Inner planets : rocky composition :: Outer planets : ?

Inner planets are rocky because they formed close to the Sun where temperatures were high and only metals, silicates, and iron could condense. Outer planets formed farther out where it was cold enough for hydrogen, helium, and ices to stick together, creating huge gas‑rich bodies. Thus outer planets are gaseous or icy giants, mainly made of hydrogen, helium, and frozen volatiles. For example, Jupiter’s mass is mostly hydrogen and helium, while Neptune contains a mix of ices and gas. This pattern shows how distance from the Sun determined the planets’ material.