A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.

 

The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and some share such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. Since 1992, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to observe similar features throughout the Milky Way Galaxy.

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and 296 known extrasolar ones.[3] The Solar System also contains at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.

 

History:
Main articles: History of astronomy and Definition of planet
See also: Timeline of solar system astronomy

The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. The concept has also now expanded to include worlds not only in our Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

Early printed rendition of a geocentric cosmological model

In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights "πλάνητες ἀστέρες" (planetes asteres: wandering stars) or simply "πλανήτοι" (planētoi: wanderers),[4] from which the today's word "planet" was derived.[5][6]

In ancient Greece as well as in ancient China, ancient Babylon and indeed all pre-modern civilisations,[7][8] it was almost universally believed that Earth was in the centre of the Universe and that all the "planets" circled the Earth. The reasons for this perception was that stars and planets appeared to revolve around the Earth each day,[9] and the apparently common sense perception that the Earth was solid and stable and that it is not moving but at rest.

The Greek cosmological system was taken from that of the Babylonians,[10] a contemporary Mesopotamian civilisation from whom they began to acquire astronomical learning from around 600 BC, including the constellations and the zodiac.[11] In the 6th century BC, the Babylonians had a highly advanced level of astronomical knowledge, and had a theory of the planets centuries before the ancient Greeks. The oldest planetary astronomical text that we possess 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.[12] The Babylonians also laid the foundations of what would eventually become Western astrology.[13] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[14] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[15] Sumerians, predecessors of Babylonians which are credited as one of the first civilizations and the inventors of writing, had identified at least Venus by 1500 BC.[10] Conversely, there is no evidence of a comparable knowledge of the planets in the earliest written Greek sources, such as the Iliad and the Odyssey.[13]

By the first century BC, 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 much of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century AD. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.[16][12]

To the Greeks and Romans there were seven known planets; each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.