The Solar System is the astronomical name for the Sun and the collection of planets, comets and other objects moving around it.
The Earth moves around the Sun in an oval shaped, or "elliptical" orbit. The other planets in the Solar System also move around the Sun in elliptical orbits.
The biggest objects that go around the Sun are called planets. It took people many years of looking carefully through telescopes to find them. No new big ones have been found for a long time, but more small objects are found every year. Most of the planets have moons that orbit around them. There are 166 of these moons in the solar system.
These are the eight planets, five dwarf planets and other large objects moving around the Sun in order from closest to the sun to farthest away from the sun (numbers sign to the picture on the right):
The planets are numbered (1) to (8) in order of distance from the sun and dwarf planets are letter (A) to (E).
There are a few main parts of the Solar System. The terrestrial inner planets (Mercury, Venus, Earth and Mars) are the four closest planets to the sun and all have rocky surface. Between Mars and Jupiter is the asteroid belt and after that are the Gas Giants, (Jupiter, Saturn, Uranus and Neptune). They are all bigger than Earth and are mostly made up of gas. This group are the four biggest planets in the Solar System.
After them is another, larger asteroid belt called the Kuiper belt. This contains many comets and asteroids as well as the dwarf planets Pluto, Haumea, Makemake, and Eris. After this scientists think there may be an even bigger asteroid belt called the Oort cloud although since it is hard to see they cant be sure.
Pluto has been called a planet since it was discovered in 1930. In 2006, astronomers decided Pluto should no longer be called a planet because it is too small. Also, its orbit is very different from the other eight planets its orbit slants a little. It is now called a dwarf planet along with four others Ceres, Haumea, Eris and Makemake.
Before the discovery of Uranus, ancient thoughts of the solar system consisted only of the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn as much as ancient records go.
Long ago, ancient scientists thought the Sun (and the whole solar system) orbited the Earth and that Venus was a star and not a planet.
Uranus, discovered in 1781
Ceres, discovered in 1801, recently (2006) defined as a dwarf planet
Neptune, discovered in 1846
Pluto, discovered in 1930
Eris, discovered in 2005
Solar System & Planets
Pluto Redefinition as a Dwarf Planet
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)