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A mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet of up to 10 Earth masses (M), smaller than Uranus and Neptune, which have about 14.5 M and 17 M, respectively. Mini-Neptunes have thick hydrogenhelium 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.[1][better source needed]

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.[2] An estimated dividing line between a rocky planet and a gaseous planet is around two Earth radii.[3][4] 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).[5][6] Similar results are confirmed by other studies.[7][8][9] 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[1] 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 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.[10] Such planets don't 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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  1. ^ a b "Gas Dwarf". Orion's Arm. Encyclopedia Galactica. 
  2. ^ Optical to near-infrared transit observations of super-Earth GJ1214b: water-world or mini-Neptune?, E.J.W. de Mooij (1), M. Brogi (1), R.J. de Kok (2), J. Koppenhoefer (3,4), S.V. Nefs (1), I.A.G. Snellen (1), J. Greiner (4), J. Hanse (1), R.C. Heinsbroek (1), C.H. Lee (3), P.P. van der Werf (1),
  3. ^ Architecture of Kepler's Multi-transiting Systems: II. New investigations with twice as many candidates, Daniel C. Fabrycky, Jack J. Lissauer, Darin Ragozzine, Jason F. Rowe, Eric Agol, Thomas Barclay, Natalie Batalha, William Borucki, David R. Ciardi, Eric B. Ford, John C. Geary, Matthew J. Holman, Jon M. Jenkins, Jie Li, Robert C. Morehead, Avi Shporer, Jeffrey C. Smith, Jason H. Steffen, Martin Still
  4. ^ When Does an Exoplanet’s Surface Become Earth-Like?,, 20 June 2012
  5. ^ Courtney D. Dressing et al. "The Mass of Kepler-93b and The Composition of Terrestrial Planets"
  6. ^ Leslie A. Rogers "Most 1.6 Earth-Radius Planets are not Rocky"
  7. ^ Lauren M. Weiss, and Geoffrey W. Marcy. "The mass-radius relation for 65 exoplanets smaller than 4 Earth radii"
  8. ^ Geoffrey W. Marcy, Lauren M. Weiss, Erik A. Petigura, Howard Isaacson, Andrew W. Howard and Lars A. Buchhave. "Occurrence and core-envelope structure of 1-4x Earth-size planets around Sun-like stars"
  9. ^ Geoffrey W. Marcy et al. "Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets"
  10. ^ Cowen, Ron (6 January 2014). "Earth-mass exoplanet is no Earth twin : Nature News & Comment". Nature. doi:10.1038/nature.2014.14477. 

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