NASA: the number of habitable planets in our galaxy is much less than is commonly believed

As is known, the habitable zone around the star is considered to be an area where water can exist in a liquid state on the surface of the planet. However, water alone is not enough; other factors are also needed, which together create favorable conditions for the emergence of life. A team of NASA scientists clarified these conditions, which led to a better understanding of where to look.

After the launch of the Kepler space telescope in 2009, scientists immediately began to look for exoplanets. With the help of the space observatory, astronomers were able to detect thousands of such objects, which made it possible to calculate the total number of Earth-like planets in the Milky Way. According to calculations, there are about 40 billion of them, mostly they revolve around stars like the Sun and around red dwarfs.

Since we know only earthly life, it is logical that scientists considered the most likely occurrence of life where there are conditions similar to those on earth. And this means the presence of liquid water and in general a kind of exoplanet, which can be considered conditionally suitable for life, our own planet.
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But, of course, it would be ideal to discover exoplanets, where complex life, multicellular organisms can arise. And here already just liquid water is not enough. Other factors are also needed, as indicated above. For example, too much carbon dioxide or carbon monoxide means the impossibility of the appearance of a complex life in the form in which we know it.

Edward Schwyterman from NASA, together with his colleagues, reviewed the conditions under which a certain area in the star-planet system could be called habitable. Given 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 you should look for indicators of a complex life outside our solar system,” says the specialist.

You can’t 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 in the course of the development of the planet - as it was with the Earth. But be that as it may, the concentration of carbon dioxide and carbon monoxide may not be very large closer to the Sun - and just in this case a complex multicellular life may arise.



But in the middle or at the outer edge of the habitable zone, the size of which was calculated earlier, the concentration of these gases to maintain the existence of water must be higher. And it is no longer suitable for the emergence of a complex life.

It is worth giving an illustrative 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 Earth, it is 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 there is a sufficiently high concentration of carbon dioxide on Kepler-62f, then the greenhouse effect could 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 time in its history. And it is not suitable for multicellular complex organisms.

According to modern scientists, in the past of the Earth there were moments when the concentration of carbon dioxide rose significantly. 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 conditions for life. As a result, Kepler-62f may not be such a good option, as previously thought.

The problem for life is also carbon monoxide. According to Shviterman and colleagues, on planets that revolve around relatively cool carbon monoxide stars there should be a lot, and this is a negative factor that reduces the likelihood of life on such planets.

As a result, astronomers came to the conclusion that we 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 looking for extraterrestrial life will search 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 habitable planets. Not billions, as now, but still it is also quite a significant number.

Ref: arxiv.org/abs/1902.04720