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How Many Earth-like Planets Have Been Found?

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Currently no Earth-like planets have been found. (“Earth analog” is another term for Earth-like planet.) However, bear in mind that at the moment, there is actually no precise definition of what would qualify as an “Earth-like planet” (although vague notions abound but there is no concensus on a precise definition). Do we mean a planet with a similar mass, size, and temperature range to Earth? How similar in each parameter, exactly? What about the compositon of a potential candidate, even if there were some precisely defined parameter ranges that were agreed upon and satisified by the candidate? How similar in composition would it have to be? What assumptions should we make for calculating the candidate planet's habitable zone. Stay tuned on this topic! (Note that the Planetary Habitability Laboratory (PHL) proposes a quantity they call the Earth Similarity Index (or ESI). However, this metric has several problems which you can read about in the article on ESI.)

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Despite the above caveats, attempts have been made to estimate the occurrence rate of Earth-like habitable planets. For example, a study by Catanzarite and Shao (2011) concluded that 1.1(+0.6,-0.3)% of sun-like stars could host a habitable planet. This was based on an early version Kepler mission exoplanet candidates sample, along with some very specific assumptions about the habitable zone and a specific host star selection recipe. Less conervative assumptions about the habitable zone gave a higher occurrence rate of 2.8(+1.9,-0.9)%. (Refer to Catanzarite and Shao (2011) for what exactly is meant by sun-like star.)

Note that the term Earth-size is not to be confused with Earth-like. The former term usually refers to a planet that has a size of 25% higher than Earth or less, and implies nothing about any other physical property.

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File under: Earth-like planets and Earth analogs; Definition of Earth-like planet.

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