Neptune is the eighth and last planet from the Sun in the Solar System. It is an ice giant. It is the fourth largest planet and third heaviest. Neptune has five rings which are hard to see from the Earth. 

Neptune is seventeen times heavier than Earth and is a little bit more massive than Uranus. Neptune is denser and physically smaller than Uranus: its greater mass causes more gravitational compression of its atmosphere. 

It was named after the Roman god of the sea, Neptune. Neptune's astronomical symbol is ♆, the trident of the god Neptune. 

Neptune's atmosphere is mostly made up of hydrogen and helium. It also contains small amounts of methane which makes the planet appear blue. Neptune's blue color is much darker than the color of Uranus. Neptune also has the strongest winds of any planet in the Solar System, as high as 2,100 km/h or 1,300 mph.

Astronomers Urbain Le Verrier and John Couch Adams discovered Neptune. They were both honored for the discovery.  It was the first planet to be discovered by mathematical calculations rather than by a telescope. In 1821, it was found that Uranus' orbit had some differences from its expected orbit. So astronomers searched for a new planet, the one we now know as Neptune.

The planet has been visited by only one spacecraft, Voyager 2 on 25 August 1989.  Neptune once had a huge storm known as the "Great Dark Spot" which was discovered in 1989 by Voyager 2.  The dark spot was not seen in 1994, and new spots were found since then.  It is not known why the dark spot disappeared.  Visits by other space probes have been proposed.

History

Discovery 
The first possible sighting of Neptune is thought to be by Galileo as his drawings showed Neptune near Jupiter. But Galileo was not credited for the discovery since he thought Neptune was a "fixed star" instead of a planet. Because of Neptune's slow movement across the sky, Galileo's small telescope was not strong enough to detect Neptune as a planet.

In 1821, Alexis Bouvard published the astronomical tables of the orbit of Uranus. Later observations showed that Uranus was moving in an irregular way in its orbit. Some astronomers thought this was caused by another large body. In 1843, John Couch Adams calculated the orbit of an eighth planet that could possibly affect the orbit of Uranus. He sent his calculations to Sir George Airy, the Astronomer Royal, who asked Adams for an explanation. In 1846, Urbain Le Verrier, who was not working with Adams, made his own calculations but also failed to get much attention from French astronomers. In the same year, John Herschel began to support the mathematical method and encouraged James Challis to search for the planet. After much delay, Challis began his unwilling search in July 1846.  Meanwhile, Le Verrier had convinced Johann Gottfried Galle to search for the planet.

Heinrich d'Arrest, a student at the Berlin Observatory, suggested the following that a newly drawn map of the sky in the region of Le Verrier's predicted area could be compared with the current sky. This map was needed to look for the change of position of a planet, compared to a fixed star. Neptune was then discovered the same night on 23 September 1846, within 1° of where Le Verrier had predicted it to be, and about 10° from Adams' prediction. Challis later found out that he had seen the planet twice in August. He did not recognize it at the time because of his careless work approach.

Crediting and naming 
When Neptune was discovered, there was also a lot of arguing between the French and the British. They could not agree who was going to receive credit for the discovery. Later, an international agreement decided that both Le Verrier and Adams together deserved credit. However, historians reviewed the topic after the rediscovery in 1998 of the "Neptune papers" (historical documents from the Royal Greenwich Observatory). It had seemingly been stolen and kept by astronomer Olin Eggen for nearly three decades and was only rediscovered (in his ownership) shortly after his death. After reviewing the documents, some historians now think that Adams does not deserve equal credit with Le Verrier.

Shortly after its discovery, Neptune was temporarily called "the planet exterior to Uranus" or "Le Verrier's planet". The first suggestion for a name came from Galle. He proposed the name Janus. In England, Challis suggested the name Oceanus. In France, Arago suggested that the new planet be called Leverrier, a suggestion which was met with a lot of opposition outside France. French almanacs quickly reintroduced the name Herschel for Uranus and Leverrier for the new planet.

Meanwhile, Adams suggested changing the name Georgian to Uranus, while Leverrier (through the Board of Longitude) suggested Neptune for the new planet. Struve gave support of that name on 29 December 1846, to the Saint Petersburg Academy of Sciences. Soon Neptune was internationally agreed among many people. It was  the official name for the new planet. In Roman mythology, Neptune was the god of the sea, identified with the Greek god, Poseidon.

