Structure of Neptune. Cutaway computer illustration showing the internal structure of the planet Neptune. Neptune is the eighth and farthest planet from the Sun. It is the fourth-largest planet by diameter and the third-largest by mass, with 17 times the mass of the Earth. At its centre is an inner core of solid rock and frozen water. Surrounding this is a frozen layer consisting of water, ammonia and methane in solid form. Around this are the inner and outer atmosphere layers, consisting of hydrogen, helium and methane gas.