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Basic Facts
Neptune The 8th planet from the sun
Color Blue-green due to presence of methane in the atmosphere
Rings Rings completely circle Neptune, thickness varies along its length
Average distance from the sun 30.06 Astronomical Units (4.5 billion km)
Diameter 49528 km
Length of year (orbital period) ~ 60,000 days (164.8 years)
Rotation period 19.1 hours
Inclination of axis 29.6 degree
Mass 1.02 x 10^26 kg (17.14 x Earth's mass)
Density 1,638 kg/cubicmeters (0.3 x Earth's density)
Average temperature 48 K (-255 degree Celsius)
Atmospheric composition Hydrogen (74%), Helium (25%), Methane (1%)
Number of observed satellites 8
Names of satellities
(accending order according to distance from Neptune)
Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, Nereid
*data from NASA's Planetary Data System

Planetary Nomenclature
Neptune Roman name for the god of the sea, equivalent to Poseidon, Greek god of the sea
Naiad The name of a group of Greek water nymphs who were guardians of lakes, fountains, springs and rivers.
Thalassa Greek sea goddess; mother of Aphrodite in some legends; others say she bore the Telchines.
Despina Daughter of Poseidon (Neptune) and Demeter
Galatea One of the Nereids, attendants of Poseidon
Larissa A lover of Poseidon
Proteus Greek sea god, son of Oceanus (or Poseidon)
Triton The merman, half man, half fish son of Poseidon (Neptune) and Amphitrite
Nereid The Mediterranean nymphs.  The Nereids were the fifty daughters of Nereus and Doris and were attendants of Neptune.