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The Earth Similarity Index (ESI or "easy scale") is a measure of how physically similar a planetary-mass object is to Earth. It is a scale from zero to one, with Earth having a value of one. The ESI was designed to measure planets, but the formula can also be applied to large natural satellites and other objects. The ESI is a function of the planet's radius, density, escape velocity, and surface temperature.〔(【引用サイトリンク】url=http://phl.upr.edu/projects/earth-similarity-index-esi )〕〔Schulze-Makuch, D., Méndez, A., Fairén, A. G., von Paris, P., Turse, C., Boyer, G., Davila, A. F., Resendes de Sousa António, M., Irwin, L. N., and Catling, D. (2011) (A Two-Tiered Approach to Assess the Habitability of Exoplanets. ) Astrobiology 11(10): 1041–1052.〕 These parameters are often estimated based on one or more known variables. Such variables depend greatly on the method of observation used. For example, surface temperature is influenced by a variety of factors including irradiance, tidal heating, albedo, insolation and greenhouse warming. Where these are not known, planetary equilibrium temperature is frequently used, or the variable is inferred from other known attributes. A planet with a high ESI (values in the range from 0.8 and 1.0) is likely to be of terrestrial rocky composition. ESI is not a measure of habitability, although given the point of reference being Earth, some of its functions match closely to those used by habitability measures. The ESI and habitable zone share in common the use of surface temperature as a primary function (and the terrestrial point of reference). According to this measure there are no other Earth-like planets or moons in the Solar System (second-ranked Mars is 0.697), although a number of exoplanets have been found with values in this range. Kepler-438b has the highest Earth Similarity of confirmed exoplanets at 0.88. On November 4, 2013, astronomers reported, based on data gathered by the ''Kepler'' spacecraft, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists.〔〔 == Formulation == The ESI is defined by the expression : where is one of the planetary properties (e.g. surface temperature), is the corresponding terrestrial reference value (e.g. 288 K) for the property, is a weight exponent for the property, and n is the total number of planetary properties. The weight exponents adjust the sensitivity of the scale and equalize their meanings across the different properties. The set of properties, their reference values, and their weight exponents are found in the following table. ESI has been split into two components to measure different aspects of physical similarity: the Interior ESI and the Surface ESI. The mean radius and bulk density constitute the Interior ESI, while the escape velocity and surface temperature constitute the Surface ESI. Global ESI is typically cited as the global measure. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Earth Similarity Index」の詳細全文を読む スポンサード リンク
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