American scientists have compiled a list of planets and satellites, which most likely could be a life. The first and second place in the list of the planets most liveable belong to Saturn’s moon Titan and the exoplanet Gliese 581g, which is believed by astronomers at a distance of 20.5 light-years from Earth in the constellation Libra. An international team of scientists has developed two types of estimating the probability of extraterrestrial life. They published their findings in the journal "Astrobiology".
In the research paper the authors identified two different indices: Index of similarity with Earth (ESI) and the habitability of the planet Index (PHI). "The first question is whether we can find conditions similar to those on other terrestrial planets, since we know empirically that those conditions suitable for life," said co-author of the analysis of Dr. Dirk Shulze-Makuch (Dirk Schulze-Makuch) from the University of Washington , USA. "The second question is that if these conditions are on exoplanets, then there may exist both known and unknown to us the various forms of life."
How is self-explanatory title, ESI index determines the degree of similarity of the planets and satellites of the Earth, taking into account such factors as size, density and distance from the parent star. Index PHI meant a number of other indicators, such as the nature of the planet’s surface, the presence of a magnetic field, its density (as measured by which the mass of the planet is divided into its volume), the presence of the atmosphere, and others.
The scientists also took into account the type of energy - necessary for the life of various organisms - whether it came from the parent star, or was formed as a result of the process is called tidal sensing, in which the gravitational interaction with another object can heat a planet or satellite from the inside.
Finally, the index PHI into account the chemical composition of planets, namely the presence or absence of these organic substances and whether they are suitable for liquid solutions of essential chemical reactions.
According to the degree of similarity between these figures with the earth, all the heavenly bodies were assigned certain factors. The higher the ratio, the more a planet or a satellite similar to the Earth. So, the maximum value for the index of similarity with the Earth was 1.00 - for Earth, unsurprisingly. The highest ratios belonged to such planets of our solar system, as Gliese 581g (whose existence some astronomers are skeptical) - 0.89, and another exoplanet orbitiruyuschey around the same star - Gliese 581d, with the value of the index ESI 0,74. Gliese 581 system has been well studied by astronomers and comprises four, and possibly five, planets orbitiruyuschih this red dwarf star.
HD 69830 d, an extrasolar planet the size of Neptune orbitiruyuschaya different stars in the constellation Feed, also reached the highest level (0.60). According to scientists, it is located in the so-called habitable zone - the region around its parent star where surface temperatures are not too high and not too low for life.
By the presence of the atmosphere (in a certain kind), the amount of light that enters the planet with its parent star, the nature of the surface and the presence of organic compounds, the most similar to planet Earth were Saturn’s moon Titan (0.64), Mars (0.59) , Jupiter’s moon Europa (0.49), as does Jupiter, Venus, Saturn, along with its satellite Enceladus. The latter ratio is 0.35, with the remaining planets in the solar system - not more than 0.37.
In recent years, the search for potentially habitable planets outside our solar system has been very successful. The space telescope "Kepler", launched into orbit by NASA in 2009, discovered more than 1,000 planets where life is possible in theory. Future telescopes will even be able to detect the presence of so-called biomarkers in the light emanating from distant planets to them, for example, is chlorophyll, a pigment that plays a key role in the growth and development of plants.