Neptune

**Neptune** is the eighth and farthest planet from the Sun in the Solar System. It is an ice giant. It is the fourth-largest planet in the system. 

Neptune's mass is 17 times Earth's mass and a little bit more than Uranus' mass. Neptune is denser and smaller than Uranus. Because of its greater mass, Neptune's gravity makes its atmosphere smaller and denser.

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

Urbain Le Verrier and John Couch Adams were the astronomers who discovered Neptune. Neptune was not discovered using a telescope. It was the first planet to be discovered using mathematics. In 1821, astronomers saw that Uranus' orbit was different from what they expected. Another nearby planet's mass was changing Uranus' orbit. They found Neptune was the cause.

*Voyager 2* visited Neptune on August 25, 1989. It was the only spacecraft to visit the planet. Neptune used to have a huge storm known as the "Great Dark Spot". *Voyager 2* discovered the spot in 1989. The dark spot was not seen in 1994, but new spots were found since then. It is not known why the dark spot disappeared. Visits by other space probes have been planned.

Neptune has five rings surrounding it. However, the rings are hard to see from Earth.

## History 
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, the co-discoverer of Neptune|left]]

### Discovery 
Galileo Galilei was the first person to see Neptune. But Galileo was not credited for the discovery. He thought Neptune was a "fixed star" instead of a planet. Because Neptune moved slowly across the sky, Galileo's small telescope was not strong enough to see that Neptune was a planet.

In 1821, Alexis Bouvard published the astronomical tables of the orbit of Uranus. Later observations showed that Uranus was orbiting in an irregular way. Some astronomers thought this was caused by another large planet. In 1846, Urbain Le Verrier made his own calculations but also failed to get much attention from French astronomers.

Heinrich d'Arrest, a student at the Berlin Observatory, suggested that a newly drawn map of the sky in the region of Le Verrier's predicted area could be compared with the current sky. It was found 1° from where Le Verrier had thought it would be. It was about 1.5° 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. After looking at 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*, but a lot of people outside France disagreed with this. French almanacs quickly reintroduced the name *Herschel* for Uranus and *Leverrier* for Neptune. Soon, *Neptune* was internationally agreed as the official name for the new planet. In Roman mythology, Neptune was the god of the sea, identified with the Greek god, Poseidon. Neptune's astrological symbol is Neptune's trident (♆).

## Structure 
### Mass and composition 
Neptune's mass is between that of the Earth and the largest gas giants. Neptune is the fourth largest planet in the Solar System and the third most massive. Neptune is a little bit more massive than Uranus, though Neptune is denser and smaller in size than Uranus. They are smaller in size than Jupiter and Saturn, and have different compositions. To search extrasolar planets, Neptune has been used as a reference to compare the size and structure of other discovered planets. 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. The color of Neptune is similar, but slightly darker, than the color of Uranus. Therefore, at its surface Neptune is the coldest planet in the Solar System. 

But in the depths of the planet, the temperature rises. The source of this heating is unclear. 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, at . 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. Another 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 1.5 Earth masses. It is made up of molten rock and metal surrounded by rock, water, ammonia, and methane. More mixtures of methane, ammonia, and water are found in the lower areas of the atmosphere.'''

### 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 not so strong as on Neptune. When Voyager flew by Neptune in 1989, powerful weather events were observed. The weather of Neptune has very active storms.

In 1989, the Great Dark Spot, an anticyclonic storm system, was discovered by NASA's *Voyager 2* spacecraft. 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 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. 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.

In August 2023, the clouds vanished. The possible reason is solar flare. 

Neptune also has similarities with Uranus in its magnetosphere. However, Uranus' magnetosphere is weaker than Neptune's magnetosphere.

Neptune has a similar axial tilt to Earth, so it will have seasons. Its seasons last about 40 years.

## Neptune's rings 
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There are five rings around the planet. Pictures showed that the ring system had several faint rings. The farthest ring, Adams, has five arcs named *Courage*, *Liberté*, *Egalité 1*, *Egalité 2*, and *Fraternité* (Courage, Liberty, Equality, and Fraternity).

The laws of motion predict that arcs will spread out into one ring in a very short time. But the arcs in Neptune's rings somehow did not. A moon's gravity may have created the arcs. Galatea is a moon just inside the Adams ring. However, in 2005, scientists figured out that Neptune's rings were more unstable than previously believed. The *Liberté* arc may disappear in roughly less than 100 years.

## Neptune's moons 
Neptune has 16 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. 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&nbsp;°C (38 K, −392&nbsp;°F).

Neptune's second known moon (by order of discovery), 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. Of these, Proteus is the second most massive Neptunian moon. It has only one quarter of 1% of the mass of Triton. 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 July 16, 2013.

 captured by Hubble. Triton is clearly visible, while others appear much smaller.]]

## Observation 
Neptune cannot be seen by just looking at the sky with the naked eye. To see it, a telescope or binoculars is needed. It can be outshined 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 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.

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

One of *Voyager 2*'s important discoveries was its very close fly-by of Triton, where it took pictures of several parts of the moon. The pictures were sent back to Earth from *Voyager 2* in 1989. 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.

### Proposed missions 
* Shensuo (used to be called Interstellar Express)—Two probes by CNSA to explore the heliosphere. The second probe would come within 1,000&nbsp;km of Neptune in 2038.
* ODINUS—This mission uses two spacecraft to study the Neptunian and Uranian systems. 2034 is the launch date.
* OSS mission—This is a mission by ESA and NASA. Its focus is to map the gravitational fields in deep space, including the Outer Solar System (up to 50 AU).
* Triton Hopper—A NIAC plan for a mission to Neptune to land on Neptune's moon Triton and then fly to several places.
* Trident—This is a finalist in the Discovery Program. It will fly close to Neptune once in 2038 and closely study its largest moon Triton.
* Neptune Odyssey—An idea for a NASA mission to observe Neptune's atmosphere and the weather, and its moon Triton. It would launch in 2033 and arrive at Neptune in 2049.

## 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)