Monday, January 14, 2008


The Solar System is the Sun and all the objects in orbit around it. The objects orbiting the Sun are the 8 planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, three dwarf planets (Pluto, Ceres and Eris), the planets’ 170 moons, and billions of smaller bodies, including asteroids, Kuiper belt objects, comets, meteoroids, and interplanetary dust.

The four planets closest to the sun—Mercury, Venus, Earth, and Mars—are called the terrestrial planets because they have solid rocky surfaces. The four large planets beyond the orbit of Mars—Jupiter, Saturn, Uranus, and Neptune—are called gas giants.

From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. But in the late 20th and early 21st centuries, many objects similar to Pluto were discovered in the outer Solar System, most notably Eris (formerly 2003 UB313), which is slightly larger than Pluto. On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a dwarf planet. According to the new rules a planet meets three criteria: it must orbit the Sun, it must be big enough for gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. Apart from Pluto, Ceres, the largest object in the asteroid belt; and Eris, which lies in the scattered disc were named in the new category of dwarf planet.

99.86% of the Solar System total mass belongs to the Sun. Of the 0.14% total mass left, more than 90% belong to the two largest planets, Jupiter and Saturn.


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