At the centre of the solar system is our sun, a massive ball of hydrogen and helium over a hundred times larger than Earth producing immense heat and enormous explosions which jettisons solar winds millions of miles into space. The nearest planet to the sun is Mercury, aptly named after the swift footed messenger god it orbits the sun in only 88 days, quicker than any other planet in the solar system. Next comes Venus, often referred to as Earth’s sister planet but underneath her thick clouds lies a hellish oven baked landscape with temperatures hot enough to melt lead. Third planet from the sun lies Earth, a striking blue sphere covered with oceans of liquid water and the only planet known to us which harbours life. The last of the rocky inner planets is Mars, consisting of a thin atmosphere this red body was once covered in oceans just like Earth but is now a desert where dust storms can engulf the entire planet.

Almost 800 million kilometres from the sun we find the first of the gas giants, Jupiter, enormous in its size it would take over a 1,000 Earths to fill its volume. Saturn is the second of the gas giants and is unlike any other planet in the solar system with its spectacularly colourful rings made from dust and ice particles. Then comes the first of the planets known as ice giants, Uranus, this large turquoise ball of gas lies tilted on its side with freezing atmospheric temperatures of -371 Fahrenheit. At 4.5 billion kilometres from the sun we find the last of the planets, Neptune, a blue ice giant where winds in the atmosphere reach over 2,000 kilometres per hour.

Beyond Neptune we reach the Kuiper Belt where we find most of the dwarf planets including Pluto, at one time considered the ninth planet in our solar system this tiny body orbits the Sun at an average distance of almost 6 billion kilometres.

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The planets of our solar system have fascinated humans for centuries, technological advances in the past 50 years have allowed us to send spacecrafts to visit the planets and study them in greater detail than ever before, as a result we are learning more and more about them. What we have also found is that some of the moons in our solar system are equally and at times even more fascinating than the planets themselves.

Jupiter’s wonderfully coloured moon Io (pictured left) is the most volcanically active object in the solar system, Saturn’s moon Titan has a thick nitrogen based atmosphere which rains methane producing liquid lakes on its surface. Another of Saturn’s moons Enceladus has volcanoes which erupt with water ice and possibly has a liquid ocean underneath its icy surface. Then there is the ice world of Europa, another of Jupiter’s fascinating moons, underneath its surface lies a vast ocean of liquid water 100 km deep possibly teeming with life.

We will learn more about these moons in the next 10 years or so, maybe by then the names of these moons will be as familiar to us as Mars or Venus.

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Human kind has often pondered whether life exists elsewhere in the solar system, and if so what kind of life? Up until the mid 20th century it was thought every planet could harbour some kind of life forms, possibly even more advanced than ourselves. In the 1930’s Orson Wells radio production of Jules Verne’s famous novel “War of the Worlds” caused widespread panic as listeners mistakenly thought it was real. This is an indication of how much the belief in advanced alien life forms from neighbouring planets was held in the common psyche.

Of course now we know this is just science fiction and that we're not under imminent attack from an alien civilisation on Mars or Neptune. So is there life anywhere in the solar system apart from Earth and who are the suspects? Well you may be surprised to learn that there are several candidates in our varied solar system.

Let's begin with Mars, the planet that captures our imagination the most. Even though we now know there are no canals on Mars or advanced civilisations, scientists involved in the Viking lunar landings still expected to find signs of life in the Martian surface. Viking 1 landed on Mars in June 1976 and analysed samples of the Martian soil, but found nothing. Subsequent missions have also found no signs of life but there's still hope, and it lies underground. The reason for this theory is that surprisingly methane is present in the Martian atmosphere and one way methane can be produced is biologically. In the warmer summer months on Mars the presence of methane increases dramatically giving more credence to the idea that there are organisms living deep under the surface, possibly around hot vents.

There are two fascinating moons around Saturn which could possibly support life. Both are very different from one another, they are Enceladus and Titan. Enceladus is a very small icy world with an area only slightly larger than Texas but under its surface there is believed to be a salt water ocean. How can there be warm water in such a cold area of the solar system? Well the tremendous gravitational force of Saturn pushes and pulls the small moon heating up its interior and melting the ice underneath its surface. If so this could provide an environment for micro-organisms or some other forms of life to exist.

