How many livable planets are there in the Milky Way?

In the late 1920s, when Hubble discovered the Andromeda galaxy 2.5 million light-years away by using variable stars, we realized that there were many independent galaxies besides the Milky Way.

This is the first time that we realize the vastness and vastness of the universe. Each of these galaxies contains at least hundreds of billions of stars, and the universe contains at least 2 trillion galaxies.

In this way, the number of stars in the universe is very large even if it is conservatively estimated, so people begin to have the idea that since the sun is just an ordinary celestial body in the universe, it has an earth suitable for life;

Then there should be many stars among these stars in the universe, and they also have a planet that can bear life. The sun can't be special. This idea is very logical and human understanding of science.

This is the case in terms of probability, and we can't think of any reason to refute the above view. This is the fundamental reason why human beings firmly believe in aliens.

So after we know the scope of the universe, we are thinking that there must be alien life in the universe, and they are very common. We think that these lives are more advanced than humans.

The reason is very simple, because the age of the universe now is 65.438+0.38 billion years old, which is very old compared with the time when humans walked out of Africa 200,000 years ago. Judging from the age of the universe, the existence time of human beings in the history of the universe is only 654.38+0 minutes, and the development of science and technology by human beings is only a few seconds.

Therefore, we humans developed very late, and we have reason to believe that there are some civilizations in the universe, which even developed hundreds of thousands, millions or even millions of years earlier than humans.

This situation is only common in our galaxy, because our earth is in the suburbs of the Milky Way, and the farther we go to the center of the galaxy, the older the stars there, the longer they exist;

Because the farther away from the center, the denser the gas cloud, the greater the disturbance of gravity, and the faster the formation of stars, so the second or third generation stars like the sun are born earlier;

Then the planets around these stars are older than the planets in the solar system, so if they gave birth to life, then they developed earlier than the earth and their technology must be more developed than ours.

Based on the above ideas, since the last century, there have been various rumors of aliens, various photos of mosaic UFOs, and various claims of aliens visiting the earth.

In addition to these unconfirmed rumors, the scientific community has not been idle. We take the initiative to send signals to outer space, hoping to get a reply from aliens.

Because we think it's easy for aliens to see our radio signals, we even worry that the way electromagnetic waves transmit signals is already a play for people;

For example, in 1970s, arecibo radio telescope sent a series of signals to M3 1 globular cluster 25,000 light years away. This cluster was chosen because of the high density of stars in it, and it is very possible to estimate aliens.

If aliens receive the signal, they will see the following picture:

In addition to sending signals, we also use detectors, such as Pioneer 10 and Pioneer 1 1 launched in 1970s, as well as Voyager 1 and Explorer 2, all of which have golden records engraved with human-related information.

At present, these detectors have flown out of the solar sphere and are flying to the center of the Milky Way. If these probes are finally captured by aliens, they will know that there are humans about 30 thousand light-years away from the center of the Milky Way.

However, since the 1990s, the way we look for aliens has changed. We are no longer stupid enough to actively expose our position, but to find out where life can be born in the Milky Way by looking for exoplanets.

In this way, we can not only determine where there should be aliens, but also know what destination we will have if humans engage in interstellar migration in the future.

1995, we discovered the first exoplanet pegasus 51b. The discovery of this planet can be said to be a milestone in astronomy, and it has also opened a new field of astronomy-exoplanet astronomy.

At this point, humans are getting farther and farther on the road to finding exoplanets. The most important event is that in 2009, NASA launched the first infrared telescope dedicated to searching for exoplanets-Kepler mission.

During eight years' work, this telescope has observed 654.38 million+stars in the Milky Way, and discovered 2600 pairs of exoplanets by using the apparent velocity method and transit method.

The 20 18 Kepler telescope was retired, and then the new telescope TISS infrared telescope took over its work. Within two years, TISS discovered 2,300 exoplanets, and now there are more than 4,000 known exoplanets.

Although this number is not worth mentioning relative to the total number of planets in the Milky Way, we can preliminarily infer the number of planets and livable planets in the Milky Way according to the types of planets and stars that have been discovered.

First of all, according to what we have found so far, it can be determined that there are far more planets than stars in the Milky Way. On average, each star corresponds to three or four planets.

In addition, NASA recently analyzed Kepler's data and published such a set of data. Among the sun-like stars discovered by Kepler, 50% of the sun-like stars have a planet with liquid water;

In other words, the temperature and atmosphere on this planet can support the existence of liquid water, and some of these planets have the same mass as the earth, and some are smaller than the earth. It is quite certain that these planets are all rocky planets.

Therefore, these planets are livable and life is likely to exist on them. The only uncertainty at present is the atmospheric composition of these planets.

The 50% mentioned above is actually a median taken by scientists. In fact, this probability can be as low as 7% (conservative estimate) or as high as 75% (optimistic estimate).

If the conservative probability of 7% is applied to the whole galaxy, scientists estimate that there are at least 300 million livable planets in the galaxy. If you are optimistic about 75%, this number will soar to 3 billion.

Moreover, NASA also said that four of the livable planets discovered now are only 30 light-years away from the Earth, and the nearest distance is only 20 light-years.

30 light years, 20 light years sounds far away, but relative to the Milky Way, or relative to the universe, this distance is very close, which can be said to be at the entrance of the village.

This new research also confirms people's conjecture about extraterrestrial life since the last century. They are really as common as we thought, because there are so many places that can support life.

If the birth of life is only related to physics and chemistry, but not to God, then we can firmly believe that one day we will find aliens, or be discovered by aliens.