PLANET

The International Astronomical Union, in his plenary assembly celebrated in Prague in August, 2006, established a definition of the term planet, at least as for the Solar system. This way, a planet is a celestial body that: (a) it orbits about the Sun; (b) it possesses enough mass as so that its own gravity dominates the present forces like rigid body, what implies a form approximately rounded determined by the balance hidrostático; (c) is the clearly domineering object in his neighborhood, having cleaned his orbit of bodies similar to him. According to this definition, Pluto stops being a planet, to happen to be the prototype of a new type of objects: the dwarf planets. Inside the category of dwarf planets are Pluto, Ceres and Eris. Therefore, the Solar system remains with eight planets: Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In our solar system there are two types of planets, gaseous and rocky, and of two types planets have been discovered orbiting other stars different from the Sun.

Saturn observed by the mission Cassini-Hyugens, with the secret Sun for the eclipse due to the disc of the planet. Although a certain material quantity exists about this gaseous giant in the shape of numerous satellites and multiple rings, Saturn dominates the whole set. Credits: Cassini Imaging Team, SSI, JPL, THAT ONE, NASA.

The gaseous planets are those constituted principally by gases, in particular hydrogen and helium. In our Solar system they belong to this category Jupiter, Saturn, Uranus and Neptune, although in the above mentioned the ice is a substantial component in his composition. The gaseous planets, depending on his formation mechanisms, do not have why to possess a rocky solid nucleus, but they can consist of a gas continuous one gradually denser that they acquire finally the properties of a fluid when it is to high pressure. In case of Jupiter and Saturn, the gaseous hydrogen in the molecular state passes the state known as «metallic hydrogen» with a few particular properties. Immense most of the extrasolar planets discovered up to the date are gaseous planets in consequence, at least partly, to that the current detection methods discriminate better against planets of major mass.
The rocky planets, also been called telluric, are the planets formed principally by silicates, in which the ambience is secondary and is influenced by the geologic activity and, in case of the Earth, by the biological activity. In the Solar system four rocky planets exist: Mercury, Venus, the Earth and Mars.

Glossary: "100 basic concepts of Astronomy”