The Earth has all the necessary conditions for the development of intelligent life. However, if somewhere near us there is another developed civilization, then it is located at a distance of several light years. But this does not mean at all that such a situation is a rule for the Universe.
We can say that here on planet Earth, in orbit around the Sun, we are the only intelligent life form. There is a possibility that a microbial form of life existed in the solar system before or now exists, but if we talk about complex, differentiated and multicellular life, then the inhabitants of our home planet are more developed than what we can find in the vicinity of our star. If any other planet is inhabited by intelligent beings, then they are at least four light years from us. But do all possible intelligent beings in the Galaxy live in the same conditions?
A lot of work needs to be done to give birth to evolution, but its components are scattered literally everywhere in space. Even if you look only for life forms that are chemically similar to us, the universe is full of all kinds of probabilities.
First of all, it is necessary to form elements heavy enough to form rocky planets, organic molecules and the building blocks of life. None of this was in the newborn universe. As a result of the Big Bang, it consisted almost entirely of hydrogen and helium; it did not contain carbon, oxygen, nitrogen, phosphorus, calcium, iron, or any other complex elements necessary for life. To obtain such, it is necessary that many generations of stars live their lives, use up their fuel, die in supernova explosions, and then the newly created elements would be processed in the next generations of stars. For the heaviest elements to form, many of which are necessary for life on Earth in huge quantities, collisions of neutron stars are necessary. And all this takes a lot of time.
Despite the fact that the Earth formed more than nine billion years after the Big Bang, the universe did not have to wait that long for life to begin in it. There are three stellar populations:
Population I: Stars like the Sun, with 1-2% of the elements heavier than hydrogen and helium. As a result of the processing of this material, star systems are formed, in which there are both gas giants and rocky planets capable of supporting life.
Population II: Mostly older and cleaner stars. They may contain only 0, 001-0, 1% of heavy elements, and most of their planets are gaseous worlds, most likely too primitive and with too low a content of heavy elements for life to arise.
Population III: The first stars in the Universe in which there are no heavy elements at all. Such objects have not yet been discovered, but in theory they are the very first stars ever to appear.
When studying the earliest galaxies with the most powerful telescopes, it will be seen that they mainly contain population II stars. Somewhat closer to us are galaxies filled with a mixture of old and young stars, rich in "metals" and practically not containing them.
The most interesting case is the Kepler-444 system. It is a Population I star (and planets revolve around it), but older than our system.Whereas Earth formed about 4.5 billion years ago, Kepler-444 is 11.2 billion years old. Consequently, the universe could well have formed a world like ours very early - at least seven billion years before the appearance of our planet. Given this possibility, and the fact that regions like the center of the Galaxy formed more heavy elements than our region, extremely quickly, it is likely that there are places in the Universe (and quite possibly even in the Milky Way) that are more prone to the emergence of intelligent life. than the Sun-Earth system.
So, given everything we know today about where the stars can be, in whose systems life can arise, how close can two alien civilizations be located in relation to each other? Where should we look for them? And what would be the answer under different circumstances? Let's consider the main features.
The same star system
It is no exaggeration to say that this is a real dream. It is likely that in the early stages of the solar system, Venus, Earth and Mars (and possibly even Thea, a hypothetical planet that collided with the Earth, resulting in the formation of the Moon) had the same favorable conditions for life. Most likely, their bark and atmosphere were full of ingredients for life and they had liquid water on them. Both Venus and Mars, at their closest approach to the Earth, approach it at distances of tens of millions of kilometers: Venus at 38 million kilometers, and Mars at 54 million. However, with regard to a class M star (red dwarf), the distances between the planets are much less - and are only one million kilometers between potentially habitable planets, for example, in the TRAPPIST-1 system. In the case of double planets, this distance can be even less. If life succeeds in such conditions, why shouldn't it arise twice in practically the same place?
Globular clusters are massive clusters of about hundreds of thousands of stars located in a sphere with a radius of several tens of light years. In the outer regions, stars are usually one light-year apart, but in the inner regions of the densest clusters, the distance between stars can be no more than the distance between the Sun and the Kuiper belt. The orbits of the planets inside these star systems should be stable even in such conditions, and given that science knows globular clusters much younger than the Kepler-444 system, they may be good candidates for life with suitable habitat conditions. Only a few hundred astronomical units is a very short distance for two civilizations.
Neighborhoods of the galactic center
As one approaches the center of the galaxy, the stellar density also increases (the number of stars per cubic parsec). Within a few light years from the center, this density is very high - so much that it can rival the cores of globular clusters. In a sense, the galactic center is a much denser environment with its huge black holes, incredibly massive stars and new star-forming clusters - all of this is not in globular clusters. However, if you look at the center of the Milky Way, it turns out that most of the stars located there are relatively young. Perhaps this is due to the variability of that environment and the harsh conditions of existence (black holes easily tear apart companion stars), but stars there rarely reach even a billion years. Despite their increased density, these stars are unlikely to have advanced civilizations. They simply do not have time to exist long enough for this.
Dense star cluster in the spiral arm
What about star clusters forming in the galactic plane? Spiral arms are denser than typical regions of the galaxy, where new stars are most often formed.The star clusters that have survived from those times often contain thousands of stars in an area only a few light years across. But again, the stars linger in such an environment for a short time. A typical open star cluster disintegrates after about a few hundred million years, and only a small fraction of such objects persist for billions of years. The stars are constantly moving in and out of the spiral arms (this also applies to the Sun). Although the distances between stars in such clusters can range from 0.1 to one light-year, they are hardly suitable candidates for life.
Let's pay attention to what we see in our vicinity: distances, usually several light years. When approaching the center of the Galaxy, this distance can be reduced down to 0.1 light years, as in open clusters. But if the distances are reduced even more, then the same problems appear as in the galactic center: collisions, interactions and other catastrophes that are likely to destroy a stable environment. Of course, sometimes the distances are shorter, but typical interstellar space is unlikely to be suitable for this. The best option in this case is to wait until another star flies by, which happens about once in a million years in the case of typical stars.
Although no one expects intelligent alien life to be found throughout the universe, like planets and stars, nevertheless, every world with suitable conditions is a chance for life. Of course, we still have to get even to the nearest system - Proxima Centauri, which is four light years from Earth - using probes of the Breakthrough Starshot program or something else, but there is a chance of detecting life in the vicinity of our star system, albeit small … And only having arrived there, we will be able to say for sure whether there is something there or not.