In Comparison to Earth
Neptune weighs 5.6834×10^26 kg, 95 times more massive than our Earth. The force of gravity on its surface is surpassed only by that of Jupiter. It is approximately 30 times farther from the Sun than our planet is. Its radius at the equator is 58232 km, four times larger than the Earth's.
Observation
One spacecraft, Voyager 2, flew by Neptune in 1989. So unlike most objects in space, astronomers and the general public have actually observed Neptune from a close distance. This video is a montage of a series of images of Neptune taken by Hubble.
Illustration of Neptune Seasons
Atmosphere
Neptune's atmosphere is primarily composed of hydrogen and helium, like Jupiter and Saturn. A small amount of methane is also present. The atmosphere comprises about 5 to 10 percent of the total mass of the planet, and extends 10 to 20 percent of the distance to the center of the planet. Lower regions of the atmosphere contain higher concentrations of methane, ammonia, and water.
Neptune's atmosphere is generally divided into four layers. Going from lower layers to upper are: troposphere, stratosphere, thermosphere, and exosphere. According to the current model, the troposhere is populated by clouds. But unlike water clouds here on Earth, these clouds are believed to be made up of different materials. Clouds higher up may be composed of methane, ones a little lower - from ammonia and hydrogen sulfide, and the lower ones are believed to be formed from ammonia, ammonium sulfide, hydrogen sulfide, and water. Even deeper, clouds of water ice may be found. The picture to the side shows an upper band of clouds that cast shadows on the lower clouds. That image was taken by Voyager 2, as it passed Neptune.
The Mantle
The lower atmosphere layer gradually changes into a liquid mantle. This mantle is believed to be made up of methane, ammonia, and water as well. Even if it was all water, swimming in it would be impossible, since the temperatures go between 2,000K and 5,000K. Such great abundance of water on other planets is very probable, because our universe was created from water, if you read Genesis Chapter 1. Existence of water on other planets does not mean there is life there, however, because life needs much more than water, to survive.
Discovery
The first person in our civilization to discover Neptune was probably Galileo, who called it a star in 1612. In 1846, the planet was named after a mythical god, Roman god of sea in particular. The name of other planets originate from the names of Greek and Roman gods as well. It may have been discovered prior to us by other civilizations too.
When planets of similar mass are discovered outside of the Solar System, they are often called "Neptunes".
Seasons?
Studies over six years have shown that Neptune might have seasons. Observations made, show a clear increase in the amount and brightness of the clouds encircling the planet.
Illustration of Seasons on Neptune
Planetary Rings
Neptune has rings, although they are not as obvious as the ones of Saturn. These rings were named as such: Adams Ring, Le Verrier Ring (after the guy who officially discovered Neptune as a planet in recent history), Galle Ring, Lassell (outward extension of Le Verrier Ring), and the Arago Ring. These rings may be made up of ice particles coated with silicates or carbon-based material.
Weather
The weather is not very pleasant on this planet. For example, when Voyager 2 was passing it, it observed a huge black spot in Neptune's atmosphere, comparable to Jupiter's Great Red Spot. Neptune has more violent winds than any other planet of the Solar System, reaching speeds of 2100km/h.
Neptune's Weather
Neptune is also one of the coldest places in the Solar System (go figure). Temperatures at cloud tops go down to -218 degrees C. Currently, the south pole of Neptune is about 10 degrees warmer than the rest of the planet. The reason for that being so is due to the axial tilt of the planet, the south pole has been exposed to the Sun for the last quarter of Neptune's year (about 40 Earth years). The slight difference in temperature is enough to cause methane at the south pole to melt, while it stays frozen throughout the rest of the planet. As it melts, methane leaks out as gas into space. As Neptune slowly moves to the other side of the Sun, the south pole will darken, and the north pole will lighten, and methane gas will leak from the north pole then.
Because Neptune orbits the Sun at much farther distance than the Earth, seasons on Neptune change in a time of 40 Earth years.
Storms
This picture was taken by Voyager 2 when it passed Neptune, and shows the Great Dark Spot of Neptune, spanning 13000 X 6600 km. This is a storm region similar to the Great Red Spot on Jupiter. But unlike Jupiter's spot, this spot disappeared after five years. Instead, a new storm similar to the Great Dark Spot was discovered in the northern hemisphere of the planet (the lower spot on the picture). Another storm, The Scooter (white spot between the two), is a white cloud group. The Small Dark Spot is a fast moving storm in the southern part of the planet.
Orbit and Rotation
Neptune is about 4.5 billion km from the sun. It orbits the sun once every 164.79 earth years. The axial tilt of Neptune is 28.32 degrees, similar to the tilt of the earth, so Neptune has similar seasonal changes to that of the earth. Its day, though, lasts only 16.11 earth hours. Neptune is not a solid body like the earth. Because it is composed of liquid and gas, different parts of the planet rotate at different speeds with respect to each other.
Moons
13 moons have been discovered to orbit Neptune. Triton is its largest moon. By mass, it comprises 99.5% of all mass orbiting Neptune. It is the coldest object in the Solar System, so far recorded, with temperatures going down to -235 degrees C. The picture to the right, is an illustration of how Neptune would look from Triton.
The second most-massive Neptunian moon is Proteus. With that being the case, it is still only 1/4 of 1 percent of the mass of Triton. Neptune's innermost moons, Naiad, Thalassa, Despina and Galatea, are so close to Neptune that they can orbit within its rings. Since astronomers couldn't come up with a better naming convention, they named the moons after the names of the lesser sea gods.
References
http://nasaimages.org/luna/servlet
http://en.wikipedia.org/wiki/Neptune