Are there any other lives in these countless stars and planets? These questions are probably one of the oldest and deepest questions of mankind, and one of the questions that scientists have been trying to find answers to.
At present, we have discovered that there are about 300 billion stars in the Milky Way galaxy alone, not to mention the entire universe, and there are some "super-Earths" that are considered to be suitable for life, just a rough estimate of 24!
So, why weren't aliens born on those planets? No, or didn't we find out?
(Imagination of planets with life, red for vegetation)
Does extraterrestrial life really exist?
The Fermi paradox, the earliest and most famous theory about extraterrestrial civilizations, was proposed by physicist Fermi in 1950, when he asked a question while exchanging reports with his colleagues about UFOs that were popular at the time, and he asked:
"Where are the aliens?" he meant, if there are many civilizations in the universe, then why haven't we found any evidence of them yet?
This question may seem simple, but it hides deep logic and reasoning. Because the scale of the universe is so large, there are hundreds of billions of stars in the Milky Way, and there are 700 stars in the observable universe, and the number of planets is even more innumerable. Even if the probability of intelligent life is only 1 in 100 million, there should be a large number of civilizations in the face of such a large number of planets.
(Schematic diagram of the Milky Way)
Because the earth is not special, whether in terms of planetary mass or characteristics, there are many similarities to the earth, which is also in line with the mediocre principle of the earth. The most closely related to the Fermi paradox is the Drake formula, which is also considered to complement and sort out this paradox.
Drake's formula was proposed in 1960 because of the limited level of technology at that time, and he wanted to use statistical methods to estimate the probability of extraterrestrial life. This formula is mainly estimated by factors such as planets in the star system, planets suitable for life, planets that can develop intelligent life, and planets that have means of communication.
(Overview diagram of Drake's formula)
Obviously, it is impossible to estimate it in this way, after all, so far human beings have not found any planets with life, let alone intelligent life, so the number of estimates made under only one Earth sample is too different, there may be several or tens of millions.
The best way to solve this paradox is obviously to find evidence of the existence of extraterrestrial civilizations, but due to the limitations of technology, humans have not been able to observe the universe more effectively, so it is still only a stage of speculation and theory. But with radio technology and the ability to build radio telescopes, humans began to search for signals beyond the solar system.
If there really is an extraterrestrial civilization, whether it is a signal they send intentionally or unintentionally, as long as it is discovered, it can be determined that an extraterrestrial civilization really exists, so scientists at that time launched a series of missions to search for radio signals such as the search for extraterrestrial civilization plan.
At the same time, it is also hoped that extraterrestrial civilizations will also discover our existence, and human beings have also carried out a lot of extraterrestrial civilization exchange programs that transmit life on earth through radio signals, such as the famous Arecibo message, through 1679 binary digital information, to convey the existence of intelligent life on the earth, because of the particularity of information, intelligent life will be easier to understand.
(The top 8 are decipherable by aliens, and the bottom 8 are undecipherable)
But many technologists believe that with existing search technology, plans to search for extraterrestrial civilizations are likely to ignore meaningful radio signals. But thanks to improvements in astronomical instruments and analytical techniques, astronomers have made new changes in the direction of the search for extraterrestrial civilizations.
In 1992, with the discovery of the first exoplanet, people also began to prefer to determine whether there is life by observing planets, and here is an explanation that planets do not emit light like stars and cannot be seen with the naked eye, so for a long time, astronomers have questioned the reality of the existence of planets outside the solar system, after all, there are no real observations.
(Universality of extrasolar planets)
Therefore, with the continuous improvement of exoplanet detection methods and the effective use of existing space technology, such as the Kepler space telescope launched in 2009, astronomers are becoming more and more likely to detect exoplanets and begin to use the Earth as a sample to find terrestrial planets suitable for life.
Because there is life on Earth, it is possible to judge whether they are in the habitable region of the star system based on the characteristics of the Earth, and it can also give us a better understanding of the distribution of habitable planets, and thus the distribution of life in the universe. This actually has a deeper meaning than just solving the Fermi paradox.
What kind of planets are better suited for life?
Therefore, the search for terrestrial planets is very important, but the universe is so big and there are so many planets, how can we quickly determine the scope suitable for life?
Because the Earth is the only planet known to have life, analyzing the location and characteristics of the Earth can bring a lot of convenience to the search for terrestrial planets. So what kind of planets are suitable for life?
