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An Earth analog (also referred to as a Twin Earth, Earth Twin, Second Earth, Alien Earth, Earth 2 or Earth-like planet) is another planet or moon with environmental conditions similar to those found on the planet Earth. The possibility is of particular interest to humans under reasoning that the more similar a planet is to Earth, the more likely it is of sustaining complex extraterrestrial life. As such, it has long been speculated and the subject expressed in science, philosophy, science fiction and popular culture. Advocates of space colonization have long sought an Earth analog as a "second home", while advocates for space and survival would regard such a planet as a potential "new home" for mankind. Before the scientific search for and study of extrasolar planets, the possibility was argued through philosophy and science fiction. The mediocrity principle suggests that planets like Earth should be common in the universe, while the Rare Earth hypothesis suggests that they are extremely rare. Philosophers have pointed out that the size of the universe is such that a near-identical planet must exist somewhere. In the future, technology may be used by humans to artificially produce an Earth analog by terraforming. The multiverse theory suggests that an Earth analog could exist in another universe or even be another version of the Earth itself in a parallel universe. On November 4, 2013, astronomers reported, based on ''Kepler'' space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarf stars within the Milky Way galaxy. 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.〔〔 Scientific findings since the 1990s have greatly influenced the scope of the fields of astrobiology, models of planetary habitability and the search for extraterrestrial intelligence (SETI). NASA and the SETI Institute have proposed categorising the increasing number of planets found using a measure called the Earth Similarity Index (ESI) based on mass, radius and temperature.〔Stuart Gary (New approach in search for alien life ) ABC Online. November 22, 2011〕 According to this measure, as of 23 July 2015, the confirmed planet currently thought to be most similar to Earth on mass, radius and temperature is Kepler-438b.〔http://phl.upr.edu/projects/habitable-exoplanets-catalog/data〕 Scientists estimate that there may be billions of Earth-size planets within the Milky Way galaxy alone.〔〔("Study: 8.8 billion Earth-size, just-right planets", ''ap.org'' )〕 ==History== Around 400 BC, Philolaus proposed that there existed a Counter-Earth, a counterbalance of the same mass to prevent the universe from being flung apart. Between 1858 and 1920, Mars was thought by many, including some scientists, to be very similar to Earth, only drier with a thick atmosphere, similar axial tilt, orbit and seasons as well as a Martian civilization that had built great Martian canals. These theories were advanced by Giovanni Schiaparelli, Percival Lowell and others. As such Mars in fiction portrayed the red planet as similar to Earth but with a desert like landscape. Images and data from the Mariner (1965) and Viking space probes, however, revealed the planet as a barren cratered world. However, with continuing discoveries, other Earth comparisons remained. For example, the Mars Ocean Hypothesis had its origins in the Viking missions and was popularised during the 1980s.〔(NASA - Mars Ocean Hypothesis )〕 With the possibility of past water, there was the possibility that life could have begun on Mars and it was once again perceived to be more Earth-like. Likewise, until the 1960s, Venus was believed by many, including some scientists, to be a warmer version of Earth with a thick atmosphere and either hot and dusty or humid with water clouds and oceans. Venus in fiction was often portrayed as having similarities to Earth and many speculated about Venusian civilization. These beliefs were dispelled in the 1960s as the first space probes gathered more accurate scientific data on the planet and found that Venus is a very hot world with the surface temperature around under an acidic atmosphere 92 times thicker the Earth's. 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 analog」の詳細全文を読む スポンサード リンク
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