The concept of Planet comes from the latin planēta, which, in turn, derives from a Greek word meaning "wandering". This is a solid celestial body that revolves around a star and becomes visible to reflected light.
The specifications made by the International Astronomical Union indicate that the planets have enough mass to allow their gravity to overcome rigid body forces, this after which they assume a hydrostatic equilibrium shape.
Furthermore, the planets are bodies having cleaned the surroundings of their orbits of planetesimals, which are solid objects contained in Protoplanetary disks.
Nowadays, considering that the solar system consists of eight planets: Earth, Mars, Jupiter, Saturn, mercury, Venus, Uranus and Neptune. Until 2006, Pluto was also considered a planet, then it has finally been classified as belonging to dwarf planets (insofar as it is not a satellite of a planet, but it has not cleaned the neighbourhood of its orbit), the image of Ceres and Eris.
Depending on their structure, the planets can be either terrestrial or telluric planets, Jovian planets. The first are small, have a solid, Rocky, surface and a high density (such as Earth and Mars). On the other hand, the second are essentially soft, their density is low and their diameter is rather big (it is the case of Jupiter and Saturn).
Relating to their movements, the planets may be lower (those who depart not too much sun and may not be in opposition) or above (their angle of elongation is unlimited and they may be in opposition).