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Introduction:

One of the big questions humans have asked for ages, and to which there is still no firm answer, is whether there is life other places in the Universe. 
 
The Christian religion, as well as many other religions, had their very firm answer to this questions. No way! Humans and all other life forms were created by a divine power, and since the Earth was the center of the Universe, there could not be life anywhere else. 
 
Scientists who believed otherwise were some centuries ago burned at the stake, such as Giordano Bruno in 1600, when he declared that the Sun was just a star around which the Earth orbited. Others, such as Galileo managed to keep his life, but was then also forced to keep his views for himself. 
 
The issue of life elsewhere is more tricky to demonstrate than the structure of our Solar System, and so far the expeditions to the Moon, Mars or other objects have not been able to do so. 
 
A way to go is therefore to look at what conditions must be in place for life to have a chance of evolving, and then try to calculate the probability of this occurring in the known Universe. 
 
Some things are essential for life as we know it. There must be liquid water and the temperature must be relatively stable. It is therefore important that the Earth rotates and moves in a fixed orbit. If these changed from time to time, there would be some huge leps in temperatures, which would wipe out many of the living organisms. 
 
Another condition is that the Earth is not constantly bombarded by huge asteroids or comets. This would create sudden changes in climate, which would also lead to extinctions of many of the life forms, and which the Earth has experienced at least five times &ndash; last time 65 million years ago when the dinosaurs disappeared. 
 
There must also be a source of energy to run the processes in the living organisms (not necessarily light - it can also be chemical energy). 
 
Other factors are maybe not essential for life in the simplest forms. But they are probably essential for life in the form of more complex organisms, since stable periods of hundreds of billions of years are probably necessary for such organisms to evolve. This is for example a shield from the radiation that comes from space - on Earth this is in the form of its magnetic field. The movement of the tectonic plates also contribute to such development of more complex life forms, since this means that nutrients keep being recycled and come back to the surface again, instead of staying buried in the Earth&#39;s crust. 
 
Scientists have so far found hundreds of planets orbiting other stars, nearly all of them large gas planets such as Jupiter. The technologies are still not developed to find the smaller earth like planets, but there is no doubt that a huge number of such planets exist - and that a great number of these have had the conditions for life to develop. 
 
We will probably only get sure evidence for this, when we one day detect signals sent from one such planet, or from one of their spaceships.

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