Earth is unique in the Solar System as being the only planet which is able to support life in all its forms: from basic living micro-organisms to highly sophisticated and intelligent human beings. There are many reasons why this happens.

REASON ONE: ATMOSPHERE 

Earth has a breathable atmosphere. Oxygen is the gas that is required for the life of most creatures. This is present in Earth' s atmosphere and also in water. Oxygen is constantly put into the atmosphere by plants and trees. Earth' s atmosphere also contains a small amount of carbon dioxide. This is a poisonous gas which makes up most of the atmosphere of planets like Venus and Mars and makes them unable to support human life.

REASON TWO: CLIMATE 

Earth has a suitable climate. This is caused by the moderate amount of carbon dioxide in the planet' s atmosphere, which is constantly refreshed whenever there is a volcanic eruption. The temperature on Earth does not go from one extreme to the other either. Mercury can be anything from 200° c below freezing to 375° c above. At 375° c, water would only exist as a gas, and the planet would be completely dry.

REASON THREE: WATER 

Earth has water! Water is believed to be the most important chemical necessary for life. It contains the oxygen needed for life. Other liquids contain poisonous elements. Water doesn' t burn skin (like liquids containing acids do) , it is drinkable, and it allows life-providing molecules to move around easily. Other moons in the Solar System, such as Europe, a moon of Jupiter, are believed to have oceans of water under its icy surface.

REASON FOUR: LIGHT 

All planets receive light from the Sun, but no planet uses it as usefully as Earth. Trees and plants on the planet produce oxygen through a process called photosynthesis. Plants need the Sun to grow. Look at plants in windows and notice how they usually seem to grow towards the Sun. Try growing a plant in a dark room and in a light room. Notice which one grows quicker. The one which has grown quickest is the one which also produces more oxygen. It is believed that if we were able to get plants to grow on another planet, such as Mars, they would begin putting oxygen into the planet' s atmosphere and increase the possibility of life. Saturn' s moon, Titan, has an atmosphere containing mainly nitrogen. If this moon was to possess oxygen too in its atmosphere, by a plant photosynthesising on it, it could have a similar atmosphere to Earth. Something else which helps the plants to photosynthesise on Earth is the length of time the planet takes to spin once on its axis. Taking just under 24 hours means that each side of the planet receives sunlight regularly. If we look at a planet like Venus, which takes 243 days to spin on its axis, it means that for a large period of time certain parts of the planet are in complete darkness. So even if the planet could support life, it would struggle to do so.

REASON FIVE: THE SUN 

All of the reasons given above for life existing on Earth are only possible because of one main reason. The Sun! Put simply, if there was no Sun, there would be no life on Earth. Where would Earth go? The Sun' s gravity keeps Earth in its orbit, but if the Sun vanished, Earth would simply float away.