NASA: the number of habitable planets in our galaxy is much less than is commonly believed
As you know, the habitable zone around the star is the area where water can exist in a liquid state on the surface of the planet. Nevertheless, water alone is not enough; other factors are also needed, which together create favorable conditions for the emergence of life. A team of scientists from NASA refined these conditions, leading to a better understanding of where to look.
After the launch of the Kepler space telescope in 2009, scientists immediately began looking for exoplanets. Using the space observatory, astronomers managed to detect thousands of such objects, which allowed to calculate the total number of earth-like planets in the Milky Way. According to calculations of about 40 billion, they mainly revolve around stars similar to the Sun and around red dwarfs.
Since we know only earthly life, it is logical that scientists considered the most probable occurrence of life where there are conditions similar to earthly. And this means the presence of liquid water and in general the likeness of an exoplanet, which can be considered conditionally suitable for life, our own planet.
But, of course, it would be ideal to detect exoplanets, where complex life, multicellular organisms can occur. And here, just liquid water is not enough. Other factors are necessary, as indicated above. For example, too much carbon dioxide or carbon monoxide means the impossibility of a complex life in the form in which we know it.
NASA Edward Schwiterman and colleagues review termsin which a certain area in the star-planet system can be called suitable for life. If we take into account the need for a not too high concentration of carbon oxides, the habitable zone should be about three quarters of the previous calculations. “Our study will seriously affect the area where it is worth looking for indicators of complex life outside our solar system,” says the specialist.
It is impossible to do without carbon dioxide - it should be enough to raise the ambient temperature, sufficient for the existence of water. Its concentration, however, may be different or change during the development of the planet - as it was with the Earth. But be that as it may, closer to the Sun, the concentration of carbon dioxide and carbon monoxide may not be very large - and just in this case a complex multicellular life can arise.
But in the middle or on the outer edge of the habitable zone, the size of which was calculated earlier, the concentration of these gases must be higher to maintain the existence of water. And this is no longer suitable for the emergence of a complex life.
It is worth giving a vivid example - the planet Kepler-62f. She is often referred to as a worthy candidate for the existence of life. This planet is three times heavier than the Earth, it is located at about the same distance from its star as Venus in our system. But since the star in that system is not too bright, the amount of radiation that reaches Kepler-62f is approximately equal to the amount of radiation that Mars receives.
Yes, if the Kepler-62f has a fairly high concentration of carbon dioxide, then the greenhouse effect can theoretically lead to an increase in temperature and the appearance of water. But in this case, carbon dioxide should be 1000 times more than on Earth at any moment in its history. And this is not suitable for multicellular complex organisms.
According to modern scientists, in the past of the Earth there were times when the concentration of carbon dioxide rose at times. And these moments are associated with the mass extinction of living organisms. Experts believe that the tolerance of organisms to carbon dioxide has its limits, and this must be taken into account when searching for candidates for earth-like planets where there are living conditions. Kepler-62f in the end may not be such a good option, as previously thought.
The problem for life is also carbon monoxide. According to Schwiterman and colleagues, there should be a lot of carbon monoxide on planets that revolve around relatively cool stars of carbon monoxide, and this is a negative factor that reduces the likelihood of life on such planets.
As a result, astronomers came to the conclusion that one can not expect the manifestation of indicators of the existence of life (and even more so, intelligent life) on planets that revolve around class M stars.
It may well be that teams of specialists who are engaged in the search for extraterrestrial life will look for it around stars that are very similar to the sun. Even so, there are hundreds of millions of candidates for the role of potentially inhabited planets. Not billions, as it is now, but still this is also a very significant number.
Ref: arxiv.org/abs/1902.04720
