Solar System [slideshow] [manual] [with notes]

The Sun
Our star, the Sun, makes up 99% of all the mass in the Solar System. Its core is so dense and hot that normally repellent nuclei fuse together in nuclear reactions that produce vast amounts of energy. The Sun is mostly hydrogen (its main fuel) and helium, and radiates charged particles called solar wind across the Solar System. Phenomena such as solar flares and sunspots are evidence of the Sun's strong magnetic field, which changes on a roughly 11-year cycle.

Mercury
The innermost planet in the Solar System is a dense, heavily cratered world that takes about 59 Earth days to fully rotate on its own axis as it travels on its 88-day journey around the Sun. It is possible to see Mercury from the Earth without a telescope or binoculars though its closeness to the Sun's bright light can make it difficult to spot. Photographed and studied at close range by the Mariner 10 and Messenger probes, Mercury is blasted by solar radiation and is not thought to be a likely place for life to flourish.

Venus
Venus, the second planet from the Sun, is an extreme place - hot and dry with surface pressures over 90 times higher than the Earth's and a super thick atmosphere mainly composed of carbon dioxide. Because the surface is hidden by sulphuric acid clouds, and the planet is similar to the Earth in size, astronomers speculated for many years that Venus might be a lush world full of life. It is now thought possible that the Sun's heat boiled away early oceans on the planet triggering a planet-warming runaway greenhouse effect that turned Venus into a hellish place.

Earth
Earth, the third planet from the Sun, is unique in the Universe as it is currently the only planet known to support life. It has a single natural satellite, the Moon, and is the fifth largest planet in the Solar System. Earth's distance from the Sun is thought to be one of the key reasons why it is home to widespread life. Our planet occupies what scientists sometimes call the Goldilocks zone. Its distance from our star means it is neither too hot, nor too cold to support liquid water - thought to be a key ingredient for life. Astronomers are searching for rocky planets like ours in the Goldilocks zones of other stars.

Mars
Mars was among the first bodies in the Solar System to be viewed through a telescope. Early astronomers saw faint surface features along with evidence of changing seasons and speculated about an advanced Martian civilisation. Though these ideas are comical today, the search for more primitive life continues. NASA's Mariner and Viking probes in the 1960s and 1970s found a cold, apparently lifeless planet with huge volcanoes and canyons and evidence of past surface floods. More recently, six-wheeled rovers have confirmed that water ice exists below the surface.

Jupiter
Jupiter, the largest planet in the Solar System, has a volume equal to more than 1,000 Earths. The fifth planet from the Sun is called a gas giant because it has no solid surface, being mainly composed of hydrogen and helium. It is believed to have liquid metallic hydrogen in its interior that generates the planet's intense magnetic field. Powerful storms such as the centuries-old Great Red Spot travel in bands across Jupiter. Its many satellites include the moons Io, Europa, Ganymede and Callisto.

Saturn
Saturn, the sixth planet from the Sun, has beautiful rings composed of ice particles. It is the second largest planet in the Solar System, yet it is the least dense - it would float in water if there were a bathtub large enough to hold it. Saturn is mainly composed of hydrogen and helium and does not have a solid surface. It has 25 satellites that measure at least 10km in diameter - the largest, Titan, is the only moon in the Solar System with a substantial atmosphere. Saturn's interior is thought to contain fluid metallic hydrogen - a substance that cannot be studied directly because it is not possible to recreate the very high temperatures and pressures at which it is predicted to form.

Uranus
The seventh planet from the Sun was first correctly identified as a planet in 1781 by William Herschel. Uranus may have been struck by a planet-sized object early in its life and knocked onto its side. Consequently, Uranus's axis is tilted 98 degrees compared with the Earth's 23.5 degrees. Like the other gas giant planets, Uranus has no solid surface. It gets its bluish surface colour from tiny frozen ammonia crystals. The atmosphere is thought to be mainly hydrogen and helium. Uranus has only been visited once, by the Voyager 2 probe.

Neptune
Neptune, the outermost planet in the Solar System, cannot be seen from Earth without a telescope or binoculars. It is covered by icy, bright blue methane clouds that whip around the globe at speeds of more than 700mph (1,100km/h). Though its diameter is four times that of the Earth's and it is 17 times as massive, it is less dense and doesn't have a solid surface. Eleven moons orbit Neptune, the largest of which is Triton.