First of all, the position of the planets suitable for life in the entire galaxy must be in the habitable zone of the galaxy, and the galaxy where we are located is too far and too close to the center of the galaxy to be suitable for life. Because the celestial bodies at the center of the Milky Way are too active, for example, the huge black hole at the center of the Milky Way and the frequent supernova explosions will release a large number of cosmic rays, which makes it difficult for complex molecules to form and is naturally not suitable for life.
If it is too far away from the center of the galaxy, the planet will lack heavy elements when it is formed, which means that the planet is not easy to become rocky planets.
So in our galaxy, the galactic habitable zone is thought to be slowly expanding to 25,000 light-years from the galactic core, including stars between 4 billion and 8 billion years old.
Taking the solar system as an example, there must first be a suitable star system, the characteristics of the stars are the most critical part, if the life of the star is too short or the heat is unstable, it is not suitable for life. Then pick a habitable zone from the right star system, just like the solar system, too far or too close to the sun.
From the perspective of the planet's own characteristics, if the mass of the planet is too small, the gravity will also become smaller, and it is not easy to form an atmosphere, resulting in a greatly reduced chance of life, just like Mars, the atmosphere is too thin to stabilize the surface temperature, resulting in the temperature being too low.
There is also the rotation of the planets, if the rotation is too slow, it will lead to the days like years, the most obvious effect is that the temperature difference between day and night is too large, just like one day on the moon is equal to nearly one month on the earth, which leads to uneven heating, one side of the temperature is too high and the other side is too low.
But this idea has also drawn criticism from some people, who believe that everything on Earth cannot be used as a criterion for selection, because it does not necessarily have to be the same as Earth to be suitable for life. Even gas planets like Jupiter may have life, i.e., different forms of life may exist in different environments.
So in 2014, Rene Heller put forward a theory, super habitable planets, also known as super Earths, that is, the Earth is not the most suitable planet for life, and therefore put forward some theories that are more suitable for life. For example, with twice the mass and volume of Earth, these planets produce a denser atmosphere. There are also stars that may be smaller and have a longer lifespan than the Sun, which is more suitable for the development of life, and so on.
How many super-Earths have been discovered so far? Is there life on them?
In 2011, Kepler's mission to search for extraterrestrial stars announced the discovery of 2,326 candidate planets, of which 680 were superEarths and 48 planets in the habitable zone.
But as research progresses, the Kepler team estimates that there are at least 50 billion stars in the Milky Way, at least 500 million of which are in the habitable zone, while some astrologists estimate that at least 30,000 planets within 1,000 light-years of Earth are habitable.
Kepler-22b, for example, was discovered 600 light-years from Earth on December 5, 2011. His radius is 2.4 times that of Earth, but until now his mass and specific composition are unknown. It is speculated that if it is as dense as the Earth, it may be 13 times the mass of the Earth, and if it is as dense as water, it will be 2.5 times the mass.
Judging by its distance from the parent star and the characteristics of the parent star, there is a possibility of life. Because scientists speculate that the average temperature would be -11 degrees if there was no atmosphere on Kepler-22b, and 22 degrees if there was a similar atmosphere.
HD 85512 b is an extrasolar planet located about 36 light-years from Earth, also known as Gliese 370b. Astronomers speculate that it is at least 3.6 times the mass of the Earth, so it is considered a super-Earth and is located right on the edge of the habitable zone.
Its surface gravity is about 1.4G, and the temperature at the top of the atmosphere is estimated at 25 degrees Celsius, which is similar to that in southern France. If it is covered with 50% clouds, water may exist in liquid form on the surface of the planet, creating an atmosphere similar to that of the Earth we live in, and making HD 85512 b habitable.
Gliese 581g was discovered in 2010 and is located about 20.5 light-years from Earth. This newly discovered planet in the "habitable zone" may be the most Earth-like exoplanet ever discovered, and it is also the first conclusive evidence of a potentially habitable planet.
When its discoverer, Stephen Walter, was asked in an interview whether life could exist on the planet, he replied: "I'm not a biologist, but in terms of the nature and Xi of life, I think there's a 100% chance of life on Gliese 581g." ”
Do you think life is possible on these planets, or what planets do you think are more suitable for life, please talk about it in the comment section!