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Definition

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IAU edit The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets. The only defining statement issued by the IAU that pertains to exoplanets is a working definition issued in 2001 and modified in 2003. An exoplanet is defined by the following criteria: Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located. Free-floating objects in young star clusters with masses below th...
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General features

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Color and brightness edit In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue. Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color, and Kappa Andromedae b, which if seen up close would appear reddish in color. Helium planets are expected to be white or grey in appearance. The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star. The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making ...
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Planet-hosting stars

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There is at least one planet on average per star. About 1 in 5 Sun-like starsa have an "Earth-sized"b planet in the habitable zone. Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method. Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets. Using data from Kepler , a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity. Some planets orbit one member of a binary star system, and several circumbinary planets have been discovered which orbit around both members of binary star. A few plane...
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