Contact Bob | About this Site | Site Map
Bob the Alien's Tour of the Solar System
The Sun Mercury Venus Earth The Moon Mars Jupiter Saturn Uranus Neptune Pluto and Dwarf Planets Comets

Why is there Life on Earth?

Intelligent life on EarthEarth 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. 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's atmosphere is kept on the planet by its pull of gravity. Mars and Mercury are too small to keep atmosphere. As a result, Mercury has no atmosphere, and Mars' atmosphere is very thin, containing gases which have not managed to escape into space yet. Earth's atmosphere is thick enough to prevent poisonous rays of radiation from getting through it (this is what has happened on Mars).

Reason Two: ClimateEarth

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 200c below freezing to 375c above. At 375c, water would only exist as a gas, and the planet would be completely dry. Venus has a surface temperature of 480c, which would be much too hot for anybody to live in. Mars, although it can reach 25c, can be as cold as -140c, a temperature which would freeze blood and water. The other planets are colder still.

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 Europa, a moon of Jupiter, are believed to have oceans of water under its icy surface. Scientists believe that, if they could find traces of water on Mars, the possibility of life existing on that planet would be greatly increased. Water on Earth can be found anywhere, in its three states. 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.

Water, ice and cloud on Earth

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.

The SunReason 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. 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.