page_title: Earth analog
section_title: Terrestrial
hierarchical_section_title: Earth analog / Attributes and criteria / Terrestrial
caption_attribution_description: Deutsch: Aufnahme der Oberfläche des Saturn-Mondes Titan, die von der Raumsonde Huygens nach deren Landung am 14. Januar 2005 aufgenommen wurde. English: Image of Titan's surface taken by the Huygens probe on 14 January 2005. فارسی: تصویر سطح تیتان که توسط کاوشگر Huygens در تاریخ 14 ژانویه 2005 گرفته شده است. این تنها عکسی است که تاکنون از یک سیاره خارج از سیستم منظومه شمسی گرفته شده است. Português: Imagem da superfície de Titã tirada pela sounda Huygens a 14 de Janeiro de 2005; a cor simulada é baseada em medições espectrais tiradas pela sonda. Українська: Зображення поверхні Титана в місці посадки зонда «Гюйгенс», 14 січня 2005 року.
context_page_description: 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. The thousands of exoplanetary star systems discovered so far are profoundly different from our solar system, supporting the Rare Earth hypothesis.
  Philosophers have pointed out that the size of the universe is such that a near-identical planet must exist somewhere. In the far 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 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. The nearest such planet may be 12 light-years away.
context_section_description: Another criterion often cited is that an Earth analog must be terrestrial, that is, it should possess a similar surface geology—a planetary surface composed of similar surface materials. The closest known examples are Mars and Titan and while there are similarities in their types of landforms and surface compositions, there are also significant differences such as the temperature and quantities of ice.
  Many of Earth's surface materials and landforms are formed as a result of interaction with water (such as clay and sedimentary rocks) or as a byproduct of life (such as limestone or coal), interaction with the atmosphere, volcanically or artificially. A true Earth analog therefore might need to have formed through similar processes, having possessed an atmosphere, volcanic interactions with the surface, past or present liquid water and life forms.
caption: Surfaces like this of Saturn's moon Titan (taken by Huygens probe) bear superficial similarities to the floodplains of Earth
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