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.The five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy.
Several thousands of planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way.
As of 1 November 2017, 3,693 known extrasolar planets in 2,768 planetary systems (including 620 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. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age.
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.
By the 1st century BC, during the Hellenistic period, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets.
These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye.
The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC.
Venus, Mercury, and the outer planets Mars, Jupiter, and Saturn were all identified by Babylonian astronomers.
it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.