A mini-Neptune (sometimes known as gas dwarf) is a gaseous planet smaller than Uranus and Neptune, up to 10 Earth masses. Those planets have thick hydrogen-helium atmospheres often with layers of deep liquid oceans (made of water, ammonia, mixture of both, or heavier volatiles), ice, or rock below. 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 thick atmosphere, ocean planet would be classified instead. An estimated dividing line between rocky planet and gaseous planet is around two Earth radii, but for mass, it can vary widely for different planet depending on their compositions. The dividing mass can vary from as low as two Earth masses to as high as 20 Earth masses.
There are several exoplanets discovered which are possibly gas dwarfs, based on known masses and densities. For example, Kepler-11f has mass of 2.3 Earth masses, yet its density is the same as Saturn, telling that this planet is a gas dwarf with solid or liquid ocean surrounded by hydrogen-helium atmosphere. Such planets should not orbit too close to the heat of their parent stars otherwise that thick atmosphere would be blown away by stellar winds. It is demonstrated in the Kepler-11 system that inner planets have higher densities than planets orbiting farther away.
See also 
- "Gas Dwarf". Orion's Arm. Encyclopedia Galactica.
- 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),
- 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
- When Does an Exoplanet’s Surface Become Earth-Like?, blogs.scientificamerican.com, 20 June 2012
- Six New Planets: Mini-Neptunes Found Around Sunlike Star, Victoria Jaggard, National Geographic News, Published February 2, 2011
- Barnes, Rory; Jackson, Brian; Raymond, Sean N.; West, Andrew A.; Greenberg, Richard (January 13, 2009). "The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?". The Astrophysical Journal 695 (2): 1006. arXiv:0901.1698. Bibcode:2009ApJ...695.1006B. doi:10.1088/0004-637X/695/2/1006.