A mini-Neptune is a planet less massive than Neptune but resembles Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans. A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having as result a total radius between 1.7 and 3.9 Earth radii. The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than and planets greater than, namely ice giants and gas giants.
Properties
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 1.6-2.0 Earth radii. 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 contain significant amounts of volatiles or H–He gas, likely acquired during formation. 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. 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. Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out. A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. Neptune like planets are considerably rarer than sub-Neptunes, despite being only slightly bigger. This “radius cliff” separates sub-Neptunes from Neptunes. This radius-cliff is thought to arise because during formation when gas is accreting, the atmospheres of planets that size reach the pressures required to force the hydrogen into the magma ocean stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas.
