Neptune is about four times larger than the Earth in diameter, and is the fourth largest planet in our Solar System. Like its near-twin Uranus, most of this bulk is atmosphere. The planet's rocky core alone however is probably about the same size as Earth.
The pressure of the overlying gases, again mostly hydrogen and helium, is more than a million bar. This produces internal temperatures of several thousand degrees Celsius, in sharp contrast to the freezing conditions on the surface.
As with Uranus, methane in Neptune's atmosphere contributes to the blue colour of the planet. As Neptune's main cloud layer lies deeper than Uranus's, light travels further into its atmosphere. This means the planet appears a brilliant blue, whereas Uranus is more green in colour.
The surface temperature on Neptune is brutally cold, around -220 °C. Rapid winds blow around and parallel to the equator, and there is evidence of large storms or vortices like those on Jupiter and Saturn. Neptune's winds are the fastest in the Solar System, reaching 2,000 km per hour. This is surprising as there are only small amounts of solar and internal energy to drive them.
Clouds on Neptune
