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Terrestrial planets
The planets closest to the sun -- Mercury, Venus, Earth and Mars -- have rocky surfaces and are called terrestrial planets. Mercury is closest to the sun, followed by Venus, Earth and Mars. Unlike all the other planets, Mercury does not have an atmosphere. It is also the smallest planet with a diameter of 3,100 miles, less than half that of Earth at 8,100 miles and Venus 7,500 miles. Mars' diameter is 4,200 miles.
Gas giants
Jupiter and Saturn are called gas giants. They do not have a solid surface and are primarily composed of hydrogen and helium. Until recently, astronomers thought that only Saturn had rings, but now they have observed rings around Jupiter too, as well as Uranus and Neptune. The gas giants are much bigger than the terrestrial planets. Jupiter is the largest with a diameter of 88,846 miles. Saturn's diameter is 74,897 miles.
Ice giants
The planets farthest from the sun are Neptune and Uranus. Both of these planets have an atmosphere that contains methane, giving them a blue color. Neptune's color is a more intense blue, due to an as yet undiscovered component. The majority of Uranus' atmosphere is helium and hydrogen. These planets are smaller than the gas giants, but still larger than the terrestrial planets. Uranus' diameter is 31,763 miles and Neptune's measures 30,775 miles.
Pluto
Astronomers once included Pluto as one of the planets of the solar system. Then the International Astronomical Union redefined the meaning of the word "planet" and now Pluto no longer qualifies. It is now known as a dwarf planet. There are two other known dwarf planets in our solar system: Ceres and 2003 UB 313. Astronomers expect to discover more than 100 additional dwarf planets.
Other bodies
Our solar system also contains moons, asteroids and comets. Moons orbit planets. They are also known as satellites. There are approximately 140 known satellites and there there are also thousands of asteroids and most of them inhabit the area between the orbits of Mars and Jupiter. Furthest out in our solar system is an area called the Oort cloud and this is where comets exist. Comets develop an atmosphere as they travel closer to the sun, then lose it as their orbit takes them away again.