Structure

Mass and composition 
At 102.413kg, Neptune's mass puts the planet between Earth and the largest gas giants. Neptune has seventeen Earth masses but just 1/18th the mass of Jupiter. Neptune and Uranus are often considered to be part of a sub-class of a gas giant known as "ice giants." It is given their smaller size and big differences in composition compared to Jupiter and Saturn. In the search for extrasolar planets, Neptune has been used as a reference to determine the size and structure of the discovered planet. Some discovered planets that have similar masses like Neptune are often called "Neptunes."  

The atmosphere of Neptune is made up mostly of hydrogen, with a smaller amount of helium. A tiny amount of methane was also detected in the atmosphere.  The methane gives Neptune its blue color.

Because of Neptune's far distance from the Sun, it gets very little heat. The average temperature on Neptune is about -201°C (−331 °F; 72 K). Therefore, Neptune is the coldest planet in the Solar System. But in the depths of planet the temperature rises slowly. The source of this heating is unknown. Neptune is the farthest planet from the Sun, yet its internal energy is strong enough to create the fastest winds seen in the Solar System. Several possible explanations have been suggested. Firstly, radiogenic heating from the planet's core. Among the explanations is the continued radiation into space of leftover heat made by infalling matter during the planet's birth. The last explanation is gravity waves breaking above the tropopause. It has also been suggested that the friction and ram pressure of the diamond hail heats up the planet.

The structure inside Neptune is thought to be similar to the structure inside Uranus.  There is likely to be a core, thought to be about 15 Earth masses. It is made up of molten rock and metal surrounded by rock, water, ammonia, and methane. This mixture is referred to as icy. It is called a water-ammonia ocean.  More mixtures of methane, ammonia, and water are found in the lower areas of the atmosphere.    

At a depth of 7,000 km of Neptune, the conditions may be such that methane decomposes into diamond crystals. These diamond crystals look like hailstones.

Weather and magnetic field 
One difference between Neptune and Uranus is the level of its meteorological activity. When the Voyager spacecraft flew by Uranus in 1986, the winds on that planet were observed to be mild. When Voyager flew by Neptune in 1989, powerful weather events were observed. The weather of Neptune has very active storms. Its atmosphere has the highest wind speeds in the Solar System. It may be powered by internal heat flow. Regular winds in the equatorial region have speeds of around 1,200 km/h (750 mph). Winds in storm systems can reach up to 2,100 km/h, near-supersonic speeds.

In 1989, the Great Dark Spot, an anticyclonic storm system, was discovered by NASA's Voyager 2 spacecraft.  On 2 November 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 why the Great Dark Spot has disappeared is unknown.  The Scooter is another storm, a white cloud group farther south than the Great Dark Spot. Its nickname was given when first noticed in the months leading up to the Voyager encounter in 1989. It moved faster than the Great Dark Spot. Later images showed clouds that moved even faster than Scooter. The Wizard's Eye/Dark Spot 2 is another southern cyclonic storm, the second strongest storm seen during the 1989 encounter. It originally was completely dark, but as Voyager came closer to the planet, a bright core developed.

Neptune also has similarities with Uranus in its magnetosphere. However, Uranus' magnetosphere is weaker than Neptune's magnetosphere. The magnetic field is strongly tilted compared to its rotational axis at 47°. It is offset at least 0.55 radii (about 13,500 kilometres, bigger than the Earth's diameter, for scale) from the planet's physical center. The unusual course may be caused by flows in the interior of the planet.

Neptune's rings 

Tiny blue-colored rings have been discovered around the blue planet. They are not as well known as the rings of Saturn. When these rings were discovered by a team led by Edward Guinan, originally, they thought that the rings might not be complete. However, this was proven wrong by Voyager 2. Neptune's planetary rings have a weird "clumpy" arrangement. The scientists think that it may be because of the gravitational contact with small moons that orbit near them.

Proof that the rings are incomplete first began in the mid-1980s, when stellar occultation were found to rarely show an extra "blink" just before or after the planet occulted the star. Pictures from Voyager 2 in 1989 solved the problem when the ring system was found to have several faint rings. The farthest ring, Adams, has three famous arcs now named Liberté, Egalité, and Fraternité (Liberty, Equality, and Fraternity).

The existence of arcs is hard to understand because the laws of motion predict that arcs spread out into a single ring in a very short time. The gravitational effects of Galatea (a moon just inner to the ring) may have created the arcs.

