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 two Earth masses 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, because otherwise their thick atmospheres would be blown away by stellar winds. It is demonstrated that the inner planets of the Kepler-11 system have higher densities than planets farther away.
- "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
- Cowen, Ron (6 January 2014). "Earth-mass exoplanet is no Earth twin : Nature News & Comment". Nature.
- 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.