Enceladus's near neighbour, well 600,000 miles away, is another of Saturn's moons, Titan. This moon is exceptional in the solar system as it is the only one with a significant atmosphere. What's even more surprising is that its atmosphere is comprised mainly of nitrogen, just like our own planet. Even more amazing is that it is the only object in our solar system apart from Earth to have large areas of liquid on its surface, not of water but liquid methane. Methane on Titan acts like water on Earth, there are methane clouds which produce methane rain and methane lakes. Titan is often compared to primordial Earth, but unfortunately it's in deep freeze with surface temperatures around -179C (-290F). It is still possible that methane based microbial life could exist there breathing hydrogen instead of oxygen.

Lastly we come to an icy moon orbiting around Jupiter called Europa. It is believed this moon, just slightly smaller than our own moon, presents the best possibilities for life in the entire solar system. The force of Jupiter's gravity produces tremendous tidal heating inside Europa, warming its interior and producing a salt water ocean which is 62 miles (100 km) deep. It is possible that there is twice the amount of liquid water on Europa than there is on Earth! It's been speculated that Europa's ocean could be teeming with life, not just bacteria but complex organisms could be swimming in the warm water. Around the year 2025 NASA hopes to land a probe on Europa's surface which will then melt through the ice and investigate the ocean for signs of life, maybe then we will found out that we are not alone in the solar system.

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Diameter of the Solar System: Unknown (possibly 2 to 3 light years)
Distance from Centre of Galaxy: 25 million light years
Orbital Period: 250 million years
Age: 4.6 billion years

Number of Planets: 8
Number of Dwarf Planets: 5
Number of Moons: 173

Rocky Planets: Mercury, Venus, Earth and Mars
Gas Giants: Jupiter, Saturn, Uranus and Neptune
Dwarf Planets: Ceres, Pluto, Haumea, MakeMake and Eris

Nearest Planet to Sun: Mercury (58 million km)
Farthest Planet from Sun: Neptune (4.5 billion km)
Farthest Man Made Object from Sun: Voyager 1 (17 billion km)

Largest Planet: Jupiter (Diameter 142,984 km)
Smallest Planet: Mercury (Diameter 4,879 km)

Largest Moon: Ganymede (Diameter 5,262 km)
Smallest Moon: S/2003 J 9 and S/2003 J 12 (Diameter 1 km)

Greatest Planetery Gravity: Jupiter (20.87 m/s²)
Greatest Planetery Density: Earth (5.515 g/cm³)
Greatest Planetery Mass: Jupiter (1.8987 x 10²⁷ kg)
Greatest Planetery Volume: Jupiter (1.4255 x 10¹⁵ km³)

Lowest Planetery Gravity: Mars (3.693 m/s²)
Lowest Planetery Density: Saturn (0.7 g/cm³)
Lowest Planetery Mass: Mercury (3.3022 x 10²³ kg)
Lowest Planetery Volume: Mercury (6.08272 x 10¹⁰ km³)

In the context of the size of the universe, the Earth and indeed the entire solar system is insignificant. Our sun is just one of 200 billion stars in our galaxy and the milky way is just one of 100 billion galaxies in the universe. The solar system exists at the far edge of our galaxy, far from the centre where there is a supermassive black hole 4 million times bigger than the sun. The milky way is in fact a giant among galaxies, spanning 100,000 light years in diameter, there are more than 30 other galaxies in our "neighbourhood" of which only Andromeda is larger.

The best explanation we have as to how the universe was created is The Big Bang theory. Around 14 billion years ago a superdense, superhot mass billions of times smaller than a proton began expanding, in time creating the stars, planets and galaxies we have in the universe today, indeed it is still expanding. It is not known how the initial mass came to be.

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Our Sun

Surface of Mars

Ice Moon Europa

Jupiter's Red Spot

Saturn's Rings

Surface of Titan (Artist's Impression)