Description:

The existence of Neptune, the last known giant, out from the Sun, was first prdicted in the 19th Century by two astronomers: John Couch Adams in England and Urbain Jean Joseph Leverrier in France. Both had calculated the position of the hypothetical eighth planet, these calculations were aimed to explain the gravitational perturbation being caused on the seventh planet, Uranus as it revolved in its orbit around the Sun. In 1846, at the Berlin Observatory, astronomer Galle observed a star-like object of the eight magnitude on the same night as he received Le Verrier's amphemerides. He almost instantly recognised it as a new planet. This proved a great triumph for theoretical astronomy.

Image: Neptune
Source: NASA/NSSDC

Neptune is the second last planet of our Solar System and has an orbital period of about 164.79 years, which means that it has not yet made a complete revolution around the sun since the time its discovery!!
Neptune is slightly small planet thanUranus, having a diameter of 49,500 km. The surface of Neptune has an atmosphere of hydrogen and appreciable quantities of methane gas, which give it the same bluish color as Uranus. Its core may be of rock-iron at about 7000 degree C. This planet also emits more energy then it receives from the Sun. It has a fast rotation period of 16 hours 3 mins. The axis of this planet is inclined at an angle of 29 degrees.
Provious of Voyager 2, encounter with this gas giant in August 1989, very little was known about this planet and even less about its to then known moons -Triton, Nereid. At the enormous distance of about 45000 million kms away from the Sun, Neptune was found by Voyager, to be an extremely tutbulent world with a great dark spot. Clouds high above the surface were photographed in great detail.Voyager discovered six new moons and thin rings encircling the planet.
Triton, the biggest of the eight Neptunian satellites, was found to have live volcanoes joining the select group of Earth and Io. While Triton takes less than 6 days to circle Neptune, Nereid, the other major satellite takes almost one year to compelete one revolution.

Interesting Facts:

-Neptune is slightly more than 17 times as massive as the Earth.
-The Sun is 1096 times as bright from the Earth as from Neptune.
-Neptune 's volume is 59 times as great as the Earth.
-The fastest winds in the Solar System have been measured on Neptune as 2400 km/hr.