Image Courtesy of NASA's Hubble Site.
Neptune is the eighth and farthest planet from the Sun. It is the fourth largest planet in diameter and the third largest in mass. Its mass is around 17 times greater than that of the Earth’s. Neptune is also slightly more massive than Uranus, but it is smaller due to its density.
The atmosphere of Neptune consists mostly of hydrogen, helium, and traces of methane. These gases make up the planet’s bluish color. An unknown component is presumed to be what causes the intensity of the blue color since Uranus has almost the exact same amount of methane. Neptune has the strongest winds in the solar system measuring around 2,100 km/h and 1,300 km/h. At the southern hemisphere of Neptune there is a large dark spot that is similar to the Great Red Spot of Jupiter. Neptune’s temperature at the cloud tops is about −210 °C (−346 °F), but at the center the temperature can reach to hotter than the Sun’s surface, about 7,000 °C (13,000 °F). This is caused by its extremely hot gases.
Neptune was also the first planet to be discovered by other means than observation. It was found mathematically. “Perturbations in the orbit of Uranus led astronomers to deduce Neptune's existence” (Wikipedia). The planet was further proven to have existed when Voyager 2 flew by it on August 25, 1989.