Neptune, fourth largest of the planets in the solar system, and eighth major planet in order of increasing distance from the sun. The mean distance of Neptune from the sun is 4.5 billion km, and its mean linear diameter is approximately 49,400 km, or about 3.8 times that of the earth.
Its volume is about 72 times, its mass 17 times, and its mean density 0.31 that of the earth. The period of rotation is about 16 hr, and the period of revolution about the sun is 164.79 earth years. The average stellar magnitude of the planet is 7.8, and it is therefore never visible to the naked eye, but it can be observed in a small telescope as a small, round, greenish-blue disk without definite surface markings.
The temperature of the surface of Neptune is about -218 degrees C , much like Uranus, which is more than 1 billion miles closer to the sun. Scientists assume, therefore, that Neptune must have some internal heat source. The atmosphere consists mostly of hydrogen and helium, but the presence of up to three percent methane gives the planet its striking blue color.
Eight known satellites orbit Neptune, two of which are observable from earth. The largest and brightest is Triton, discovered in 1846, the same year Neptune was first observed. Triton, with a diameter of 2705 km, is only slightly smaller than earth's moon. It has a retrograde orbit-that is, opposite its primary's direction of rotation-unlike any other major satellite in the solar system. Despite its extreme coldness, Triton has a nitrogen atmosphere with some methane and some form of haze, and it displays an active surface of geysers that spout an unknown subsurface material.
Nereid, the second satellite, has a diameter of only about 320 km. Neptune is also circled by five thin rings. Further, its magnetic field is tilted more than 50 degrees to the rotation axis.