Mean densities of major and dwarf planets are possible to calculate by the values of the planets’ distance to the Sun, the mean densities of massive natural satellites of planets are computable by the satellite’s distance to the Sun and the primary. The article hypothesizes that the mean density of a body was affected by the gravitational field during the body formation in the formation point, and the gravity was influenced by the Sun and a hypothetical supermassive belt in the region beyond Neptune, and by the primary also, in case of the natural satellites. The mean densities obtained by the traditional methods and through the newly proposed approach characterize different life stages of celestial bodies, and the comparative analysis of these mean density values can be a useful tool in studying migration of the bodies in the Solar System and in other systems of planets.