Wednesday, November 26, 2008

The Solar System

The solar system is the stellar system comprising the Sun and the retinue of celestial objects gravitationally bound to it: currently eight official planets (according to the International Astronomical Union) and their 162 known moons,[1] as well as dwarf planets, asteroids, meteoroids, planetoids, comets, and interplanetary dust. From 1930 to 2006 there were nine official planets, but Pluto's status was changed to that of a "dwarf planet" on August 24, 2006 by the IAU.

The principal component of the solar system is the Sun; a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally. Because of its large mass, the Sun has an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation, including visible light. The Sun's two largest orbiting bodies, Jupiter and Saturn, account for more than 90% of the system's remaining mass. (The currently hypothetical Oort cloud, should its existence be confirmed, would also hold a substantial percentage).

In broad terms, the charted regions of the solar system consist of the Sun, four rocky bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas giant planets, and an outer belt of small, icy bodies known as the Kuiper belt. One planet, Pluto, is also a member of the Kuiper belt. In order of their distances from the Sun, the major planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All planets but two are in turn orbited by natural satellites (usually termed "moons" after Earth's Moon) and the largest are encircled by planetary rings of dust and other particles. The planets (with the exception of Earth) are named after gods and goddesses from Greco-Roman mythology.The definition of the term "planet" was decided by the International Astronomical Union, in a vote on August 24, 2006. This new definiton most directly affects Pluto, which is now not considered one of major planetary bodies, leaving our Solar System with eight major planets.

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