The Solar System as you've never seen it before

For decades we've believed that nine planets orbit our Sun. Now, with the discovery of 'Xena', all that has changed. Steve Connor presents a guide to our cosmic neighbours

VENUS

Orbit 108,200,000km from the Sun

Diameter 12,104km

Discovered Prehistoric times

The Greeks called it Aphrodite, the Babylonians Ishtar - the goddess of love. It is the brightest of the planets and the third most luminous object after the Sun and the Moon. It is sometimes referred to as the sister planet of Earth - they are similar in size and mass but its environment is one of the least hospitable for life. Any water boiled away long ago thanks to a runaway greenhouse effect. Temperatures can soar to 500C - hot enough to melt lead.

MARS

Orbit 227,940,00km from the Sun

Diameter 6,794km

Discovered Prehistoric times

Named after the Roman god of war, probably because of its red colour - it is sometimes referred to as the Red Planet. Early astonomers thought that Mars's system of "canals" indicated life. Mariner 4 in 1965 was the first spacecraft to visit Mars, which is currently being scrutinised by several European and American space probes. If these investigations indicate that liquid water existed there in the past, it may also have once harboured life.

SATURN

Orbit 1,429,400,000km from the Sun

Diameter 120,536km

Discovered Prehistoric times

Galileo was the first person to observe Saturn's distinctive rings with a telescope in 1610. The origin of the rings is still a mystery, but astronomers believe that they are constantly being regenerated, possibly by the breakup of larger rocks in the solar system. Although the rings look continuous, they are made of innumerable small particles each in independent orbit, ranging in size from a centimetre to several metres. Saturn has 34 named satellites or moons.

URANUS

Orbit 2,870,990,000km from the Sun

Diameter 51,118km

Discovered 13 March 1781

The astronomer William Herschel discovered Uranus while systematically searching the sky with his telescope, but thought it was just another star or possibly a comet. The blue colour of the planet is because its methane-rich atmosphere absorbs red light. Like other gas planets, Uranus has bands of cloud that blow around producing streaks. The planet is composed of rocks and frozen gases that are distributed throughout the body - it has no rocky core.

NEPTUNE

Orbit 4,504,000,000km from the Sun

Diameter 49,532km

Discovered 23 September 1846

The existence of Neptune was predicted after it was noticed that the orbit of Uranus (discovered 65 years earlier) was not as it should be - it was being perturbed by another object. Neptune's most prominent feature was the Great Dark Spot in the southern hemisphere. The spot vanished after it was first spotted by the Voyager craft in 1989, only to reappear in the northern hemisphere, showing the dynamic nature of the planet's icy atmosphere.

PLUTO

Orbit 5,913,520,000km from the Sun

Diameter 2,274km

Discovered 1930

The farthest planet from the Sun - until last weekend - is also the smallest. Pluto is even smaller than seven of the solar system's planetary moons, including our own. In fact, it is so small that some astronomers believe it should be reclassified as an asteroid rather than a planet. Pluto is the only planet not to have been visited by a spacecraft. Its exact composition is unknown, but it is likely to be 70 per cent rock and 30 per cent water ice.

MERCURY

Orbit 57,910,000km from the Sun

Diameter 4,880km

Discovered Sumerians, 3rd millennium BC

The Greeks gave the planet two names - Hermes as an evening star and Apollo as a morning apparition - although they knew that it was the same object. The temperatures on Mercury, the closest planet to the Sun, are the most extreme to be found anywhere in the solar system, ranging from minus 157C to 527C. The planet's thin atmosphere consists of atoms blasted from its surface by the intense solar wind.

EARTH

Orbit 149,600,000km from the Sun

Diameter 12,756km

Discovered Only understood to be a planet in the 16th century, thanks to Copernicus. Photographs from space earned it the sobriquet "pale blue dot". There are hundreds of names for our home planet, such as Tellus, the Roman earth goddess, and Gaia, the Greek equivalent. Earth is a classic rocky planet, with a core composed mostly of molten iron. About two-thirds of its surface is covered by water, and its temperature and atmosphere are perfect for life.

JUPITER

Orbit 778,330,000km from the Sun

Diameter 142,984km

Discovered Prehistoric times

A massive planet, more than twice the mass of all the other planets combined and some 318 times the mass of Earth. Named after the Roman king of the gods - Zeus in Greek - Jupiter is the fourth-brightest object in the sky. Technically, it is a "gas giant" - a planet with a dense atmosphere that forms around a rocky core. Jupiter's main feature is its Great Red Spot, a swirling, gaseous vortex close to its equator.

2003 UB313 "XENA"

Orbit c16,093,000,000km from the Sun

Diameter c2,300km

Discovered 29 July 2005

Astronomers from the US discovered the new planet after studying telescopic images of the entire northern sky. The planet is slightly bigger than Pluto and three times further away. It is the largest object detected in the solar system since the discovery of Neptune and its moon Triton in 1846. Astonomers have designated the new planet by the code 2003 UB313, and it has yet to be officially named, although unofficially is is being called "Xena" after the warrior princess in the television series. Its wide, ellipical orbit carries it to the distant reaches of the solar system, some 100 times further from the Sun than the Earth. It orbits the Sun once every 560 years. In 280 years it will be at its closest point to Earth, 38 times the distance between us and the Sun. Whether "Xena" will continue to be classed as a true planet awaits further deliberations by the International Astronomical Union, the official body that will decide the issue - and what to eventually call it.