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.
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 or possibly a thick gas envelope.
A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.
- Three regimes of extrasolar planets inferred from host star metallicities, Buchhave et al.
- 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. Bibcode: 2005ApJ...621.1049T.
- Mass-radius relationships for exoplanets, Damian C. Swift, Jon Eggert, Damien G. Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, and Gilbert W. Collins
- 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. Bibcode: 2015Natur.522..321J. http://www.nature.com/nature/journal/v522/n7556/full/nature14494.html. Retrieved 16 July 2015.
- Earth-mass exoplanet is no Earth twin – Gaseous planet challenges assumption that Earth-mass planets should be rocky
- *Mass-Radius Relationships for Very Low Mass Gaseous Planets, Konstantin Batygin, David J. Stevenson, 18 Apr 2013