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.
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. However, its smaller presence on Earth is useful as it helps to moderate the planet's temperature and is absorbed by plants during photosynthesis to produce oxygen.
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. Venus has a surface temperature of 480°c, which would be much too hot for anybody to live in. Mars, although it can reach 25°c, is usually freezing and can be as cold as -140°c, a temperature which would freeze blood and water. Other planets are colder still.
Earth has water! Water is considered to be the most important chemical necessary for life. It contains the oxygen needed for life. Other liquids can 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 Europa, a moon of Jupiter, are believed to have oceans of water under its icy surface. Scientists believe that the presence of water on other objects in the solar system greatly increases the chances of life existing on them. Water on Earth is abundant and can be found in its three states of matter. It can be frozen, taking the form of ice. It can be liquid, seen in seas and oceans and lakes. It can also be a gas, seen as clouds. In the picture below, we can see water in its three states; a solid, a liquid and a gas. The blue glow at the top of the Earth is the planet's thin atmosphere.
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. This process is sometimes referred to as terraforming and is a requirement for the possible existence of human beings on other planets. 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. But Mars, with a day length similar to that of Earth, and quite a bit of light, it could be a possibility.
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. Technically, Earth probably wouldn't exist either! Because of Earth's ideal distance from the Sun, it receives the perfect amount of heat and light to allow life to be created and to support it. Imagine what would happen if the Sun suddenly vanished. How would you keep warm? How would you see? How would you get food and drink? How would plants and trees grow? How would they photosynthesise? Where would Earth go? The Sun's gravity keeps Earth in its orbit, but if the Sun vanished, Earth would simply float away.
The Sun is good, but it isn't completely good. In fact, too much Sun can be very very bad. The Sun continually pumps out radiation. Heat and light are two examples of this radiation and, as we have seen, are essential to life existing on Earth. But there is a another type of radiation that the Sun produces which we can't see or feel. This is ultraviolet radiation or UV rays. Only about 1% of the ultraviolet radiation that the Sun sends to Earth actually reaches the surface. Small amounts of exposure to UV rays are beneficial. They cause the body to produce Vitamin D, which has several health benefits, and it has been suggested that even a few minutes of exposure to sunlight will cause the body to produce enough natural Vitamin D for the whole day*. UV rays are also what causes skin to tan. But even so, too much exposure to it can cause sunburn or have even more serious long-term effects. What prevents most of these dangerous UV rays from reaching the surface is the ozone layer. The ozone layer is a part of Earth's atmosphere situated in an area known as the stratosphere. The ozone layer starts at between 10 to 17 kilometres (6 to 10 miles) from Earth's surface and extends up to 50 kilometres (30 miles) high. Ozone is able to absorb most ultraviolet radiation so prevents it from reaching the surface. However, some manmade products use chemicals which can damage the ozone layer if allowed out. These chemicals are known as CFCs (chlorofluorocarbons) and are often used in refrigeration. They were also used in spray cans. It is the presence of chlorine in these CFCs which is particularly harmful. If chlorine is able to get into the stratosphere, it is able to break down ozone. And breaking down ozone reduces the amount of protection that the ozone layer can provide. So it is important that you look after the ozone layer because, after all, it looks after you.
As we've just mentioned, the Sun sends out lots of radiation. It spreads this radiation across the entire solar system carried through what is known as the solar wind. Some radiation is good but a lot of it is bad. Protecting Earth from the bad radiation is its atmosphere. Earth's atmosphere also contains the oxygen which is needed for a large number of living spieces to breathe. But what protects the atmosphere? Why, Earth's magnetic field, of course! At the core of Earth is a load of molten iron. Iron is a very magnetic element, and it causes Earth to act as one giant magnet. It's basically why Earth is said to have a north pole and a south pole. The radiation that the Sun sends out is electromagnetic radiation. As Earth's iron core produces a magnetic field, it is able to repel most of the electromagnetic radiation from the Sun. By doing so, it prevents the solar wind from stripping Earth of its protective atmosphere.