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
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
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
Metric: 24,764 km
English: 15,388 miles
Scientific Notation: 2.4764 x 105 km
By Comparison: 3.883 x Earth
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
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.
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
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
By Comparison: 0.514 x Earth
Orbital Inclination to Ecliptic
Equatorial Inclination to Orbit
By Comparison: 1.261 x Earth
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
Metric: -214 °C
English: -353 °F
Scientific Notation: 59 K
Hydrogen, Helium, Methane
Scientific Notation: H2, He, CH4
By Comparison: Earth's atmosphere consists mostly of N2 and O2.