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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.
To the Greeks and Romans there were seven known planets, each presumed to be circling 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.
Several thousands of planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way.
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.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 (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), 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.