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 Sun's planets can be subdivided into two main groups. The inner four terrestrial planets have a rocky composition. They are relatively small in size. The outer four planets are known as gas giants, being considerably larger and having thick gaseous atmospheres.
Table of the planets
|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.
Planets and astrology
The orbits of the planets also have a bearing on astrology, believers in which claim that the position of the planets within various constellations of stars 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.
- 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.