A mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet smaller than Uranus and Neptune, up to 10 Earth masses. Those planets have thick hydrogen–helium atmospheres, probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles). Mini-Neptunes have small cores made of low-density volatiles. Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune. Without a thick atmosphere, it would be classified as an ocean planet instead. An estimated dividing line between a rocky planet and a gaseous planet is around two Earth radii, but for mass, it can vary widely for different planets depending on their compositions. The dividing mass can vary from as low as one Earth mass to as high as 20 Earth masses.
Several exoplanets have been discovered that are possibly gas dwarfs, based on known masses and densities. For example, Kepler-11f has a mass of 2.3 Earth masses, yet its density is the same as that of Saturn, implying that it is a gas dwarf with a liquid ocean surrounded by a thick hydrogen–helium atmosphere and only a small rocky core. The even smaller Kepler-138d, having only roughly Earth's mass, is also a suspected gas planet due to its relatively large diameter (~ 20500 km) and its consequently low density. Such planets should not orbit too close to their parent stars, otherwise their thick atmospheres would be evaporated by heat or blown away by stellar winds. It is demonstrated that the inner planets of the Kepler-11 system have higher densities than planets farther away.
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