Fermin paradox - Alien life not possible?

In the year 1950 , Enrico Fermi was the supposition of the existence of a single alien civilization capable of interstellar travel at speeds below the speed of light . He assumed that civilization concerned, for whatever reason, the conquest of our galaxy , the Milky Way . He hypothesized that leapfrogging, colonizing a planet for hundreds or thousands of years, and sending dozens of ships to new conquests.

According to the calculations of Fermi, the entire galaxy would be under the influence of this hypothetical alien civilization after only a few hundred million years, the low speed of vessels being largely offset by the exponential increase the number of vessels of colonization. Enrico Fermi then expressed what would become the Fermi paradox: "  If aliens exist, but where are they?  ". A million years representing only little across the galaxy, they should be ubiquitous and it should be impossible not to see them.

Since its enunciation, several hypotheses have been advanced to explain the Fermi Paradox:

The trend in the colonization of the galaxy by an alien civilization is an assumption anthropocentric is difficult to demonstrate. "This is not because we would they would need."

The humanity is not necessarily aware of the existence of all life forms that surround it. The recent discovery of thermophilic organisms and traces of methane in the Martian atmosphere are examples. In addition, an advanced alien civilization could possibly hide and avoid contact with humanity according to the zoo hypothesis of John A. Ball . "It's not because we do not see that they are not there."

The late appearance of the heavy elements (heavier than the lithium ) in the Universe , necessary for the development of life as it is currently known, was limited in time the appearance of extraterrestrial civilizations.