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.
Six of the planets are orbited by one or more natural satellites.
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.
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 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.
They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.
In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars.