Neptune as seen from Voyager 2.
Diameter 49,528 km
30,775 miles
Discovered by Johann Galle
Urbain Le Verrier
Discovery date September 23, 1846
Moons 14
Size Ranked 4th in the Solar System

Neptune is the eighth planet from the Sun, and it is the outermost major planet in our Solar System. Neptune is slightly smaller but more massive (dense) than Uranus.


Neptune is the god of the sea in Roman mythology, a brother of Jupiter and Pluto. He is analogous but not identical to the god Poseidon of Greek mythology. The Roman conception of Neptune owed a great deal to the Etruscan god Nethuns.


After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846. Seventeen days later, its largest moon, Triton, was also discovered.


Neptune orbits much further away from the Sun than the Earth, so its orbit takes much longer. In fact, Neptune takes 164.79 years to orbit around the Sun.


Deep down inside Neptune, the planet might have an actual solid surface. At the very core of the gas/ice giant is thought to be a region of rock with roughly the mass of the Earth. But temperatures at this region would be thousands of degrees; hot enough to melt rock.


The atmosphere of Neptune is similar to all the large planets in the Solar System; it mostly consists of hydrogen and helium, with trace amounts of methane, water, ammonia and other ices. But unlike the other gas planets in the Solar System, Neptune's atmosphere has a larger proportion of the ices.


Neptune's Bright Streak

The image provides obvious evidence of vertical relief in Neptune's bright cloud streaks.

Neptune has the wildest and strangest weather in the entire Solar System. It has huge storms with extremely high winds. Its atmosphere has dark spots which come and go, and bright cirrus-like clouds which change rapidly. Neptune has an average temperature of -353 Fahrenheit (-214 Celsius).

Great Dark SpotEdit

Neptune's Great Dark Spot

Close up view of the Great Dark Spot.

Neptune's Great Dark Spot, accompanied by white high-altitude clouds, as seen by the Voyager spacecraft. Neptune's atmosphere is made up predominately of hydrogen and helium, with some methane. The methane is part of what gives Neptune its brilliant blue tint, as it absorbs red light and reflects bluer colors.


Neptune has 14 known moons, which are named for minor water deities in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, just 17 days after the discovery of Neptune itself; over a century passed before the discovery of the second natural satellite, Nereid. The moon orbiting farthest from its planet in the Solar System is Neptune's Neso, which has an orbital period of about 26 Julian years.


Voyager 2 at Neptune

Voyager 2 at Neptune in Celestia.

Voyager 2 is the only spacecraft that has visited Neptune. The spacecraft's closest approach to the planet occurred on August 25, 1989. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1's encounter with Saturn and its moon Titan.

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