At Last, How Many Alien Civilizations are There?

 During the space age, 1961 was a special year: the Russian cosmonaut Yuri Gagarin became the first man to orbit Earth, while the American astronomer Frank Drake developed the now famous Drake Equation. This equation estimates the number of detectable extraterrestrial civilizations in our Milky Way galaxy, supposing our present electromagnetic detection methods. The Drake equation states:

N=  N_sx f_px n_ex f_lx f_ix f_cx f_L

N = number of alien civilizations in the Milky Way
N_s  = estimated number of stars in the Milky Way;
f_p = fraction or percentage of these stars with planets on its orbits;
n_e = average number of these planets with potential to host life as we know it;
f_l = percentage of these planets that actually develop life;
f_i = percentage of these planets that actually develop intelligence on human level;
f_c = percentage of these civilizations that actually develop electromagnetic radiation emitting technologies;
f_L  = percentage of these civilizations that keep emitting electromagnetic signals to space. This factor is extremely dependent on the lifetime a civilization remains electromagnetic communicative.

Looking to the Drake equation factors, it is obvious that none can be precisely determined by modern science. More than that, as we move from the left to right in the equation, estimating each factor becomes more controversial. The later terms are highly speculative, and the values one may attribute to each of them might tell more about a person’s beliefs than about scientific facts.