Neptune (IPA: [ˈnɛptun]) is the eighth and furthest planet from the Sun in our solar system. It is the fourth largest planet by diameter and the third largest by mass; Neptune is 17 times the mass of Earth and is slightly more massive than its near twin Uranus, which is 14 Earth Masses, but slightly smaller due to its higher density. The planet is named after the Roman god of the sea. Its astronomical symbol (♆, Unicode U+2646) is a stylized version of Poseidon's Trident. A portion of its orbit (approximately 10 of every 250 years) lies farther from the Sun than the dwarf planet Pluto's.

Neptune's atmosphere is primarily composed of hydrogen and helium, with traces of methane that account for the planet's blue appearance. Neptune's blue color is much more vivid than that of Uranus, which has a similar amount of methane, so an unknown component is presumed to cause Neptune's intense color. ^[1] Neptune also has the strongest winds of any planet in the solar system, with estimates as high as 2,500 km/h or 1,500 mph. At the time of the 1989 Voyager 2 flyby, it had in its southern hemisphere a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune's temperature at its cloud tops is usually close to −210℃ (−346°F), one of the coldest in the solar system, due to its long distance from the sun. Neptune's center is about 7,000℃ (13,000°F), however, hotter than the sun's surface. This is due to extremely hot gases and rock in the center.

Faint azure colored rings have been detected around the blue planet, but are much less substantial than those of Saturn. When these rings were discovered by a team led by Edward Guinan, it was thought that they might not be complete. However, this was disproved by Voyager 2. Neptune possesses thirteen confirmed moons. Neptune's largest moon, Triton, is notable for its retrograde orbit, extreme cold (38K), and extremely tenuous (14 microbar) nitrogen/methane atmosphere.

Discovered on September 23, 1846, Neptune is notable for being the first planet discovered based on mathematical prediction rather than regular observations. Perturbations in the orbit of Uranus led astronomers to deduce Neptune's existence. It has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL and the California Institute of Technology.^[2]

Naming

Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name came from Galle. He proposed the name Janus. In England, Challis put forth the name Oceanus, particularly appropriate for a seafaring people. In France, Arago suggested that the new planet be called Leverrier, a suggestion which was met with stiff resistance outside France. French almanacs promptly reintroduced the name Herschel for Uranus and Leverrier for the new planet.

Meanwhile, on separate and independent occasions, Adams suggested altering the name Georgian to Uranus, while Leverrier (through the Board of Longitude) suggested Neptune for the new planet. Struve came out in favor of that name on December 29, 1846, to the Saint Petersburg Academy of Sciences.^[3] Soon Neptune became the internationally accepted nomenclature. In Roman mythology, Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except for Uranus, were named in antiquity.

The planet's name is translated literally as the sea king star in the Chinese,^[4] Korean, Japanese, and Vietnamese languages (海王星 in Chinese characters, 해왕성 in Korean).

In India the name given to the planet is Varuna (Devanagari वरुण) the god of the sea in Vedic / Hindu mythology, the equivalent of Poseidon/Neptune in the greco-roman mythology.

Physical characteristics

Relative size

At 1.0243×10²⁶ kg Neptune is an intermediate body between Earth and the largest gas giants: it is seventeen Earth masses but just 1/18th the mass of Jupiter. It and Uranus are often considered a sub-class of gas giant termed "ice giants", given their smaller size and important differences in composition relative to Jupiter and Saturn. In the search for extra-solar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes"^[5] just as astronomers refer to various extra-solar "Jupiters."

Composition

Orbiting so far from the sun, Neptune receives very little heat with the uppermost regions of the atmosphere at −218 °C (55 K). Deeper inside the layers of gas, however, the temperature rises steadily. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space.

The internal structure resembles that of Uranus. There is likely to be a core consisting of molten rock and metal, surrounded by a mixture of rock, water, ammonia, and methane. The atmosphere, extending perhaps 10 to 20 percent of the way towards the center, is mostly hydrogen and helium at high altitudes (80% and 19%, respectively). Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere. Gradually this darker and hotter area blends into the superheated liquid interior. The pressure at the center of Neptune is millions of times more than that on the surface of Earth. Comparing its rotational speed to its degree of oblateness indicates that it has its mass less concentrated towards the center than does Uranus.

Magnetic field

Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres) from the planet's physical center. Comparing the magnetic fields of the two planets, scientists think the extreme orientation may be characteristic of flows in the interior of the planet and not the result of Uranus' sideways orientation.

Weather

One difference between Neptune and Uranus is the level of meteorological activity. Uranus is visually quite bland, while Neptune's high winds come with notable weather phenomena. Neptune's atmosphere has the highest wind speeds in the solar system, thought to be powered by the flow of internal heat, and its weather is characterized by extremely violent hurricanes, with winds reaching up to 2000 km/h.

In 1989, the Great Dark Spot, a cyclonic storm system the size of Eurasia, was discovered by NASA's Voyager 2 spacecraft. The storm resembled the Great Red Spot of Jupiter. However, on November 2, 1994 the Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere. The reason for the Great Dark Spot's disappearance is unknown. One possible theory is that heat transfer from the planet's core disrupted the atmospheric equilibrium and disrupted existing circulation patterns. The Scooter is another storm described as a white cloud south of the Great Dark Spot. The Wizard's eye (Great Dark Spot 2) is a southern hurricane, the second most intensive hurricane known to exist on the planet.

