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.

Eighth planet from the sun in our solar system.

Giant, frigid planet with a hazy atmosphere and strong winds. Composed mostly of ice, hydrogen and helium. May have a small, rocky core, and an icy mantle that blends into the atmosphere. Horrendous winds near the spot were measured by Voyager 2 to be about 1,500 miles per hour (2,400 kph). These are the strongest recorded winds in our solar system.

Eight moons.  Three main moons, Nereid, Proteus, and Triton, plus five smaller moons. Triton and Proteus orbit close to Neptune; Nereid is in a distant orbit.

Triton is the only moon in our Solar System with a retrograde orbit (orbiting in the opposite direction than its primary, Neptune, is rotating). Coldest measured object in our Solar System, and Nereid is the Solar System object with the most eccentric orbit.

Has very thin, dark rings located in clumps. Rings composed of small rocks and dust. Almost circular (unlike Uranus' elliptical ring system). Three distinct rings, named Adams, Le Verrier and Galle (after the discovers of Neptune) plus a wide plateau of dust adjacent to the Le Verrier ring (this wide sheet of orbiting dust is co-orbital with the moon Galatea).

Rings have been very difficult to detect because they are not uniform in thickness and density. The thicker part of the rings are called ring arcs; these are the parts of the rings that are more easily detected. The Adams ring has three prominent arcs (named Liberty, Equality and Fraternity). The gravitational pull of Neptune's moons may cause the unevenness of the rings. Some of Neptune's smaller moons may 'shepherd' the inner rings with their gravitational forces.

Neptune's blue color is caused by the methane (CH₄) in its atmosphere; this molecule absorbs red light.

Neptune cannot be seen using the eyes alone.

Neptune was the first planet whose existence was predicted mathematically.  Neptune's existence was predicted in 1846, after calculations showed perturbations in the orbit of Uranus. The calculations were done independently by both J.C. Adams and Le Verrier. Neptune was then observed by J.G. Galle and d'Arrest on September 23, 1846.

About 30,775 miles (49,528 km) in diameter. This is 3.88 times the diameter of the Earth. If Neptune were hollow, it could hold almost 60 Earths.

Fourth largest planet in our Solar System (after Jupiter, Saturn, and Uranus). 

Neptune's mass is about 1.02 x 10²⁶ kg. This is over 17 times the mass of the Earth.

Gravity on Neptune is only 1.19 times of the gravity on Earth. A 100-pound person would weigh 119 pounds on Neptune. 

Each day on Neptune takes 19.1 Earth hours.

A year on Neptune takes 164.8 Earth years; it takes almost 165 Earth years for Neptune to orbit the sun once.

Neptune was discovered in 1846, it has not yet completed a single revolution around the sun.

Neptune is about 30 times farther from the sun than the Earth is.

Occasionally, Neptune's orbit is actually outside that of Pluto; because of Pluto's highly eccentric (non-circular) orbit.  During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.

At aphelion (the point in Neptune's orbit farthest from the sun) Neptune is 4,546,000,000 km from the sun, at perihelion (the point in Neptune's orbit closest from the sun) Neptune is 4,456,000,000 km from the sun.

Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun (this is few degrees more than the Earth).

Neptune has seasons.  Each season lasts 40 years; the poles are in constant darkness or sunlight for 40 years at a time. 

The mean temperature is 48 K.

Neptune was visited by NASA's Voyager 2 in August, 1989. Before this visit, virtually nothing was known about Neptune.

Neptune was named after the mythical Roman god of the seas. Neptune's symbol is the fishing spear.

Neptune's Great Dark Spot is an Earth-sized hurricane in the thick methane atmosphere of Neptune. The size, shape, and location of the spot vary greatly over time; it even disappears and reappears occasionally. The storm spins counterclockwise.