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The eighth planet from the Sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky.

When Uranus didn't travel exactly as astronomers expected it to, two mathematicians, working independently of each other, proposed the position and mass of another, as yet unknown planet that could account for Uranus' orbit.

Although "the establishment" ignored the predictions, a young astronomer decided to look for the predicted planet. Thus, Neptune was discovered in 1846. Seventeen days later, its largest moon, Triton, was also discovered.

Nearly 4.5 billion kilometers from the Sun, Neptune orbits the Sun once every 165 years, and therefore it has not quite made a full circle around the Sun since it was discovered. 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.

Neptune has the smallest diameter of our solar system's giant gas planets (including Jupiter, Saturn, and Uranus), so called because they have no solid surfaces. Even so, its volume could hold nearly 60 Earths. Neptune's atmosphere extends to great depths, gradually merging into water and other "melted ices" over a heavier, approximately Earth-sized liquid core. Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun. Its seasons last an incredible 41 years. During the southern summer, the south pole is in constant sunlight for about 41 years, and in northern summer, the north pole is in constant sunlight for about 41 years. Neptune's atmosphere is made up of hydrogen, helium, and methane, the last of these giving the planet its blue color (because methane absorbs red light). 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 south-ern hemisphere. This hurricane-like "Great Dark Spot" was large enough to contain the entire Earth; spun counterclockwise; and moved westward at almost 1,200 km per hour. Recent images from the Hubble Space Telescope show no sign of the "Great Dark Spot," although a comparable spot appeared in 1997 in Neptune's northern hemisphere.

The planet has several rings of varying widths, confirmed by Voyager 2's observations in 1989. The outermost ring, Adams, contains five distinct arcs (incomplete rings) named Liberté, Equalité 1, Equalité 2, Fraternité, and Courage. Next is an unnamed ring coorbital with the moon Galatea, then Le Verrier, Lassell, Arago, and Galle. 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 planet's rotation direction, and is gradually getting closer until it will collide with the planet in about 10 to 100 million years, forming vast rings around Neptune that will rival or exceed Saturn's extensive ring system. Triton is the coldest body yet visited in our solar system; temperatures on its surface are about -235 °C. Despite the deep freeze, Voyager 2 discovered great geysers of gaseous nitrogen on Triton.


Neptune: Facts & Figures

Discovered By: Johann Galle

Date of Discovery: 1846

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.


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