Researchers from the Pennsylvania State University (USA) have proposed new borders of the habitable zone for earth-type planets. Using as a basis the corrected absorption coefficients of molecules of water and carbon dioxide, they came to the conclusion that potentially "habitable" planets should be looked for farther away from a star than it was previously considered.

Such work is very important for the planning of future projects aimed at searching for extrasolar planets, but, unfortunately, so far, we lack the understanding of how climate machines are organized; both the authors of the work and its critics accept this.

The driving force of the search for planets outside the Solar system is an attempt to find life outside its only present day shelter. The number of "new worlds" is growing with great rapidity: almost 700 objects are for sure identified as exoplanets around other stars, and more than 2000 "candidates", discovered by the Kepler telescope, are waiting for confirmation. There is no doubt that in the near future this number will grow. It is necessary to sort this abundance, so that later to choose the most interesting objects for a careful study: those that are most likely to become a habitat of living organisms.

The position of the planet in the so-called "habitable zone" may become the selection criterion. "Habitable zone" is the region of space around the star, inside of which the planet of the earth's mass and of a similar composition of atmosphere (consisting of nitrogen, water, and carbon dioxide) can preserve liquid water on its surface. As is known, there is no liquid water on the surface of the two closest to the Earth planets - Venus and Mars.

Not only the temperature of the star, but also the composition and properties of the atmosphere of the planet influence the borders of the habitable zone. All these characteristics are included in the model, on the basis of which the distance is calculated later that is usually measured in astronomical units equaling the distance from the Sun to the Earth (1.5 million kilometers).

The new work of the group of researchers from Pennsylvania State University, NASA, the Washington University and the University of Bordeaux (France) cast doubt on the previous model (its description appeared in an article in 1993) developed by George Casting (who is also included in the group of the authors of the new article). The previous model predicted that the internal and external borders of the habitable zone of our Sun lie between 0.95 and 1.67 astronomical units. However, it is worth noting that rather a simple model of the planet, devoid of clouds, was used in this model. Whatever it was, it turned out that the Earth is on the very edge of the district suitable for life.

However, as indicated by the first author of the new study, Ravi Kumar Kopparapu, Casting's model used outdated, less exact absorption coefficients of solar radiation by molecules of water and carbon dioxide - the most important greenhouse gases. These data are essential for the understanding of how the radiation of the star affects the photochemistry of the atmosphere and, indirectly, the temperature of the surface and retention of water. In addition, the model possessed other inaccuracies, and it did not consider the M-type stars (rather cold and small stars, red dwarfs).

The new work tries to correct these inaccuracies. First of all, the most recent data about the absorption of solar radiation were included in the model. Then, some of the physical mechanisms of radiation's behavior in the atmosphere were clarified. In this work, the researchers provided several conclusions at once: how the habitable zone will be located around the stars of different types and what kind of zones there will be for different planets.

It is interesting, what "zone limitations" this model puts as regards the distance from the Earth to the Sun. It turns out that our planet barely fits within the necessary scope: the internal boundary of the "habitable zone" lies at a distance of 0.99 astronomical units from the Sun, and the external boundary - at a distance of 1.70 astronomical units, that is in general a little farther than Casting's model allowed.

In addition, according to the new model, several exoplanets, for example, HD85512b, fall out of the "habitable zone", although earlier it was supposed that they are in it.

The authors point out, that the model can hardly be considered complete due to the absence of clouds in the atmosphere, which may reflect solar radiation, thereby preventing the heating of the planet, and also serve as a "blanket" for the internal heat, not letting it vanish into space. With the addition of clouds, the borders of the zone can expand in both directions. Besides, experts add that the planet itself can heat due to internal processes; this is also not taken into account in this model.

So, what does the study give us, besides being another rough estimate?

The authors point out that on the basis of the received results it is possible to more accurately "target" the instruments designed for searching for potentially habitable planets, especially around the K- and M-class stars. For example, the future James Webb telescope (JWST) is potentially capable of registering the spectrum of an Earth-like planet orbiting an M-class star. As the JWST is not intended primarily for the study of exoplanets, such researches should be very accurately directed.

The article is published in the Astrophysical Journal. In addition to the text, the authors have also developed a "calculator of habitable zones" - an online program, which helps to calculate the boundaries of the zone for main-sequence stars with temperatures of 2600-7200 degrees Kelvin. Of course, this modeling is based only on today's understanding of the problem. But without these inevitably approximate attempts we can hardly understand the possibility of life in the Universe.

Source: Voice of Russia