Earth-based observations in 2005 appeared to show that Neptune's rings are more unstable than thought before. It looks like the Liberté ring may disappear in less than 100 years.

Neptune's moons 

Neptune has a total of 14 known moons. As Neptune was the Roman god of the sea, the planet's moons were named after lesser sea gods or goddesses.  

The largest moon of Neptune  is Triton. Triton was discovered  on  October 10, 1846 by British astronomer William Lassell. Unlike all other large planetary moons, Triton orbits in the other direction to the other moons. This shows the moon was probably captured and maybe was once a Kuiper belt object. It is close enough to Neptune to be locked into a synchronous orbit. It is also slowly moving into Neptune and may one day be torn apart when it passes the Roche limit. Triton is the coldest object that has been measured in the Solar System, with temperatures of −235°C (38 K, −392 °F). 

Neptune's second known moon (by order of distance), the odd moon Nereid, has one of the most unusual orbits of any satellite in the Solar System. Nereid is so far from Neptune that it requires 360 Earth days to make one orbit. It causes the largest elliptical orbit and the largest deviation from a circular path. It is also considered that Nereid may be a captured asteroid or Kuiper Belt object.  

From July to September 1989, Voyager 2 discovered six new moons of Neptune. Of these, Proteus is the second most massive Neptunian moon. It has only one quarter of 1% of the mass of Triton. Neptune's closest four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be inside Neptune's rings. The next farthest out, Larissa was originally discovered in 1981 when it had occulted a star.  The moon was credited for causing Neptune's ring arcs when Voyager 2 observed Neptune in 1989. Five new unusual moons discovered between 2002 and 2003 were announced in 2004. The latest moon, Hippocamp, was discovered from examining Hubble Telescope images on 16 July 2013.

Observation 
Neptune can not be seen by just looking at the sky with the naked eye. To see it, a telescope or binoculars is needed. This is because Neptune has a normal brightness between magnitudes +7.7 and +8.0. It can be out-shined by Jupiter's Galilean moons, the dwarf planet Ceres, and the asteroids 4 Vesta, 2 Pallas, 7 Iris, 3 Juno and 6 Hebe. A telescope or strong binoculars will show Neptune as a small blue dot that looks similar to Uranus. The blue color comes from the methane in its atmosphere. Its small size in the night sky has made it difficult to study visually. Most telescopic data was quite limited until the arrival of the Hubble Space Telescope and large ground-based telescopes with adaptive optics.

The average distance between Neptune and the Sun is about 4.5 billion km. Therefore Neptune completes its orbit in every 164 years. On 12 July 2011, Neptune completed its first orbit since its discovery in 1846.

Exploration 

Currently, only one spacecraft has visited Neptune. NASA's Voyager 2 probe made a quick flyby of the planet with its closest encounter on 25 August 1989. 

One of Voyager 2'''s important discoveries was its very close fly-by of Triton where took pictures of several parts of the moon. The probe also discovered the Great Dark Spot. However, it had now disappeared after the Hubble Space Telescope took pictures of Neptune in 1994. Originally thought to be a large cloud or cyclonic storm system. It was later guessed to be a hole in the visible cloud deck.

The pictures sent back to Earth from Voyager 2 in 1989 became the basis of a PBS all-night program called Neptune All Night''.

Proposed missions 
 Interstellar Express — A pair of probes by CNSA that would explore the heliosphere. The second would fly by Neptune in 2038 at a distance of 1,000 km.
 ODINUS —  A mission idea based on a twin spacecraft mission to investigate the Neptunian and Uranian systems. Launch date would be 2034.
 OSS mission — A proposed joint flyby mission by ESA and NASA. Its main focus would be to map the gravitational fields in deep space, including the Outer Solar System (up to 50 AU).
Triton Hopper — A NIAC study of a mission to Neptune with the goal of landing, and flying from site to site, on Neptune's moon Triton.
Trident — A finalist in the Discovery program, would perform a single flyby of Neptune in 2038 and closely study its largest moon Triton.
 Neptune Odyssey — it is an idea for a mission to observe Neptune and its moons; The idea is to launch in 2033 and arrive at Neptune in 2049; That is what the idea is in 2021.

References

Other websites 

 NASA's Neptune fact sheet
 Neptune Profile  by NASA's Solar System Exploration
 Planets - Neptune A kid's guide to Neptune.
 Mission to Neptune Under Study   (Universe Today)
Neptune by amateur (The Planetary Society)

 
Solar System planets