Claim: Venus is a celestial object.

A planet is an astronomical body 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.This definition is drawn from two separate IAU declarations ; a formal definition agreed by the IAU in 2006 , and an informal working definition established by the IAU in 2001/2003 for objects outside of the Solar System . The official 2006 definition applies only to the Solar System , whereas the 2003 definition applies to planets around other stars . The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference .  The term planet is ancient , with ties to history , astrology , science , mythology , and religion . Several planets in the Solar System can be seen with the naked eye . 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 -LRB- IAU -RRB- 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 and Vesta -LRB- each an object in the solar asteroid belt -RRB- , and Pluto -LRB- the first trans-Neptunian object discovered -RRB- , 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 . At about the same time , by careful analysis of pre-telescopic observation data collected by Tycho Brahe , 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 .   Several thousands of planets around other stars -LRB- `` extrasolar planets '' or `` exoplanets '' -RRB- have been discovered in the Milky Way . As of , known extrasolar planets in planetary systems -LRB- including multiple planetary systems -RRB- , 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 . 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 .