In theory, habitable planets could number in the millions. A significant part of the civilizations inhabiting them, in theory, should be highly developed.
Reserved forest
The first and most pleasant version for people, explaining the strange silence of the Universe, is the “space zoo”. According to this assumption, the territory of human habitation, for some higher intergalactic reasons, has been declared a protected area, all contacts with which are prohibited. Rather, there is contact, but one-way.
They know about us, they are watching us, perhaps even touched by our timid attempts to get out of the earthly cradle. For a highly developed civilization, such observations are invaluable: in our infancy, they can consider their past.
This version was described in 1973 in the book by John Ball, which was called the Zoo Hypothesis. The idea of a "space nursery" is very closely intertwined with it. We, they say, are too young for contact with "adult" civilizations, we have not even learned how to speak galactically - in short, we need to grow up and wiser. But, perhaps, by the time we and our cosmic "brothers in mind" are ripe for a meeting, neither we nor they will need it.
Both hypotheses have one weak point. Let's say aliens simply do not want to communicate with us, but this does not explain why we do not see traces of their activities anywhere: bases, buildings, ships, etc.
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And yet it is unique…
On this insist the authors of the study "Rare Earth" Peter Ward and Donald Brownlee. In their opinion, a number of non-trivial conditions must be fulfilled in order for life similar to the earth to arise on some planet:
- The planet should be located in a belt where water can be in a liquid state.
- The planet cannot be behind the gas giants, otherwise they will constantly knock it out of its usual orbit.
- The planet must have a massive satellite, such as our Moon, that would stabilize the planet's axis of rotation.
- In addition, the planet must have sufficient quantities of substances necessary for life, the crust must have a well-defined composition and structure, and the star must have the correct temperature cycle.
At the same time, the famous American evolutionary psychologist Jeffrey Miller believes that the mind is not at all a necessary product of evolution. This is the exception rather than the rule, and billions of worlds can be inhabited by non-sentient beings with whom we cannot have any contact.
However, this hypothesis contradicts the same "Copernican principle", according to which the Earth is not unique, and there must be many star systems and planets in the Universe with conditions similar to those on Earth. Besides, here we mean a life similar to the earthly, but who said that the “assortment” of intelligent beings is limited to humanoids built in our likeness?
However, the search for an alien civilization within our Galaxy alone, according to many scientists, is much more laborious than the search for a needle in a haystack. If there are 150-300 thousand straws in an ordinary stack, then the stars in the Milky Way range from 200 billion to 300 billion. True, other scientists object to this: we are not looking for any “needles”, but those that themselves can signal themselves … Roughly speaking, not needles, but key rings that respond to a whistle. But nevertheless, we still do not find anything….