There may be more than 300 million livable planets in the Milky Way, so why are we the only ones on the earth?

The Milky Way is like a discus, 100000 light-years wide and 100000 light-years thick. The Milky Way contains hundreds of billions of stars like our sun, and scientists have discovered dozens of terrestrial planets, although they have only detected a small part of these hundreds of billions of stars. If there is a planet similar to the earth in a thousand stars, there are hundreds of millions of such planets in the Milky Way. If there is one Earth-like planet in a thousand, there are 100,000 Earth-like planets in a hundred million. If one of1000 livable planets gave birth to human beings, there should be about 1000 animals (such as human beings) with high intelligence in1000 livable planets.

Because the Milky Way is very vast, the earth is about 35,000 light years away from the center of the Milky Way. Assuming that there is human life outside the solar system, the average distance is 10000 light years. What does this mean? Our nearest star, Centauri, is 4.22 light years away. At the current speed of manned spacecraft, it will take 70 thousand years to reach it. Therefore, at present, for people living in the Milky Way, interstellar travel is impossible. Even if a radio signal is sent, it will take thousands of years to reach a planet with human civilization, let alone manned interstellar travel. In addition, in such a vast space, the radio signal is so weak that it is unknown whether it can reach another planet.

In addition, humans on every planet are slow to move. Even if there is a more developed scientific and technological civilization than ours, it will be hundreds of thousands of years ahead of the earth. If hundreds of thousands of years later, because the distance is too far, it is normal for them to emit space radio signals. More importantly, even the most civilized planet can't survive the impact of asteroids like the earth 65 million years ago. It can be said that it is back to the barbaric era, all civilizations have been eliminated, and everything has to start again.

Therefore, it is not necessarily only the planets in our galaxy that have humans, but it is entirely possible that other planets also have humans. Just because they are too far away to be suitable for interstellar navigation, coupled with the restrictions of civilization and radio waves, we earthlings have not found any traces of them so far, but this does not mean that we have not.