The Solar System formed about 4.5 billion years ago, when a huge cloud of cosmic gas and dust collapsed, forming a disc of infalling material. The middle formed the Sun, and most of the rest formed the planets. Initially, all the planets consisted of the same mixture of solids, liquids and gases. However, the heat given off by the newborn Sun roasted most of the volatile materials off the inner planets: Mercury, Venus, Earth and Mars, while the outer planets: Jupiter Saturn, Uranus and Neptune, were far enough from the Sun to hang onto all their material. That is why the four inner planets are rocky, and the outer planets are mostly gas, and are termed "gas giants". Each of these components has its own characteristics and history.

Sun
The Sun is a yellow dwarf star at the centre of our solar system. Containing 99.8% of our solar system's mass, if the Sun were a hollow sphere, one million Earths could fit inside it.

The Sun is roughly 4.6 billion years old, about halfway through its expected life cycle. It will spend its latter years as a red giant, eventually expanding well beyond the orbit of Venus. Later still, it will eject part of its material back into space. The remainder will collapse into a tiny white dwarf star, half the Sun's mass and about the size of Earth. It will then slowly cool and fade into invisibility.

Mercury

This closest planet to the Sun also has the speediest orbit as it races around the Sun. Due to its proximity to the Sun, it suffers from temperature extremes: from a scorching 200°C in the daytime down to a frigid -400°C in the night.

Venus

Named after the Roman goddess of love and beauty,Venus is normally visible at sunrise or sunset, giving rise to the misnomer Morning or Evening Star. At its brightest, Venus is the brightest astronomical object in the sky after the Sun and the Moon. Unable to stray far from the Sun in the sky, it is an extremely cloudy planet, with acid rain and carbon dioxide skies that obscure our view of the planet's surface.

Moon
Earth's one natural satellite has long captivated us, with its dark maria (seas) and lighter-coloured highlands. Even a small telescope reveals a surface pocked with craters. Due to changing angles between the Earth, Sun and Moon, we observe the lunar phases.

Occasionally, we may be fortunate enough to view a lunar or solar eclipse, in which the Moon is a key player. Tidal action of Earth's seas is actually pushing the Moon 3.8 cm farther from Earth each year.

The Moon received its first human visitor on July 20^th, 1969, when Neil Armstrong took "one small step."

Mars
Mars was named for the Roman god of War, due to its blood-red colour. During the month of August 2003, Mars was closer to Earth than at any time in the last 60,000 years. Sometimes called the "Red Planet", the red of the surface is actually rust (iron oxide).

Mars is home to the largest known volcano in the solar system, Olympus Mons, which is nearly 3 times the height of Mount Everest. Exploration by unmanned spacecraft will soon determine whether liquid water or life have ever existed or still exist today on our sister planet.

Jupiter
Weighing more than twice as much as all the other planets of our solar system combined, Jupiter is called the King of the Planets for good reason. It is the first of the gas giants, and so has no solid surface except for a small rocky core. Not only is this giant superior in size, it also has more known satellites than any other planet -- 61 moons at last count.

The Great Red Spot, a giant hurricane on Jupiter that is big enough to swallow two Earths, has been blowing for over 300 years. Jupiter takes just under 12 Earth years to make one orbit around the Sun. Consequently, the giant spends roughly a year in each sign of the Zodiac, making it relatively easy to spot in the night sky.

Saturn
The second largest planet, Saturn is the root of the English word "Saturday". First observed with a telescope in 1610 by Galileo, the planet's rings were mistaken for ears, though later examination revealed several ring divisions. While each of the gas giants has ring systems of its own, Saturn's rings are by far the most stunning. They appear solid, but are actually millions of pieces of icy rock in a band just under one kilometre thick.

Saturn is the flattest of the planets and also the least dense. This enormous planet would float in a bathtub big enough to hold it.

Uranus
Originally overlooked as just another star due to its faintness, Uranus was later re-examined by William Herschel, making it the first planet to be discovered in modern times. It is also a gas planet, though unlike Jupiter or Saturn, Uranus contains methane, which gives it an eerie bluish hue.

Unlike most other planets, which spin on an axis perpendicular to the plane of the ecliptic (the plane of the solar system), Uranus' axis is parallel. This makes it appear to be lying on its back.

Neptune
The existence of Neptune was predicted from an anomaly in the orbit of Uranus, which appeared to be due to the gravitational pull of a distant planet beyond its orbit. Neptune is the second most distant planet of our solar system, though this is not always true. Due to the unusual orbit of Pluto, it occasionally comes inside the orbit of Neptune, making Neptune the most distant planet at these times.

Neptune is the home of some of the fastest windstorms anywhere in the Solar System, with winds often reaching speeds of 2000 km/h. Like Jupiter's Great Red Spot, Neptune's Great Dark Spot is a large storm with a diameter the size of Earth.

Pluto
There has been much debate as to whether Pluto should be classified as a planet. While it orbits the Sun, it is a lone icy body, out of place amongst a group of gas giants. It is also outmatched and outsized compared to several satellites (moons) in our solar system. Pluto is the only planet in the solar system not yet visited by spacecraft.

Sedna
Sedna, named after an Inuit Goddess, was discovered on 14 November, 2003, when two astronomers found a new object, beyond Pluto, and almost as big. Further observations showed this object to be very unusual. Instead of going round the Sun in an almost circular path, Sedna ranges between 13 and 130 billion kilometres from the Sun. It takes 10,500 years to complete a single orbit.