Neptune  

http://en.wikipedia.org/wiki/Neptune

     Neptune is the eighth planet from the sun. It was discovered on September 23, 1846. It is named after the Roman god of the sea. The Great Dark Spot on Neptune contains super fast winds of up to 2100 km/h. Neptune has 13 known moons the largest being Triton. The blue appearance is caused by traces of methane in the upper atmosphere absorbing the red end of the light spectrum. Neptune has rings around it even though they are less substantial than the ones of Saturn. Its distance from the Sun makes its atmosphere one of the coldest places of the solar system. Surprisingly, Neptune radiates internal heat. Its mass is equivalent to 17 earth masses. Neptune is never visible to the naked eye, because its brightness is between a magnitude of  +7.7 to +8, which is not bright enough to be seen from a great distance.

Structure:

http://www.planetsalive.com/?planet=Neptune&tab=B

     The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of the Earth. The pressure at the center is 7 Mbar (700 GPA), millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.

     The atmosphere gives away to a superheated liquid mantle because there is no crust. The temperatures in the mantle reaches between 2,000 K to 5,000 K. The mantle's weight is about 10 to 15 Earth masses and is mainly made of water, ammonia, and methane. This mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid has a high electrical conductivity. It is called as a water-ammonia ocean. Its depth is 7,000 kilometers (4,300 mi). The conditions here may be such that methane turns into diamond crystals that then rain down towards the core.

     Neptune does not have a surface crust like the other outer planets. Instead its atmosphere starts right after the mantle. The atmosphere is made of 80% hydrogen, 19% helium, and the other one percent being small amounts of methane, water, ammonia, and other ices. The Great Dark Spot contains the fastest winds in the solar system. The upper level clouds form where it is cold enough for methane to condense. Deeper down it might become to 0°C where clouds of water might form.