OUR SUN	MERCURY	VENUS	EARTH	MARS
JUPITER	SATURN	URANUS	NEPTUNE	PLUTO

NEPTUNE

         Discovered in 1846 by Galle following predictions by Leverrier based on analysis of the observed motion of Uranus, the planet in many ways resembles that discovered by Herschel. It is 3.8 times bigger than the Earth and 17.2 times as massive.

         Neptune circles the Sun every 168 years. Its telescopic appearance and colour are similar to those of Uranus. Its spectrum is dominated by absorption bands of methane and hydrogen. As in the case of Uranus, the tiny disk appears featureless. The spin period is about 18.2 hours.

         The temperature of the upper atmosphere is higher than that of Uranus, suggesting a local, non-solar source of energy. It has been speculated that the extra heating may come from tides generated by Neptune's large satellite, Triton, which orbits the planet once every 5.9 days in a retrograde sense. While the exact size of Triton is uncertain, the diameter is between 2,800 and 3,600 km, making Triton one of the largest satellites in the solar system. It is possible that the satellite has a thin atmosphere that contains methane.

         One implication of Triton's close retrograde orbit is that tides must be reducing the size of the orbit, so that Triton is slowly spiralling in towards Neptune. One estimate is that Triton will crash into the planet in about 100 million years!!!

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