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Photo of Neptune taken by Voyager 2

Photo of Neptune taken by NASA's Voyager 2 spacecraft in 1989.

Neptune is the 8th planet from the Sun with an average distance of 30 AU or 4.498 x 109 km. It takes 164.86 Earth years to orbit the Sun, and rotates at a rate of 1 rotation every 16.11 hours. It has a mass of 1.024 x 1026 kg or 17.15 Earth masses. It is more massive than Uranus, but has a smaller diameter. It has 14 confirmed moons, the largest of which are Triton, Nereid and Proteus. The average temperature of Neptune at the cloudtops is about 55 K (−218 °C or −360 °F).

Neptune is the only planet to have been discovered by calculation, rather than by observation. By the 1830s, astronomers who had been carefully observing Uranus' motion noticed that Uranus motion could not be predicted using Newton's Laws, and they theorized that there was another planet exerting gravitational forces on Neptune. Several groups of astronomers began searching for it and on September 23, 1846 it was observed for the first time. Galileo may have observed Neptune as well, but did not identify it as a planet because its motion against the background stars, at 2° per year, is so slow.  Neptune has a ring system with 3 main rings. One of the rings seem to be decaying and may disappear within a century. Neptune, like Uranus, has an unusual magnetic field, which is oriented at a 47° angle to its axis of rotation.

Neptune was visited by Voyager 2 in 1989.

Neptune Factbox
Average distance from Sun 30.066 AU = 4.498 x 109 km = 3.6 million lengths of California
Number of moons 14
Average orbital speed 5.5 km/s
Orbital period 164.86 years
Rotation period 16.11 hours
Inclination of equator to orbit 29.56°
Diameter 49,528 km = 3.883 Earth diameters
Mass 1.024 x 1026 kg = 17.15 Earth masses
Average density 1638 kg/m3
Surface gravity (Earth = 1) 1.1
Albedo 0.51
Average temperature at cloudtops -218°C = -360°F = 55 K
Atmosphere 79% hydrogen, 18% helium, 3% methane