Drake equation explained - How likely is Alien life ?

 N = R x f_p x n_e x f_l x f_i x f_c x L

R * average star formation rate per year in our galaxy:

The average star formation rate is through empirical observations such as by the so called Hubble Space Telescope and will be estimated reasonably well estimated 4 to 19 When considering is important to note that a star of medium magnitude is needed. Stars that are bigger and brighter than the sun consume their energy in less than a billion years, allowing for the development of life on suitable planets not enough time. It is therefore looking for stars that are similar to our sun, because it is believed that the development of life as on Earth takes about a billion years. About 70 percent of the stars are faint red dwarfs. Although these stars have a life, which is higher by one order of magnitude as that of the sun, while its luminosity, mass and gravity much lower, making the Habitable Zone is located very close to the central star and planet in this zone, therefore strong tidal forces would be exposed. In addition, red dwarfs are prone to sudden changes in solar activity, which would be detrimental to the development of life.
Furthermore, approximately every second formation a double or multiple star system. It is here to rotate two or more stars orbiting each other, specifically around their common center of gravity. Physical simulations have shown that planets in such systems, a highly unstable orbit, and crash sooner or later in one of the sun or are ejected entirely from the system (three-and many-body problem). Exceptions are planets that are far from their suns so far that the gravitational force acts between the two stars on the planet as a single star and the planet by re-train has a more stable (two-body problem). The probability of a multiple star system over a longer period planets is very small.
When two galaxies collide, which happens due to the large gaps while collision-free, but they usually lose so much cosmic gas, that of the resulting galaxy (mostly ring galaxy)  caused virtually no stars anymore.
fp fraction of stars with planetary systems:
How many stars in our galaxy have a planetary system? Observations show that about half of all stars may have planetary systems like our own sun. Since 1995, with very sensitive detectors by measuring the radial velocities of solar-like stars already discovered 563 extrasolar planets in 472 systems (as of 19 July 2011).With increasing accuracy of the instruments, new methods and better resolution telescopes will be even more accurate measurements possible. So far, we can only find extrasolar planets that are very large (several Jupiter masses) and / or are very close to their sun. In both cases, there are probably very inhospitable conditions.

ne number of planets within the ecosphere:
The Ecosphere is the field in the solar system in which physical conditions do not preclude the emergence of life from the outset. A planet may, depending on the size of the sun, not be too close nor too far from its star. Is he too far gone, he's just too cold, he is too close, he is too hot and the solar wind blows away the atmosphere.In our solar system are Venus, Mars and Earth in the ecosphere. 2007 first two exoplanets have been discovered, which could be in the habitable zone of HD 209458b, and called by its discoverers as Earth-like Planet Gliese 581 c. Whether the conditions are really friendly live there but is controversial among scientists.
fl fraction of planets with life:
On how many planets there's life in the ecosphere? For this factor there is no scientifically verifiable numbers, because so far there is only the example of our solar system. Future with more sensitive telescopes are expected also to exoplanets traces of life, such as oxygen in the atmosphere prove to you.
fi fraction of planets with intelligent life:
If developed on a planet of life, it must not evolve into intelligent life. Even for this factor, there is no scientifically verifiable numbers. There can be only our solar system as an example. This raises the question of how intelligence is defined.
fc share planet with an interest in interstellar communication:
How many intelligent civilizations are interested in communicating with other individuals? Because only if you are interested in communication, there is the opportunity for us to find them. It is believed that extraterrestrial intelligent beings also go to the search for life.
Lifetime L of a technical civilization in years:
As a technical civilization refers to a civilization that is capable of a radio signal from space to receive and send signals into space. Life on planet is threatened by external and internal factors. A complete destruction can be triggered by events that have already several times in Earth's history led to mass extinction. These drastic climate changes such as massive volcanic eruptions and impacts from comets or asteroids. It is also conceivable that self-destruction of a technological civilization and the destruction of a technical civilization by another intelligent species, such as a virus.
Since the lifetime is limited by stars, is also the limited lifetime of a civilization in their solar system.Civilizations outside of the solar systems would be switched on independently sufficient solar energy sources; Perpetua mobilia are impossible under the laws of thermodynamics.