How many Earths are there in the Galaxy?

The answer is, obviously, that nobody knows it. Still, at least. But that does not prevent from being able to estimate the number bearing in mind what we know about the exoplanetas detected up to the date. After all, the search of planets similar to ours is one of the priorities of the modern astronomy.
The planets similar to the Earth receive the name of exotierras, that is to say, a terrestrial extrasolar planet placed in the habitable area of his star. Normally a terrestrial planet is defined as as any world with 1-10 times the mass of the Earth, although other definitions suggest a low limit of 0,3 terrestrial masses. The definition of habitable area is more complex, since it is defined as the area about a star where liquid water of stable form can exist in the surface of a planet. This implies that the principal factors to be born in mind are the luminosity and the effective temperature of the star, what in turn depends on the spectral type and age of the same one. Nevertheless, the definition of habitable area also depends on the characteristics of the planet (period of rotation, atmospheric composition, inclination of the axis, etc.), slightly very complicated to calculate exactly.
If only we bear in mind the characteristics of the star, the area of the habitable area changes enormously according to the spectral type: for example, the habitable area about a red dwarf star (type M) has only a fifth part of the size of the habitable area about a star of solar type (type G). Bearing in mind these variables, we can use numerical methods to calculate the probability of finding a terrestrial planet in his habitable area.
And this is precisely what it has made the team of Jianpo Guo (National Astronomical Observatories, in China), which Métodos Monte Carlo has used to calculate the probability of the existence of exotierras. Supposing that our galaxy has approximately 300 billion stars, we can include in the simulation all those whose masses they are understood between 0,08 and 4 solar masses. The low limit is the correspondent to the minimum of a star (so that supported nuclear fusion exists) and the Superior is the correspondent to stars whose life is of a minimum of 200 million years, minimal time so that the life appears. The result is that the number of terrestrial planets in Vïa Láctea is of approximately 45,5 billions: 11 548 millions about type stars M, 12 930 millions in those of type K, 7622 millions in those of type G and 5556 millions in those of type F.
If we bear in mind that most of the stars of the Galaxy is of type M, the results can seem surprising, since there is more exotierras about stars type K, but it is necessary to bear in mind the small size of the habitable area about the red dwarfs, what explains the low probability of finding a planet in this area.
Although the article of Guo has enough slightly rigorous points (ArXiv is like that) and polemic - including the intelligent calculation of alive worlds - it does not stop being a much brought near estimation of the possible number of planets similar to ours. Until point will correspond to the reality?

More information:

• Probability Distribution of Habitable Terrestrial Planets in Zones around Host Stars, Jianpo Guo et to. (March, 2010).