Neptune is very similar to Uranus. It's made of a thick soup of water, ammonia, and methane over an Earth-sized solid center. Its atmosphere is made of hydrogen, helium, and methane. The methane gives Neptune the same blue color as Uranus.
Created with images by WikiImages - "neptune planet solar system" • WikiImages - "neptune planet solar system" • skeeze - "solar system planets space" • topquark22 - "Uranus" • NASA Hubble - "Hubble Sees New Dark Spot on Neptune"