Neptune. Computer artwork of a possible scene below clouds on the planet Neptune. Neptune, a gas giant, is the eighth planet from the Sun. The large distance from the Sun means Neptune's outer layers are around -218 degrees Celsius. Most of the atmosphere is hydrogen and helium, with traces of water and methane. The temperature and pressure rise with depth, and the atmosphere merges into a liquid region of rock, water, methane and ammonia. Deep below this is a rocky core. From space, Neptune appears blue due to the absorption of red light by atmospheric methane. Neptune has the fastest winds in the solar system.