Neptune is the fourth largest of the nine planets and the eighth in distance from the Sun.  Its orbit lies about 4.5 billion km out from the Sun and 4.3 billion km from the Earth. It has a diameter of just over 49,000 km, making it almost 4 times larger than the Earth.  However its overall mass is more than 17 times that of the Earth.  It takes over 164 years to complete one orbit around the Sun but rotates about its own axis once every 16 hours.

Neptune is a planet of very low temperatures. The temperature on the surface ranges from a high  -216 to an incredibly low  -360 degrees centigrade.  Its atmosphere is made up almost entirely of three gases - hydrogen, helium and methane.
Eight known satellites orbit Neptune but only two of them are observable from earth.  The largest and brightest of these is Triton, discovered in 1846, the same year that Neptune was first observed.  Triton is only slightly smaller than the Earth's moon.  Nereid, the second observable satellite, is much smaller with a diameter of only 200 miles.
Like Saturn, Neptune also has  a series of rings around the planet which are made of concentrated particles. These rings are a much smaller feature than the rings of Saturn.

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