Size: 49 500 km diameter (3.9 x Earth)
 Surface Temperature: -240°C
  • 86 parts Hydrogen
  • 19 parts Helium
  • 1 part Methane
 Spin: 16 hours 7 minutes
 Year: 163 years 9 months
  • Naiad
  • Thalassa
  • Depoina
  • Galatea
  • Larissa
  • Proteus
  • Triton
  • Nereid

Neptune is the eighth or, occasionally, the ninth planet from the Sun due to Pluto's eccentric orbit, and the outermost gas giant in our solar system. Although the smallest of the gas giants, Neptune is more massive than Uranus: its stronger gravitational field has compressed it to a higher density. The planet is named after the Roman god of the sea because of its blue methane clouds. Its astronomical symbol is a stylized version of the god's trident.

Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier.

Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining known 14 moons were located telescopically until the 20th century. The planet's distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar.

Neptune is similar in composition to Uranus, and both have compositions that differ from those of the larger gas giants, Jupiter and Saturn. Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, but contains a higher proportion of "ices" such as water, ammonia, and methane. However, its interior, like that of Uranus, is primarily composed of ices and rock, and hence Uranus and Neptune are normally considered "ice giants" to emphasize this distinction. Traces of methane in the outermost regions in part account for the planet's blue appearance.

In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns. For example, at the time of the Voyager 2 flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (580 m/s; 1,300 mph).

Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K (-218° C). Temperatures at the planet's centre are approximately 5,400 K (5,100° C). Neptune has a faint and fragmented ring system (labelled "arcs"), which was first detected during the 1960s and confirmed by Voyager 2.

Temperatures on Neptune Are Mysteriously Fluctuating, And It Doesn't Make Sense   Science Alert - April 11, 2022
An unprecedented analysis of changes in the temperature of Neptune has revealed something rather strange taking place at the most distant planet in our Solar System. Far-flung Neptune, which orbits the Sun at about 30 times the distance of Earth, takes an accordingly long time to make its trek around the star: about 165 Earth years, in fact, meaning each season on Neptune lasts for over 40 Earth years. A new study compiling 17 years' worth of observations of Neptune's temperature instead shows the opposite taking place, with readings indicating a mysterious globally averaged drop of about 8 °C between 2003 and 2018, as evidenced by a significant decline in atmospheric radiation from 2003 on.


Galileo's astronomical drawings show that he had first observed Neptune on December 27, 1612, and again on January 27, 1613; on both occasions Galileo had mistaken Neptune for a fixed star when it appeared very close (in conjunction) to Jupiter in the night sky. Believing it to be a fixed star, he cannot be credited with its discovery. At the time Galileo first observed Neptune on December 28, 1612, it was stationary in the sky because it had just turned retrograde that very day; because it was stationary in the sky and only beginning the planet's yearly retrograde cycle, its motion was far too slight to be detected with Galileo's small telescope. Had Neptune been moving at its regular/average speed when Galileo first observed it in 1612 and 1613, he would have most likely realized that it was a planet and not a fixed star due to Neptune's relatively rapid normal motion along the ecliptic compared to the extremely slow motion of the fixed stars.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize some perturbing body. In 1843, John Couch Adams calculated the orbit of an eighth planet that would account for Uranus' motion. He sent his calculations to Sir George Airy, the Astronomer Royal, who asked Adams for a clarification; Adams began to draft a reply but never sent it.

In 1846, Urbain Le Verrier, independently of Adams, produced his own calculations but also experienced difficulties in encouraging any enthusiasm in his compatriots. However, in the same year, John Herschel started to champion the mathematical approach and persuaded James Challis to search for the planet.

After much procrastination, Challis began his reluctant search in July 1846. However, in the mean time, Le Verrier had convinced Johann Gottfried Galle to search for the planet. Though still a student at the Berlin Observatory, Heinrich d'Arrest suggested that a recently drawn chart of the sky, in the region of Le Verrier's predicted location, could be compared with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. Neptune was discovered that very night, September 23, 1846, within 1° of where Le Verrier had predicted it to be, and about 10° from Adams' prediction. Challis later realized that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.

