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's 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 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.

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's orbit. They found Neptune was the cause.

Voyager 2 visited Neptune on 25 August 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. These rings are hard to see from the Earth.

History

Discovery 
Galileo Galilei was the first person who saw Neptune. He saw it on 28 December 1612 and 27 January 1613. His drawings showed points near Jupiter where Neptune is placed. But Galileo was not credited for the discovery. He thought Neptune was a "fixed star" instead of a planet. Because Neptune slowly moved 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 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. George Airy asked Adams for an explanation. In 1846, Urbain Le Verrier made his own calculations but also failed to get much attention from French astronomers. Airy saw his calculations and encouraged James Challis to search for the planet. Challis began his 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 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 discovered the same night on 23 September 1846. 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. Neptune became the first planet to be discovered by mathematical calculations instead of a telescope.

Crediting and naming 
When Neptune was discovered, the French and the British could not agree on who would get credit for the discovery. Later, an international agreement decided that both Le Verrier and Adams 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 by astronomer Olin Eggen for nearly three decades. It was only found again (in his ownership) shortly after his death. 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 the new planet.

Meanwhile, Adams suggested changing the name Georgian to Uranus, while Le Verrier (through the Board of Longitude) suggested Neptune for the new planet. Struve supported the name Neptune 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. 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 17 times the mass of Earth, but just  the mass of Jupiter. Neptune is a little bit more massive than Uranus, though Neptune is denser and physically smaller than Uranus. Neptune and Uranus are often considered to be part of a sub-class of a gas giant known as "ice giants". They are smaller in size than Jupiter and Saturn, and have differences from them in composition. In the search for extrasolar planets, Neptune has been used as a reference to compare 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. The color of Neptune is much darker than the color of Uranus.

Because of Neptune's far distance from the Sun, it gets very little heat. The average surface temperature on Neptune is about −201°C (−331 °F; 72 K). 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. 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 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. 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.

In August 2023, the clouds vanished. The possible reason is solar flare.  30 years of Neptune' weather observations by Hubble Space Telescope showed that cloud activity is related to solar cycles. 

Neptune also has similarities with Uranus in its magnetosphere. However, Uranus's 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 has a similar axial tilt to Earth, so it will have seasons. Seasons last about 40 years.

Neptune's rings 

There are five rings around the planet. They are not as well known as the rings of Saturn. The rings were discovered by a team led by Edward Guinan in 1968. Then, in the mid-1980s astronomers thought that the rings might not be complete.  Stellar occultations were found that rarely showed an extra "blink" just before or after the planet moved in front of the star. However, Voyager 2 showed that they were complete. The planetary rings of Neptune have a weird "clumpy" arrangement. Scientists think that it may be because of the gravitational contact with small moons that orbit near them. Pictures showed that the ring system had several faint rings. The farthest ring, Adams, has five arcs now named Courage, Liberté, Egalité 1, Egalité 2 and Fraternité (Courage, Liberty, Equality, and Fraternity).

Arcs are a puzzle. The laws of motion predict that arcs spread out into one ring in a very short time. But the arcs in Neptune's rings did not. A moon's gravity may have created the arcs. Galatea is a moon just inside the Adams ring.

In 2005, scientists figured out that Neptune's rings were more unstable than had been believed. The Liberté arc may disappear in less than 100 years.

Neptune's moons 

Neptune has 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 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. 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 discovered in 1981 when it had covered up 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 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 once 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 it 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—Two probes by CNSA to explore the heliosphere. The second probe would come within 1,000 km of Neptune in 2038.
 ODINUS—This mission uses two spacecraft to study the Neptunian and Uranian systems. 2034 is 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)