gas planet mini-Neptune envoAstro transitional planet These planets have a mass up to 10 Earth masses, notably smaller than Uranus or Neptune, which are about 14.5 and 17 Earth masses, respectively. A gas planet which has an atmosphere composed of hydrogen and helium - probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles) - and a small core made of low-density volatiles.