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A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having as result a total radius between 1.7 and 3.9 Earth radii (). The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than and planets greater than , namely ice giants and gas giants.〔(Three regimes of extrasolar planets inferred from host star metallicities ), Buchhave et al.〕 No gas dwarfs are known to exist in the Solar System, but they are common in other planetary systems. Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out.〔(Mass-radius relationships for exoplanets ), Damian C. Swift, Jon Eggert, Damien G. Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, and Gilbert W. Collins〕 The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope.〔(Earth-mass exoplanet is no Earth twin – Gaseous planet challenges assumption that Earth-mass planets should be rocky )〕 A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.〔 *(Mass-Radius Relationships for Very Low Mass Gaseous Planets ), Konstantin Batygin, David J. Stevenson, 18 Apr 2013〕 == References == 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「Gas dwarf」の詳細全文を読む スポンサード リンク
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