The outermost layer of Neptune is the atmosphere, forming about 5-10% of the planet’s mass, and extending up to 20% of the way down to its core.Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere. Unlike Uranus, Neptune’s composition has a higher volume of ocean, whereas Uranus has a smaller mantle. Like the other gas/ice giants, Neptune is believed to have a solid core, the composition of which is still subject to guesswork. However, the theory that it is rocky and metal-rich is consistent with current theories of planet formation.