Voyager 2 captured this convergence of atmospheric features on Neptune.
Voyager 2 captured this convergence of atmospheric features on Neptune.
The eighth planet from the Sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had recorded it as a fixed star during observations with his small telescope in 1612 and 1613.) When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus' orbit. 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.

Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, due to Pluto's unusual elliptical orbit, Neptune is actually the farthest planet from the Sun for a 20-year period out of every 248 Earth years.

The main axis of Neptune's magnetic field is 'tipped over' by about 47 degrees compared with the planet's rotation axis. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune's magnetosphere undergoes wild variations during each rotation because of this misalignment. The magnetic field of Neptune is about 27 times more powerful than that of Earth.

Neptune's atmosphere extends to great depths, gradually merging into water and other 'melted ices' over a heavier, approximately Earth-sized solid core. Neptune's blue color is the result of methane in the atmosphere. Uranus' blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color that we see. The cause of Neptune's bluish tinge remains a mystery.

Despite its great distance from the Sun and lower energy input, Neptune's winds are three times stronger than Jupiter's and nine times stronger than Earth's. In 1989, Voyager 2 tracked a large oval dark storm in Neptune's southern hemisphere. This hurricane-like 'Great Dark Spot' was observed to be large enough to contain the entire Earth, spun counterclockwise, and moved westward at almost 1,200 kilometers (750 miles) per hour. (Subsequent images from the Hubble Space Telescope showed no sign of the Great Dark Spot photographed by Voyager. A comparable spot appeared in 1994 in Neptune's northern hemisphere but had disappeared by 1997.) Voyager 2 also photographed clouds casting shadows on a lower cloud deck, enabling scientists to visually measure the altitude differences between the upper and lower cloud decks.

The planet has six rings of varying thicknesses, confirmed by Voyager 2's observations in 1989. Neptune's rings are believed to be relatively young and relatively short-lived.

Neptune has 13 known moons, six of which were discovered by Voyager 2. The largest, Triton, orbits Neptune in a direction opposite to the direction of the planet's rotation. Triton is the coldest body yet visited in our solar system - temperatures on its surface are about -235 degrees Celsius (-391 degrees Fahrenheit). Despite this deep freeze at Triton, Voyager 2 discovered geysers spewing icy material upward more than 8 kilometers (5 miles). Triton's thin atmosphere, also discovered by Voyager, has been seen from Earth several times since, and is growing warmer - although scientists do not yet know why.

Neptune: Facts & Figures
Discovered By
Johann Galle
Date of Discovery
Average Distance from the Sun
Metric: 4,498,252,900 km
English: 2,795,084,800 miles
Scientific Notation: 4.4982529 x 109 km (30.069 A.U.)
By Comparison: 30.069 x Earth
Perihelion (closest)
Metric: 4,459,630,000 km
English: 2,771,087,000 miles
Scientific Notation: 4.45963 x 109 km (29.811 A.U.)
By Comparison: 29.820 x Earth
Aphelion (farthest)
Metric: 4,536,870,000 km
English: 2,819,080,000 miles
Scientific Notation: 4.53687 x 109 km (30.327 A.U.)
By Comparison: 30.326 x Earth
Equatorial Radius
Metric: 24,764 km
English: 15,388 miles
Scientific Notation: 2.4764 x 105 km
By Comparison: 3.883 x Earth
Equatorial Circumference
Metric: 155,597 km
English: 96,683 miles
Scientific Notation: 1.55597 x 105 km
Metric: 62,526,000,000,000 km3
Scientific Notation: 6.2526 x 1013 km3
By Comparison: 57.7 x Earth's
Metric: 102,440,000,000,000,000,000,000,000 kg
Scientific Notation: 1.0244 x 1026 kg
By Comparison: 17.147 x Earth's
Metric: 1.76 g/cm3
By Comparison: 0.317 x Earth
Surface Area
Metric: 7,640,800,000 km2
English: 2,950,100,000 square miles
Scientific Notation: 7.6408 x 109 km2
By Comparison: 14.980 x Earth
Equatorial Surface Gravity
Metric: 10.71 m/s2
English: 35.14 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 110 pounds on Neptune.
Escape Velocity
Metric: 85,356 km/h
English: 53,038 mph
Scientific Notation: 23,710 m/s
By Comparison: Escape velocity of Earth is 25,022 mph.
Sidereal Rotation Period (Length of Day)
0.67125 Earth days
16.11 hours
By Comparison: 0.673 x Earth
Sidereal Orbit Period (Length of Year)
164.79 Earth years
60,190 Earth days
Mean Orbit Velocity
Metric: 19,720 km/h
English: 12,253 mph
Scientific Notation: 5,477.8 m/s
By Comparison: 0.490 x Earth
Orbital Eccentricity
By Comparison: 0.514 x Earth
Orbital Inclination to Ecliptic
1.769 degrees
Equatorial Inclination to Orbit
29.58 degrees
By Comparison: 1.261 x Earth
Orbital Circumference
Metric: 28,142,000,000 km
English: 17,487,000,000 miles
Scientific Notation: 2.8142 x 1010 km
By Comparison: 30.44 x Earth
Effective Temperature
Metric: -214 C
English: -353 F
Scientific Notation: 59 K
Atmospheric Constituents
Hydrogen, Helium, Methane
Scientific Notation: H2, He, CH4
By Comparison: Earth's atmosphere consists mostly of N2 and O2.
Neptune's Moons