How many habitable planets are there in the Milky Way?
At the end of the 1920s, as Hubble used variable stars to discover the Andromeda galaxy as far away as 2.5 million light-years away, we only knew that there were a large number of independent galaxies in the universe besides the Milky Way;
This is the first time we have realized the vastness and hugeness of the universe. Each of these galaxies contains at least hundreds of billions of stars, and the universe contains at least 200 billion galaxies;
The number of stars in the universe is very large even if it is a conservative estimate, so people have begun to think that since the sun is just an ordinary celestial body in the universe, it has something suitable for life.Earth;
Then there should be a lot of stars in the universe that also have a planet that can carry life. It is impossible for the sun to be special. This idea is very logical and human's understanding of science.
In terms of probability, this is the case, and we can't think of any reason to refute the above view. This is the root cause of humans' belief in aliens.
So after we know the scope of the universe, we are thinking that there must be extraterrestrial life in the universe, and their existence is very common, and we think that these lives are more advanced than human beings with a high probability.
The reason is simple, because the universe is now 13.8 billion years old, which is very old compared to the time when humans came out of Africa 200,000 years ago. If we look at the age of the universe, humans have existed in the history of the universe.But 1 minute, human development of science and technology is only a few seconds.
So we humans developed very late, we have reason to believe that there are some civilizations in the universe, they even developed hundreds of thousands of years, millions of years, or even tens of thousands of years earlier than mankind.
This situation is very common only for our Milky Way, because our earth is in the suburbs of the Milky Way, and the more we go to the center of the Milky Way, the older the stars there and the longer they exist;
Because the closer to the center, the denser the gas cloud, and the greater the gravitational disturbance, the faster the speed of star formation, the earlier the birth of a second or third generation star like the sun;
Then the planets around these stars are older than the solar system, so if they give birth to life, they will develop earlier than the earth, and the technology must be more advanced than ours.
Based on the above thoughts, rumors of various aliens have appeared since the last century, photos of various UFOs with mosaics, and various theories that the Earth has been visited by aliens are not uncommon;
In addition to these unverifiable rumors, the scientific community is not idle. We actively send signals to outer space, hoping to get a reply from aliens;
Because we think that it is easy for aliens to watch our radio signals, we even worry about the way that electromagnetic waves are used to transmit signals, which is already left by others;
For example, the Arecibo Radio Telescope in the United States in the 1970s actively sent a series of signals to the M31 globular star cluster 25,000 light-years away. The reason for choosing this star cluster is because the density of stars in it is very large, and it is estimated to existThe possibility of aliens is relatively high.
If aliens receive the signal, they will see the picture below:
In addition to sending signals, we also use detectors, such as the Pioneer 10 and 11 detectors launched in the 1970s, and the Voyager 1 and 2 detectors, which have a golden record engraved with human-related information;
At present, these probes have already flown out of the heliosphere and are heading towards the center of the Milky Way. If these probes are eventually captured by aliens, they will know that they are about 30,000 light-years away from the center of the Milky Way.Human existence.
However, since the 1990s, the way we look for aliens has changed. We are no longer stupid enough to actively expose our position, but by searching for exoplanets to determine where life can be born in the galaxy.
This way we can not only know with certainty where there should be aliens, but also know what destinations we have if humans engage in interstellar migration in the future.
In 1995, we discovered the first exoplanet 51b Pegasus. The discovery of this planet can be said to be a milestone in astronomy and also opened up a new field of astronomy, exoplanet astronomy.
So far, human beings have gone further and further on the road to search for exoplanets. The most significant event during this period was that in 2009, NASA launched the first infrared telescope dedicated to search for exoplanets, the Kepler Space Telescope.
This telescope has observed 100,000 stars in the Milky Way during the eight years of its work, and discovered 2,600 pairs of exoplanets using the direction-of-sight velocity method and transit method;
The Kepler telescope was retired in 2018, and the new telescope TISS infrared telescope took over its work. In two years, TISS discovered another 2,300 exoplanets, and now there are more than 4,000 exoplanets known to mankind.
Although this number is not worth mentioning relative to the total number of planets in the Milky Way, we can preliminarily deduce based on the types of planets and stars that have been discovered, the number of planets in the Milky Way, and how many suitableHome planet.
First of all, based on what has been discovered so far, we can be sure that the number of planets in the Milky Way is far more than the number of stars. On average, each star corresponds to three to four planets.
Moreover, NASA has 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;
That is to say, 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 very certain that these planets are rocky.
So these planets are habitable, and life is likely to exist on them. At present, the only uncertainty is the atmospheric composition of these planets.
The 50% mentioned above is actually an intermediate value taken by the scientist. In fact, this probability can be as low as 7% conservative estimate, or as high as 75% optimistic estimate.
If the conservative estimate of 7% is applied to the entire galaxy, then scientists estimate that there are at least 300 million habitable planets in the galaxy. If it is an optimistic 75%, then this number will soar to 3 billion.
And NASA also said that among the habitable planets discovered now, 4 of them are only 30 light-years away from the earth, and the nearest is only 20 light-years away.
30 light-years, 20 light-years sounds very far, but relative to the Milky Way galaxy, or relative to the universe, the distance is very close, it can be said that it is at the gate of the village.
This new research also allows us to once again confirm people's conjectures about extraterrestrial life since the last century. They are really as common as we thought, because there are so many places to support life.
If the birth of life is only related to physics and chemistry, and has nothing to do with God, then we can firmly believe that one day we will find aliens or be discovered by aliens.
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