Gas dwarf

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. 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.

The smallest known extrasolar planet that might be a "gas planet" 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.

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