Planet

450px-1e7m_comparison_Uranus_Neptune_Sirius_B_Earth_Venus.pngICAL - PRE ASTRO
A planet (from Ancient Greek ἀστὴρ πλανήτης (astēr planētēs), meaning "wandering star ") is an astronomical 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. The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen 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 , Vesta (each an object in the Solar asteroid belt ), and Pluto (the first-discovered trans-Neptunian object ), 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 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 gas giants 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 gas giants, Jupiter , Saturn , Uranus , and Neptune. Six of the planets are orbited by one or more natural satellites.

More than a thousand planets around other stars ("extrasolar planets " or "exoplanets ") have been discovered in the Milky Way: as of 1 May 2014, 1786 known extrasolar planets in 1106 planetary systems (including 460 multiple planetary systems), ranging in size from Earth to gas giants larger than Jupiter. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way. Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone.

MODERN
Union Science does not use the term Extra solar planet to describe a planet outside the Sol System. An Exo Planet is a planet sized object not associated to a solar system. Also called dark planet, wanderer or Non star bound natural object (NSBO ).

Planets are classified in four Rock Core, Gaseous planets (Terran Astronomers keep using the term Jovian ), Garden world and X-Class.
 * Rock panets are sub classified into : Hot, Warm, Temperate, cold and very cold. Condtions such as atmophere, tectonic activity, gravitation and other aspects  further specify the type.
 * Garden Worlds are rock planets that are capable of sustaining a bio sphere based on  Carbon and NiOx breathers.
 * Gaseous planets are also sub classified : Sub Jovian, Jovian , Super Jovian , Failed Sun
 * X-Class: Planet sized objects that can not be classified with any of the other terms. Failed Planets, Destroyed Planets , Proto Planets and organic objects[1]

[1] Currently only one such object is known. Gore II