Mini-Neptune

A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles).

[2] Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune.

[4][5] Planets with larger radii and measured masses are mostly low-density and require an extended atmosphere to simultaneously explain their masses and radii, and observations show that planets larger than approximately 1.6 Earth-radius (and more massive than approximately 6 Earth-masses) contain significant amounts of volatiles or H–He gas, likely acquired during formation.

[17] This is thought to arise because, during formation when gas is accreting, the atmospheres of planets of that size reach the pressures required to force the hydrogen into the magma ocean, stalling radius growth.

[20] However, more recent evidence suggests that it may be more dense than previously thought, and could be an ocean planet instead.