Perhaps, in the entire history of the Universe, there was no other intelligent, technologically advanced type of creatures other than humans. When you consider that there may be 400 billion stars in the Milky Way, each with three potentially inhabited worlds, and the universe has about two trillion galaxies, intelligent life seems to be quite common. But intuition may well let us down, because our assumptions are often unscientific. The magnitude of the unknowns that may be hidden in abiogenesis, evolution, long-term adaptation and other factors does not allow us to draw up an accurate equation of life. There is an astronomical number of possibilities for the development of intelligent, technologically advanced life, but huge uncertainties make it quite possible that humans are the only cosmic inhabitants.
In 1961, scientist Frank Drake presented the first equation that predicts how many civilizations conquering space can be in the universe. He relied on a series of unknowns that he could estimate approximately, and ultimately named the approximate number of technologically advanced civilizations that existed in the past and present, in our galaxy and in the observable universe. 55 years later, today some of these values allow us to make more accurate predictions.
First, our understanding of the size and scale of the universe has greatly improved. Now we know, thanks to observations of space and ground-based observatories, covering the entire spectrum of electromagnetic wavelengths, how large the universe is and how many galaxies there are. We have begun to better understand how stars form and function, and the deeper we look into the cosmic abyss, the more accurately we estimate the number of stars in the universe. There were many stars in the Universe - about 1,024 - and, based on this number, we can estimate the chances of the appearance of life in 13.8 billion years.
We are used to wondering how many stars there are planets nearby, while solid and with quite an interesting atmosphere, similar to ours, and how many such planets are at a suitable distance from their star so that there is liquid water on the surface. For a long time, we were surprised only by this. But thanks to the Kepler space telescope, we have learned a lot:
80-100% of stars have a planetary system or planets;
20-25% of these systems have a planet in the "habitable zone" in which water will remain liquid on the surface;
10-20% of these planets are similar to Earth in size and mass;
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Thus, there will be about 1022 potentially habitable terrestrial planets in the Universe with suitable conditions.
Moreover, almost all of these planets will be enriched with heavy elements and ingredients necessary for life. Looking at the interstellar medium, at clouds of molecular gas, at the centers of distant galaxies, we see all the elements of the periodic table - carbon, nitrogen, oxygen, silicon, sulfur, phosphorus, copper, iron, and much more. Looking at meteors and asteroids in our own solar system, we find not only these elements, but also their organic formations - sugars, benzene rings, and even amino acids. In other words, the Universe should not just contain 1022 potentially habitable planets, but 1022 planets with elements necessary for life.
Abiogenesis - when raw materials associated with organic processes suddenly turn into "life".
Life must exist and survive billions of years on the planet in order to acquire such properties as complexity, multicellularity, differentiation and "intelligence".
Finally, intelligent life must become a technological civilization in order to either announce its presence in the universe, or go outside its own home and explore the universe, or hear and discover other forms of intelligence in the universe.
When Carl Sagan introduced Cosmos in 1980, he stated that it would be wise to give each of these three steps a 10% chance of success. If this were correct, there would be more than 10 million intelligent alien civilizations in the Milky Way galaxy.
There are those who argue that in total, these three steps are less likely to happen than 10-22. But this in itself is a ridiculous statement, not based on anything. Abiogenesis can be common; it could have occurred many times on Earth, Mars, Titan, Europa, Venus, Enceladus, or even outside our solar system. But it can be such a rare process that even if we created one hundred clones of the young Earth - or a thousand, or a million, or more - our world could become the only planet on which life appeared.
And even if life does emerge, how likely is it that it will survive and flourish for billions of years? Will a catastrophic warming scenario like Venus be the norm? Or a scenario of catastrophic freezing and atmospheric losses, like on Mars? Or will life end up poisoning itself with its existence, as it did on Earth two billion years ago? And even if life survives for billions of years, how often will Cambrian explosions occur when huge, multicellular, macroscopic plants, animals, and fungi dominate the planet? This can be a relatively common or rare scenario, occurring in 10% of cases, or hardly at all.
And even if we assume all this, how likely is it that a technologically advanced, tool-firing, rocket-launching species like a human would appear? Complex reptiles, birds and mammals, which can be considered intelligent in many respects, have existed for tens and hundreds of millions of years, but modern humans appeared less than a million years ago, and became "technologically advanced" in the last century. Will there be a 10% chance that, having overcome the previous stages of development, you will become a space civilization? It is hard to believe. And we don't really know.
We know that intelligent life in the Universe should appear quite often (1022) And we know that there is a small chance of becoming a civilization conquering space. But we don't know if this chance is 10-3, 10-2, or 10-50. We need data. And no assumptions or statements can replace them. We need to find life to know for sure about its existence. Everything else is nothing more than ordinary speculation.
Ilya Khel