Unique among the gas giants is the presence of high clouds casting shadows on the opaque cloud deck below. Though Neptune's atmosphere is much more dynamic than that of Uranus, both planets are made of the same gases and ices. Uranus and Neptune are not strictly gas giants similar to Jupiter and Saturn, but are rather ice giants, meaning they have a larger solid core and are also made of ices. Neptune is very cold, with temperatures as low as -224°C (-372°F) recorded at the cloud tops in 1989.

Exploration of Neptune

The closest approach of Voyager 2 to Neptune occurred on August 25, 1989. Since 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.

The probe also discovered the Great Dark Spot, which has since disappeared, according to Hubble Space Telescope observations. Originally thought to be a large cloud itself, it was later postulated to be a hole in the visible cloud deck.

Neptune turned out to have the strongest winds of all the solar system's gas giants. In the outer regions of the solar system, where the Sun shines over 1000 times fainter than on Earth (still very bright with a magnitude of -21), the last of the four giants defied all expectations of the scientists.

One might expect that the farther one gets from The Sun, the less energy there would be to drive the winds around. The winds on Jupiter were already hundreds of kilometres per hour. Rather than seeing slower winds, the scientists found faster winds (over 1600 km/h) on more distant Neptune.

Scientists now know why this is the case —if enough energy is produced, turbulence is created, which slows the winds down (like those of Jupiter). At Neptune however, there is so little energy, that once winds are started, they meet very little resistance, and are able to maintain extremely high velocities.

Planetary rings

Neptune has a faint planetary ring system of unknown composition. The rings have a peculiar "clumpy" structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.

Evidence that the rings are incomplete first arose in the mid-1980s, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings. The outermost ring, Adams, contains three prominent arcs now named Liberté, Egalité, and Fraternité (Liberty, Equality, and Fraternity). The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs.

Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the centre of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.^[6]

New Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. In particular, it seems that the Liberté ring might disappear in as little as one century. The new observations appear to throw our understanding of Neptune's rings into considerable confusion.^[7]

Name of ring	Radius (km)	Width (km)	Notes
1989 N3R ('Galle')	41,900	15	Named after Johann Galle
1989 N2R ('Leverrier')	53,200	15	Named after Urbain Le Verrier
1989 N4R ('Lassell')	55,400	6	Named after William Lassell
Arago Ring	57,600	-	Named after François Arago
Liberté Ring Arc	62,900	-	"Leading" arc
Égalité Ring Arc	62,900	-	"Equidistant" arc
Fraternité Ring Arc	62,900	-	"Trailing" arc
Courage Ring Arc	62,900	-
1989 N1R ('Adams')	62,930	<50	Named after John Couch Adams

Natural satellites

Neptune has 13 known moons. The largest by far, and the only one massive enough to be spheroidal, is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons, Triton has a retrograde orbit, indicating that it was captured, and probably represents a large example of a Kuiper Belt object (although clearly no longer in the Kuiper Belt). It is close enough to Neptune to be locked into a synchronous orbit, and is slowly spiraling inward and eventually will be torn apart when it reaches the Roche limit. Triton is the coldest object that has been measured in the solar system, with temperatures of 38.15K (-235°C, -392°F).

Triton, compared to Earth's Moon
Name   (Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km)	Orbital period (days)
Triton	ˈtraɪtən	2700 (80% Luna)	2.15×1022 (30% Luna)	354,800 (90% Luna)	-5.877 (20% Luna)

Neptune's second known satellite (by order of distance), the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the solar system.

From July to September 1989, Voyager 2 discovered six new Neptunian moons. Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity. Although the second most massive Neptunian moon, it is only one quarter of one percent of the mass of Triton. Neptune's innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be within Neptune's rings. The next farthest out, Larissa was originally discovered in 1981 when it had occulted a star. This was attributed to ring arcs, but when Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004.^{[8] [9]} As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.

Appearance and visibility from Earth

Neptune is never visible with the naked eye. The brightness of Neptune is between magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. With the use of a telescope it appears as a small blue-green disk, similar in appearance to Uranus; the blue-green color comes from the methane in its atmosphere. Its small apparent size has made it almost impossible to study visually; even observatory data was fairly poor until the advent of adaptive optics.

With an orbital period of 165 years, Neptune will soon return to the approximate position in the sky where Galle discovered it. This will happen three different times. These are April 11, 2009, when it will be in prograde motion; July 17, 2009, when it will be in retrograde motion; and finally for the last time for the next 165 years, on February 7, 2010. This is explained by the concept of retrogradation. Like all planets in the solar system beyond Earth, Neptune undergoes retrogradation at certain points during its synodic period. In addition to the start of retrogradation, other events within the synodic period include astronomical opposition, the return to prograde motion, and conjunction to the Sun.

In its orbit around the Sun, Neptune will return to its original point of discovery in August 2011.

Voyager flyby

In 1989, Voyager II flew by Neptune and the images relayed back to Earth became the basis of a PBS all-night program called Neptune All Night.