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A distant blue world
The planet Neptune is considerably larger than the Earth (17 times heavier) and is much further away from the sun (30 times further). The year there lasts 165 earth-years, although the day is shorter at 16 hours (on average).
With being so far from the sun, the temperatures in the upper atmosphere are much colder. Estimates vary around -218 degrees Celsius (55 Kelvin), but deeper down it's much hotter.
The gravity is slightly higher than on the Earth (see gravity on different planets).
Although Neptune is regarded as being very cold (based on the upper atmosphere), there's a lot of weather, with speeds in the hydrogen/helium/methane atmosphere reaching 1000MPH. There are dynamic storm systems which appear as surface features.
Neptune was discovered by Le Verrier in 1846, by knowing approximately where to look based on calculations from the quirks of the orbit of Uranus. Although Galileo had seen the planet Neptune in 1612, he didn't know it was a planet.
There have been ideas there is another Neptune-sized planet out there, but finding it would require noticing significant perturbations in the orbit of Neptune. As Neptune has only completed one orbit since its discovery, the data may or may not be in yet.
Neptune has at least 13 moons, and some faint rings.
There isn't a "surface of Neptune" exactly, so for convenience the radius of the planet is based on the visual size of the planet's atmosphere, which appears blue at a distance. The planet Neptune is often assumed to be similar to Uranus , but the similarities are over-stated!
The best pictures of Neptune currently are the ones from Voyager 2, which went by in 1989.
Here are some helpful references:
Photo of Neptune by NASA released as public domain. http://en.wikipedia.org/wiki/File:Neptune.jpg
There are solar orbits larger than Neptune. There's Pluto, and several others.