A mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet of up to 10 Earth masses (M⊕), less massive than Uranus and Neptune, which have about 14.5 M⊕ and 17 M⊕, respectively. Mini-Neptunes 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). These planets 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. In fact, empirical observations are showing that planets larger than approximately 1.6 Earth-radius (more massive than approximately 6 Earth-masses) contain significant amounts of volatiles or H–He gas (such planets appear to have a diversity of compositions that is not well-explained by a single mass–radius relation as that found in rocky planets). Similar results are confirmed by other studies. As for mass, the lower limit can vary widely for different planets depending on their compositions; the dividing mass can vary from as low as one to as high as 20 M⊕.
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 M⊕, yet its density is the same as that of Saturn, implying that it is a gas dwarf with a thick hydrogen–helium atmosphere and only a small rocky core. The even smaller Kepler-138d, having only roughly Earth's mass, is also suspected to be a gas planet due to its relatively large diameter (~20,500 km) and therefore low density. Such planets are not found orbiting too close to their parent stars, where 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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