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A mini-Neptune or sub-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 hydrogenhelium atmospheres, probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles).[1]

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 1.6-2.0 Earth radii.[3][4] 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 are showing 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.[5][1] Such planets appear to have a diversity of compositions that is not well-explained by a single mass–radius relation as that found for denser, rocky planets.[6][7][8] Similar results are confirmed by other studies.[9][10][11] 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.

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  1. ^ a b D'Angelo, G.; Bodenheimer, P. (2016). "In Situ and Ex Situ Formation Models of Kepler 11 Planets". The Astrophysical Journal. 828 (1): id. 33. arXiv:1606.08088. Bibcode:2016ApJ...828...33D. doi:10.3847/0004-637X/828/1/33.
  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. ^ D'Angelo, G.; Bodenheimer, P. (2013). "Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks". The Astrophysical Journal. 778 (1): 77 (29 pp.). arXiv:1310.2211. Bibcode:2013ApJ...778...77D. doi:10.1088/0004-637X/778/1/77.
  6. ^ Benjamin J. Fulton et al. "The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
  7. ^ Courtney D. Dressing et al. "The Mass of Kepler-93b and The Composition of Terrestrial Planets"
  8. ^ Leslie A. Rogers "Most 1.6 Earth-Radius Planets are not Rocky"
  9. ^ Lauren M. Weiss, and Geoffrey W. Marcy. "The mass-radius relation for 65 exoplanets smaller than 4 Earth radii"
  10. ^ 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"
  11. ^ Geoffrey W. Marcy et al. "Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets"

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