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OUTER SOLAR SYSTEM

Jupiter, Saturn, Uranus and Neptune are built from mostly gas; their moons are made from a mixture of rock and ice. Together these planets are called the Jovian planets and form the outer solar system. Jupiter has 63 known moons, Saturn has 33 moons, Uranus has 27 moons, Neptune has 13 moons. Four planets have rings: Jupiter, Saturn, Uranus and Neptune - though Saturn's are most clearly visible from Earth. All solar system planets have been visited by spacecraft from Earth.

Pluto was considered a planet from the time of its discovery in 1930 until 24 August 2006 when it was excluded as a planet by the International Astronomical Union (more below).

The size of each outer planet and Pluto compared to Earth and Australia

Jupiter is a big ball of gas, composed of mostly hydrogen and helium. It radiates more energy into space than it receives from the Sun, suggesting the core is about 20,000C. It contains more matter than all of the other planets combined and more than one thousand Earths could fit inside. Orbiting Jupiter is a thin ring and 63 known moons, including the large moons Callisto, Europa, Ganymede and Io. Jupiter is 12 times wider than Earth, making it the largest planet. A large storm on its surface, called the Great Red Spot, has lasted for more than 400 years and is bigger than the Earth.

Saturn is the sixth planet from the Sun and is the second largest in the solar system. Its very fast rotation has flattened the planet at the poles, giving it an oval shaped appearance. Orbiting the planet are 33 known moons. The beautiful system of rings is made of thousands of separate rings. The origin of the rings is not clear; they may have been formed from a moon that was shattered by the impact of a comet or asteroid. They are thought to be composed of pieces of ice ranging in size from a few centimetres to a few metres in size. A space probe called Cassini arrived at Saturn on 1 July 2004 and will spend four years studying Saturn.

Discovered by William Herschel in 1781, it was named after the Roman god of the sky. In 1986 the Voyager 2 spacecraft encountered Uranus and its satellites. It has an atmosphere of mainly hydrogen with 10% helium, beneath which are clouds of methane ice, and further down, clouds of ammonia and water. The planet lies on its side, so its poles face towards the sun. Uranus has 27 moons and is surrounded by a ring.

Neptune was discovered in 1846 and was named after the Roman god of the seas. Its blue colour is probably caused by the red in the Sun's light being selectively absorbed by methane in Neptune's upper atmosphere. Triton is Neptune's largest moon. In August 1989 Voyager 2 flew past Neptune and its moons.

Pluto was discovered in 1930 by Clyde Tombaugh and named after the god of the underworld. It was considered a planet until a meeting of the International Astronomical Union on 24 August 2006, when the definition was changed to state that a planet must:
a) orbit the Sun
b) be in hydrostatic equilibrium, ie it must be big enough for gravity to have pulled it into a near spherical shape
c) dominate its region of space, ie there are no other objects of comparable size crossing its orbit.
This definition excluded Pluto - so Pluto was redefined as a dwarf planet.

Pluto is smaller than the solar system planets - even smaller than Australia. Pluto is made from rock and ice. Every 248 years, Pluto completes one orbit around the Sun. In 1978 a Moon named Charon was discovered orbiting Pluto.

A spacecraft called New Horizon was launched in 2006, and is expected to reach Pluto and Charon in 2015 and proceed beyond, into the Kuiper belt (of which Pluto is now considered to be a part). This will be the first spacecraft to reach Pluto.