In the aftermath of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is now being re-evaluated by historians with the rediscovery in 1998 of the "Neptune papers" (historical documents from the Royal Greenwich Observatory), which had apparently been misappropriated by astronomer Olin Eggen for nearly three decades and were not rediscovered (in his possession) until immediately after his death. After reviewing the documents, some historians now suggest that Adams did not in fact deserve equal credit with Le Verrier.


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.

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.

In the Chinese, Korean, Japanese, and Vietnamese languages the planet's name is literally translated as the sea king star.

Physical Characteristics

Orbiting so far from the sun, Neptune receives very little heat - in fact the uppermost regions of the atmosphere are "218 -C (55 K). Because Neptune is a gas giant, there is no solid surface; as one ventures deeper and 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. Neptune's atmosphere has the highest wind speeds in the solar system, up to 2000 km/h, thought to be powered by this flow of internal heat.

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, but has increasing concentrations of methane, ammonia, and water as it approaches and finally blends into the 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.

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.

Great Dark Spot

The Great Dark Spot was a dark spot on Neptune similar to Jupiter's Great Red Spot. Although it looked nearly the same as Jupiter's spot, it was not thought to be a storm but instead an atmospheric hole similar to the hole in Earth's ozone layer.

The spot was detected by NASA's Voyager 2 probe, which passed Neptune in 1989. It was comparable in size to Jupiter's spot, and was located in the southern hemisphere. Winds measured in the spot were up to 1,500 miles an hour (the highest of any planet). The spot appeared to change as the spacecraft flew by, and does so in many pictures of it. The spot was believed to have been rotating counterclockwise. Over the Great Dark Spot were concentrated regions of crystal methane and frozen water particle clouds that resemble cirrus clouds on Earth.

When the Hubble Space Telescope viewed Neptune again in June 1994, the spot had vanished. However, another spot very much like the old one appeared in the planet's northern hemisphere later that year. It is currently not known why the Great Dark Spot appeared in the first place; whether it is a common occurrence on Neptune or a bizarre phenomenon remains to be seen.

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. The Great Dark Spot, a cyclonic storm system the size of Eurasia, was captured by Voyager 2 in the 1989 flyby. The storm resembled the Great Red Spot of Jupiter, but was shown to have disappeared in June 1994. However, a newer image of the planet taken by the Hubble Space Telescope on November 2, 1994, revealed that a smaller storm similar to its predecessor had formed over Neptune's Northern Hemisphere. Unique among the gas giants is the presence of high clouds casting shadows on the opaque cloud deck below.

The Moons of Neptune

Neptune has 14 known moons, which are named for minor water deities in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, just 17 days after the discovery of Neptune itself; over a century passed before the discovery of the second natural satellite, Nereid. The moon orbiting farthest from its planet in the Solar System is Neptune's Neso and has an orbital period of about 26 Julian years.

Triton is unique among moons of planetary mass in that its orbit is retrograde to Neptune's rotation and inclined relative to Neptune's equator, which suggests that it did not form in orbit around Neptune but was instead gravitationally captured by it. The next-largest irregular satellite in the Solar System, Saturn's moon Phoebe, has only 0.03% of Triton's mass. The capture of Triton, probably occurring some time after Neptune formed a satellite system, was a catastrophic event for Neptune's original satellites, disrupting their orbits so that they collided to form a rubble disc. Triton is massive enough to have achieved hydrostatic equilibrium and to retain a thin atmosphere capable of forming clouds and hazes.

The Rings of Neptune

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.

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.

Ghostly rings of Neptune shine in new James Webb Telescope images   Live Science - September 21, 2022
When it comes to planetary rings, Saturn is the undisputed poster child. But now a new contender enters the, er, ring - courtesy of a stunning new image taken by the James Webb Space Telescope (JWST). In the new picture, our solar system's eighth planet Neptune shimmers like a glorious crystal ball, with a stack of gauzy rings wrapped magically around it.

