Astronomy:Gas dwarf

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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 (1.7–3.9 R). 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 1.7 R and planets greater than 3.9 R, namely ice giants and gas giants.[1]

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.[2][3]

The smallest known extrasolar planet that might be a gas dwarf is Kepler-138d, which is less massive than Earth but has a 60% larger volume and therefore has a density (2.1(+2.2/-1.2) grams per cubic centimetre) that indicates either a substantial water content[4] or possibly a thick gas envelope.[5]

A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.[6]


  1. Three regimes of extrasolar planets inferred from host star metallicities, Buchhave et al.
  2. Feng Tian; Toon, Owen B.; Pavlov, Alexander A.; De Sterck, H. (March 10, 2005). "Transonic hydrodynamic escape of hydrogen from extrasolar planetary atmospheres". The Astrophysical Journal 621: 1049–1060. doi:10.1086/427204. Bibcode2005ApJ...621.1049T. 
  3. Mass-radius relationships for exoplanets, Damian C. Swift, Jon Eggert, Damien G. Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, and Gilbert W. Collins
  4. Jontof-Hutter, D; Rowe, J (18 June 2015). "Mass of the Mars-sized Exoplanet Kepler-138b from Transit Timing". Nature 522: 321–323. doi:10.1038/nature14494. PMID 26085271. Bibcode2015Natur.522..321J. Retrieved 16 July 2015. 
  5. Earth-mass exoplanet is no Earth twin – Gaseous planet challenges assumption that Earth-mass planets should be rocky
  6. *Mass-Radius Relationships for Very Low Mass Gaseous Planets, Konstantin Batygin, David J. Stevenson, 18 Apr 2013

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