Are they among us? Are they hiding in some distant galaxy? Have they already visited us and, disappointed, have they left? On the existence of aliens – in particular of intelligent extraterrestrial life forms – literature, cinema, music and science have said practically everything and the opposite of everything. And net of more or less imaginary, suggestive or founded speculations, the only certainty is that, at the moment, these blessed aliens have not yet shown up: if there really are intelligent life forms beyond ours, we have not been able to reveal them or make any contact with them. Yet we have not spared – nor will we skimp – any effort: here is a summary of what we have learned in over half a century of research, where we are looking and what we hope to find.

The reasoning may seem paranoid, and even a little presumptuous: after all, how can we think of interpreting the will of an intelligence so superior to ours? If you seem to have heard this before, you’re right.

However, in fact perhaps there is a key to the reading. For astrobiologists at the University of Cambridge, life, anywhere in the Universe, can evolve only by following the laws of evolution, which in a nutshell we can summarize in „what is suitable, survives“. It follows that every suitable organism that succeeds in reproducing itself will try to route its offspring along the same path, both biological and cultural.

Gordon Cooper, one of the first astronauts to travel in space from 1958 to 1963 and part of the crew of the NASA Project Mercury, has the same idea. Cooper firmly stated that aliens not only exist, but have also landed on Earth. He is sure of it, he explained, given that in 1951 during a training he found himself face to face with a UFO.
Not only Cooper and Mitchell, other NASA engineers are convinced that extraterrestrials are already among us. Among them Dake Slayton, famous for being the first man to take part in a space mission. Slayton revealed that he had contact with a UFO in 1951 and has since supported the hypothesis that aliens exist. Finally we find Brian o’Leary, an astronaut selected to take part in a mission to Mars, who has openly declared that he has had contact with alien life forms.

The lack of signs „does not mean that we are alone“, is Carroll-Nellenback’s answer. „It only means that habitable planets are probably rare and difficult to reach.“
To simulate the spread of aliens in the Milky Way, the researchers used numerical models that take into account the proximity of the hypothetical extraterrestrial civilization to other star systems, the speed of any interstellar probes, the distance they could cover and the frequency of launches. If an alien civilization had landed on Earth millions of years ago, the researchers write, there would probably be no more traces of its passage.
It could also be that the aliens passed around the Earth after the appearance of the man, however deciding not to visit us because they would have had no chance of surviving (the so-called ‚Aurora effect‘, from the homonymous novel by Kim Stanley Robinson). Among the various hypotheses, also that for which the aliens could deliberately avoid the planets that already host life, with an opposite attitude compared to the spirit of conquest typical of human beings.

Matter there is little to do. If we really want to talk about science, and not science fiction, we must take courage and start with math. And in particular from Drake’s equation, a probabilistic formula formulated in 1961 by the American astronomer Frank Drake and used to estimate the number of extraterrestrial civilizations existing and able to communicate in our galaxy.

Here she is: N = R * x fp x ne x fl x fi x fc x L
As you can see, the equation contains seven factors: the average annual rate of formation of new stars in the Milky Way; the fraction of stars that possess planets; the average number of planets that are in the so-called habitable zone, that is, at the right distance from their star, the one that would allow the presence of liquid water on their surface; the fraction of these planets on which life actually developed; the fraction of planets that host intelligent life; the fraction of planets in which intelligent life has evolved enough to be able to communicate with the rest of us; the temporal duration of existence of these civilizations.

-linate