On planet Earth, orbiting the Sun, we are the only intelligent life. Somewhere else in the solar system, microbial life may well exist, but intelligent, complex, diverse, and multicellular life is unlikely. Intelligent aliens, if they inhabit another world, are at least four light years away. Is it an accident or a pattern? How close could two independent intelligent civilizations be in the Universe, if we forget about interstellar travel and assume that they evolved in different star systems and are at least a little "life"? Globular clusters may have a high density of stars, but would the increased density interfere with habitability? An astrophysicist in a dense globular cluster would have a completely different view of the universe and the search for exoplanets.
For life to appear, many conditions must be met, but the main ingredients for it are essentially everywhere. Even if we limit ourselves to finding life that is chemically similar to ours, the universe will be full of possibilities.
Atoms can assemble into molecules, including organic molecules and biological processes, both on planets and in interstellar space. Perhaps life began not on Earth, but not on the planet at all.
It is necessary that enough heavy elements are formed, of which solid planets, organic molecules and the building blocks of life. The universe was born without them. After the Big Bang, the universe was 99.9999999% hydrogen and helium. There was no carbon, oxygen, nitrogen, phosphorus, calcium, iron and, in general, any complex elements necessary for life. For them to appear, many generations of stars had to be born and die, which burned out their fuel and died supernovae, transforming the heavy elements created into a new generation of stars. For the heaviest elements, a merger of neutron stars is necessary, and without these elements there would be no life on Earth and our bodies could not exist. The gears of astrophysics had to work at full power.
Even though the Earth formed 9 billion years after the Big Bang, the universe didn't have to wait that long. We classify stars into three groups:
Population I: Sun-like stars, 1-2% heavier than hydrogen and helium. This material is well processed and creates a mixture of gas giants and solid planets in solar systems that can support life.
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Population II: These are mostly old stars. Their content of heavy elements can be 0.001-0.1% of the sun, and the worlds around them are mostly diffuse, gaseous. There may be too few heavy elements for life, and they will be primitive.
Population III: the first stars in the universe that were completely free from heavy elements. We have not yet found such, but theoretically they exist (and existed).
If you look at the first galaxies, they are full of Population II stars. But in our vicinity we see a mixture of young and old, metal-rich and metal-poor stars.
One of the most important lessons learned from the Kepler mission was the Kepler-444 system. It is a Population I star (with planets around it), but much, much older than Earth. Our world is 4.5 billion years old, and Kepler-444 is 11.2 billion years old, which implies that the universe could have formed an Earth-like world a long time ago, 7 billion years before Earth formed. Given this possibility, as well as the fact that there are more metal-rich luminaries in the center of our galaxy than in the regions, it may well be that somewhere in the Universe (and maybe even in the Milky Way) there is a system with intelligent life.
So, given everything we know about where suitable stars could be, how close could two alien civilizations be? Where to find them? Under what circumstances? Let's take a look at five of the most likely options picked up by Ethan Siegel.
The same solar system
This is just a dream. In the early days of the solar system, it is likely that Venus, Earth, and Mars (and perhaps even Theia, the hypothetical planet that collided with the Earth and formed the Moon) were all in favorable conditions for life. They had a crust and atmosphere full of ingredients for life, and they also once had liquid water on the surface. Venus and Mars, on their closest approach, are 38 million and 54 million kilometers from Earth, respectively. But in red dwarf systems (M-class), the planets are separated by significantly smaller distances: about 1 million kilometers between potentially habitable worlds in the TRAPPIST-1 system, for example. Moons near giant worlds may be even closer. If life develops successfully under certain conditions, why not repeat it twice in the same place?
Within a globular cluster
Globular clusters are massive collections of hundreds of thousands of stars enclosed in a sphere several tens of light years in radius. In the outer regions, the spheres of the stars are separated by light years, but in the inner, densest clusters, the distance between the stars can be as much as from the Sun to the Kuiper belt. The orbits of planets in such stellar systems must be stable even in dense conditions, and given what we know of globular clusters that are less than 11.2 billion years old, like Kepler-444, there could be many candidates for life. Several astronomical units are a surprisingly short distance between two civilizations, isn't it?
Near the galactic center
The closer you get to the center of the galaxy, the denser the stars become. Within the central few light years, the density of stars is extremely high, even when compared to the cores of globular clusters. In a sense, the galactic center is extremely dense because it contains black holes, huge clusters of masses and star formations that are not found in globular clusters. But the problem with the stars we see in the center of the Milky Way is that they are too young. Perhaps due to the instability of the region, stars rarely live even a billion years. Despite the increased density, such stars are unlikely to acquire advanced civilizations. They just don't live.
In a dense cluster of stars or a spiral arm
What about star clusters that form in the galactic plane? The arms of a spiral galaxy are denser than other regions and are where new stars tend to appear. Star clusters that remain from those eras often contain thousands of stars located in a region just a few light years away. But again, stars don't stay in such conditions for long. A typical open cluster of stars disintegrates after a few hundred million years, and only a few live for billions of years. The stars move along the spiral arms constantly, including our Sun. And while the stars in the arm can converge up to 0.1 light years, they are unlikely to be good candidates for life.
Interstellar distribution
So we're back to what we're seeing in our own area: distances of several light years. As you get closer to the center of the galaxy, you can reduce this distance to what you saw in an open cluster: 0.1-1 light years. But if you get even closer, there will be a problem that we observed too close to the center of the galaxy: mergers, interactions and other catastrophes that destroy a stable environment. You can get closer, but usually interstellar space does not allow this. In the best case, you can wait until another star passes by, and this happens every few million years.
Overall, while we don't expect intelligent alien life to be as ubiquitous and ubiquitous as planets and stars, every such world that meets the right conditions is rare. And every time you get that chance, success is unlikely. The number of possibilities that can become reality is very limited. But now we at least know what to expect if we find a bunch of other advanced civilizations in the Universe.
Ilya Khel