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 Franćois Arago
Liberté Ring Arc 62,900 - "Leading" arc
Égalité Ring Arc 62,900 - "Equidistant" arc
Fraternite Ring Arc 62,900 - "Trailing" arc
Courage Ring Arc 62,900 -
1989 N1R ('Adams') 62,930 50 Named after John Couch Adams

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.

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.

Neptune's Clouds

Neptune's Clouds Have Strangely Disappeared For The First Time In Almost 30 Years   IFL Science - August 21, 2023

Neptune’s clouds have almost disappeared, images from space and Earth confirm. The cause remains somewhat mysterious, but the timing suggests it has something to do with the strength of the current solar cycle. The announcement raises the priority of a project where NASA is calling on amateur astronomers to assist it with observations of Neptune and Uranus. It seems the impact of solar maxima can extend a surprisingly long way into space.


In Roman Mythology Neptune (Greek: Poseidon) was the god of the Sea.

Poseidon, or Neptune was the god of the sea, earthquakes and horses. Although he was officially one of the supreme gods of Mount Olympus, he spent most of his time in his watery domain. Poseidon was brother to Zeus and Hades. These three gods divided creation between them. Zeus became ruler of the sky, Hades got dominion of the Underworld and Poseidon was given all water, both fresh and salt.

Although there were various rivers personified as gods, these would have been technically under Poseidon's sway. Similarly, Nereus, the Old Man of the Sea, wasn't really considered on a par with Poseidon, who was known to drive his chariot through the waves in unquestioned dominance. Poseidon had married Nereus's daughter, the sea-nymph Amphitrite.

In dividing heaven, the watery realm and the subterranean land of the dead, the Olympians agreed that the earth itself would be ruled jointly, with Zeus as king. This led to a number of territorial disputes among the gods. Poseidon vied with Athena to be patron deity of Athens. The god demonstrated his power and benevolence by striking the Acropolis with his three-pronged spear, which caused a spring of salt water to emerge. Athena, however, planted an olive tree, which was seen as a more useful favor. Her paramount importance to the Athenians is seen in her magnificent temple, the Parthenon, which still crowns the Acropolis. The people of Athens were careful, all the same, to honor Poseidon as well.

Poseidon was father of the hero Theseus, although the mortal Aegeus also claimed this distinction. Theseus was happy to have two fathers, enjoying the lineage of each when it suited him. Thus he became king of Athens by virtue of being Aegeus's son, but availed himself of Poseidon's parentage in facing a challenge handed him by King Minos of Crete. This monarch threw his signet ring into the depths of the sea and dared Theseus to retrieve it. The hero dove beneath the waves and not only found the ring but was given a crown by Poseidon's wife, Amphitrite.

Poseidon was not so well-disposed toward another famous hero. Because Odysseus killed the Cyclops Polyphemus, who was Poseidon's son, the god not only delayed the hero's homeward return from the Trojan War but caused him to face enormous perils.

Poseidon similarly cursed the wife of King Minos. Minos had proved his divine right to rule Crete by calling on Poseidon to send a bull from the sea, which the king promised to sacrifice. Poseidon sent the bull, but Minos liked it too much to sacrifice it. So Poseidon asked Aphrodite, the goddess of love, to make Minos's queen, Pasiphae, fall in love with the bull. The result was the monstrous Minotaur, half-man, half-bull.

As god of horses, Poseidon often adopted the shape of a steed. It is not certain that he was in this form when he wooed Medusa. But when Perseus later killed the Gorgon, the winged horse Pegasus sprang from her severed neck.

Poseidon sometimes granted the shape-shifting power to others. And he ceded to the request of the maiden Caenis that she be transformed into the invulnerable, male warrior Caeneus.


Neptune is the ruler of Pisces

Modern Western astrologers associate the planet Neptune with illusion, deception, religions, spirituality, the mass media, music, drugs, extreme sensitivity, psychic phenomena and altered mental states. The discovery of Neptune in 1846 coincided with the discovery of anesthetics and hypnotism around this period. Neptune dissolves boundaries and barriers, enabling us to connect with the universe. It's about inspiration, imagination, compassion, ideals and communion. But, the other side of Neptune is about confusion, illusion and diffusion.