NASA: The number of habitable planets in our galaxy is much smaller than is commonly believed.
As you know, the habitable zone around a star is the area where water can exist in a liquid state on the surface of the planet. However, water alone is not enough, and other factors are needed, which together create favorable conditions for the emergence of life. A team of scientists from NASA clarified 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. With the help of the space observatory, astronomers were able to detect thousands of such objects, which allowed us 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 similar to the Sun and around red dwarfs. Since we know only terrestrial 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 the similarity of an exoplanet, which can be considered conditionally suitable for life, our own planet.
But, of course, it would be ideal to find exoplanets where complex life, multicellular organisms, can arise. And here, just liquid water is not enough. Other factors are also needed, as mentioned above. For example, too much carbon dioxide or carbon monoxide means that it is impossible for complex life to appear as we know it.
NASA’s Edward Schwieterman and his colleagues reviewed the conditions under which a certain region in the star-planet system can be called habitable. If we take into account the need for a not too high concentration of carbon oxides, then the habitable zone should be about three-quarters of the previous calculations. “Our study will have a major impact on the area where it is worth looking for indicators of complex life outside our Solar system,” says the expert.
It is impossible to do without carbon dioxide at all — it should be enough to raise the ambient temperature sufficient for the existence of water. Its concentration, however, can be different or change in the course of the development of the planet — as it was with the Earth. But anyway, closer to the Sun, the concentration of carbon dioxide and carbon monoxide may not be very large — and just in this case, 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 to maintain the existence of water should be higher. And this is no longer suitable for the appearance of a complex life.
It is worth giving a clear example — the planet Kepler-62f. She is often mentioned as a worthy candidate for the existence of life. This planet is three times heavier than Earth, and 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 roughly equal to the amount of radiation that Mars receives.
Yes, if Kepler-62f has a sufficiently 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, there should be 1,000 times more carbon dioxide than on Earth at any point in its history. And this 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 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 conditions for life. In the end, Kepler-62f may not be as good an option as previously thought.
Carbon monoxide is also a problem for life. According to Schwieterman and colleagues, there should be a lot of carbon monoxide on planets that orbit relatively cool stars, 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 orbit M-class stars.
It may well be that teams of specialists who are searching 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 potentially habitable planets. Not billions, as it is now, but it is still quite a significant number.