology and history
The word "planet" came from the greek πλανήτ- (planēt-), meaning "wanderers." Before the invention of the telescope, there were seven planets visible in the sky; the ancient Greeks considered each one to be dedicated to a deity:
- Phainon, "the shiner," dedicated to Cronos;
- Phaethon, also meaning "shining," brighter than the first, dedicated to Zeus;
- Pyroeis, "fiery", the red one, dedicated to Ares;
- Phosphoros, "light bringer", the brightest one, dedicated to Aphrodite;
- Stilbon, the "gleamer," dedicated to Hermes; 
as well as the Sun and Earth's Moon, the biggest wanderers in the sky. Philosophers in antiquity created many models of the universe, of which the best-known is Aristotle's. In his system, each planet belongs to one crystalline sphere, concentric with the others. Inner the seven planets' spheres are the material spheres (grosso modo, the Earth); outside, the "sphere of fixed stars."
In the Roman Empire, the planets became known directly by the name of the associated deity - not the Greek one, but the corresponding Latin god. So, Phainon became Saturnus; Phaethon, Jupiter; Pyroeis, Mars; Phosphoros, Venus; and Stilbon, Mercurius. These Latinized names remain current in European languages. Also in the Latin tradition, as well as the Germanic, the seven days of the week have the names of the seven planets.
After the Copernican Revolution, however, the definition of a planet changed: they are all the bodies that directly orbit the Sun; the Earth thus turns into a planet, the Moon becomes Earth's satellite instead of a planet, and the so-called "Solar System" then has six planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn). Later, new bodies, unseen by the naked eye, were baptized as planets, following the Latin-name tradition: Uranus was added in 1781, though not named till later; the first four minor planets to be discovered (Ceres, Pallas, Juno and Vesta) were originally recognized by most authorities as planets; Neptune was discovered in 1846 and numerous additional minor planets starting about the same time; some authorities originally added all these to the list of planets, but this became less popular as more were discovered, and the official numbering of minor planets, started in 1852, began with Ceres; as a result, most authorities from that time on excluded minor planets from the listing of planets, leaving only 8; Pluto was added in 1930.
But many other small bodies orbit the Sun, such as the bright comets and the pallid asteroids. The crisis in definition started in the 1980's and 1990's, when many bodies as big as -- or, in a few cases, bigger than -- Pluto started to be discovered beyond Neptune's orbit. Called Transneptunian objects, they sparked a discussion about what could be a good and precise definition of "planet." The discussion ended on August 24, 2006, when the IAU approved a resolution that officially defines the term. 
Planets are believed to form from the disk of dust that surrounds a newly formed star. Gravitational forces pull the dust into lumps. The lumps gradually collide and join to make larger lumps or asteroids. These join further to make larger and larger planets until the planet has cleared its orbit of objects.
Within the solar system, three classes of planets, based on material composition, are recognized.
- The terrestrial planets: Mercury, Venus, Earth, and Mars. Rocky, relatively dense planets with densities ranging between 3.9 and 5.5 g/cc.
- The gas giants: Jupiter and Saturn, with densities of 1.3 and 0.69 g/cc, respectively. Composed mainly of the gases hydrogen and helium.
- The ice giants: Uranus and Neptune, with densities of 1.3 and 1.6 g/cc, respectively. Composed mainly of water, methane, and ammonia, with a small rocky core and an atmosphere of mostly hydrogen and helium.
At one time, Uranus and Neptune were classified as gas giants along with Jupiter and Saturn. Once it became clear that they were compositionally different, they were given a new classification as ice giants.
Planets and culture
Believers in western astrology claim that the position of the planets within various sections of the ecliptic can be an indicator of events on Earth. By carefully measuring the orbits and positions of the planets and calculating their future paths, it is believed, one can predict future events. The scientific community, however, almost universally considers astrology to be a pseudoscience or superstition, and there is no widely accepted scientific evidence for its validity.
In Chinese astrology, the five visible planets are associated with the five Chinese "elements" as follows:
- Mercury is associated with water,
- Venus with metal,
- Mars with fire,
- Jupiter with wood, and
- Saturn with earth.
In science fiction, visitors to (or invaders of) Earth typically originate from a different planet. The planet Mars was often used as an the alien home world in early science fiction stories. Notable examples include the 1898 H. G. Wells novel, The War of the Worlds, the 1961 Robert A. Heinlein novel Stranger in a Strange Land, and the 1960s American television show My Favorite Martian.
In the late 1800s, the professional astronomer Percival Lowell put forth the idea that intelligent beings lived on Mars, as evidenced by canals that he believed he had observed through a telescope, though it turned out that what he saw were not actual canals. Nonetheless, the idea of intelligent life on Mars was persistent enough that a 1938 radio adaptation of The War of the Worlds, by Orson Welles, caused significant panic among many radio listeners who tuned in to the program and thought that Earth really was under attack by Martians. Eventually, as public perception of intelligent life on Mars became a less and less plausible basis for fictional stories, it became more common for fiction writers to use non-existent planets from outside the solar system as the home worlds of alien visitors. Examples include:
- An unnamed planet as the home world of the invaders in the 1955 Jack Finney novel The Body Snatchers as well as in several movie adaptations of Finney's book.
- Ork, the home world of Mork in the 1970s American TV show Mork and Mindy
- Remulak, the home world of the Coneheads in a series of recurring sketches on the American TV show Saturday Night Live in the 1970s, and later in the 1993 movie Coneheads
- Vogsphere, the home world of the Vogons, the aliens who destroy Earth (also known for writing the most horrible poetry) in the 1979 Douglas Adams novel The Hitchhiker's Guide to the Galaxy
- An unnamed planet as the home world of E.T. in the 1982 movie E.T. the Extra-Terrestrial
- Melmac, the home world of ALF in the 1980s American TV show ALF
- Vulcan on Star Trek. While not the only planet hosting intelligent life in the Star Trek universe, it is the one from which the first visitors to Earth came, as depicted in the 1996 film Star Trek: First Contact.
- An unnamed planet as the home world of the main characters in the American TV show 3rd Rock from the Sun, which ran from 1996 to 2001.
The chemical elements uranium, neptunium, and plutonium are named after the planets Uranus and Neptune, and the dwarf planet Pluto. It is unclear whether the element mercury is named after the planet or the Roman god of the same name.
A 1975 record album by Paul McCartney and Wings is titled Venus and Mars, as is one of the songs on the album.
The 1992 book by John Gray, Men Are from Mars, Women Are from Venus: The Classic Guide to Understanding the Opposite Sex, was written as a guide to help heterosexual couples better understand the differences in how people of the opposite sex (typically) think and communicate.
- IAU 2006 General Assembly: Result of the IAU Resolution votes (2006). Retrieved on 2007-04-30.
- Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union. IAU (2001). Retrieved on 2006-05-25.
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- James Evans (1998). The History and Practice of Ancient Astronomy. Oxford University Press, 296-7.