Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune. Without a thick atmosphere, they would be classified as an
ocean planet instead. An estimated dividing line between a rocky planet and a gaseous planet is around . 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 show that planets larger than approximately , and more massive than approximately (
Earth masses), contain significant amounts of volatiles or H–He gas, likely acquired during formation. The lower limit for mass can vary widely for different planets depending on their compositions; the dividing mass can vary from as low as to as high as . 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 ( ). This is thought to arise because, during formation when gas is accreting, the atmospheres of planets of 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. ==Examples==