Ice Giant

Ice Giants are giant planets similar in composition to Uranus and Neptune. In contrast to gas giants, they are only about 20% hydrogen and helium, with the rest of their composition being heavier elements and compounds such as ammonia, water and methane. Like gas giants, they are organised into layers: Theories suggest that immense pressures could separate the carbon atoms from the methane molecules that are abundant in ice giants and force them into diamonds, creating diamond hailstorms, diamond oceans and possibly even diamond icebergs.
 * A rocky core
 * A fluid-ice mantle of ammonia, water and methane
 * An outer envelope of hydrogen, helium and methane (which gives them a bluish tint)

Ice Giants, like gas giants, have magnetic fields (which, like those of gas giants, can extend beyond some of their moons), but as a result of convection currents in their fluid mantles, as they lack a metallic layer that can be ionized like a gas giant’s.

Ice Giants are found in between the H2O and H/He bands on the Mass-Radius graph, and range in radius from about 2 R⊕ to 6 R⊕.

Worldbuilding in Practice
"Aerope is a fairly standard ice giant of 0.1 MJ and 0.36 RJ, orbiting the F class star Agamemnon every 194 years at a distance of 37.3 AU. Despite only having 13 moons, Aerope has enough mineral wealth to warrant its own colony, and boasts the Aerope 6 space station, a galaxy-renowned spaceport infamous for its relaxed laws on gambling and prostitution."