Neptune

**Neptune** is the eighth and last planet from the Sun. It is an *ice giant*, as it has an outer layer of hydrogen, helium, methane, ammonia and water. Beneath this layer lies a thick mantle of chemical ices and a small core of rocky material. The atmosphere of Neptune is much stormier than Uranus' with winds up to 1,250 mph (2,000 km/h). It's color is blue.

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 °C (−346 °F), one of the coldest in the solar system, due to its long distance from the sun. However, Neptune's center is about 7,000 °C (13,000 °F), hotter than the sun's surface. This is due to extremely hot gases and rock in the center. 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.

## Pluto's strange orbit

A portion of the dwarf planet Pluto's highly eccentric orbit brings it nearer to the Sun than Neptune, whose orbit is nearly circular. As a result, for approximately 13 to 20 years out of the every 248 (Pluto's orbital period), Neptune lies farther from the Sun than Pluto. The most recent occurrence of this phenomenon began 7 February 1979 and ended 11 February 1999.

## Discovery

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 meantime, 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 wake 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 only rediscovered (in his possession) immediately after his death. After reviewing the documents, some historians now suggest that Adams does not deserve equal credit with Le Verrier.

## 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. 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, 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" 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. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core, the continued radiation into space of leftover heat generated by infalling matter during the planet's birth, and gravity waves breaking above the tropopause.

The storms, Scooter and the Wizard's Eye still exist. The internal structure resembles that of Uranus. There is likely to be a core, believed to be of around 15 Earth masses, consisting of molten rock and metal surrounded by a mixture of rock, water, ammonia, and methane. The intense pressures keep the icy component of this surrounding mixture as solids, despite the large temperatures near the core. 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 typical level of meteorological activity. When the Voyager spacecraft flew by Uranus in 1986 that planet was visually quite bland, while Neptune exhibited notable weather phenomena during its 1989 Voyager fly-by. 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 dynamic storm systems, with winds reaching up to around 2100 km/h, near-supersonic speeds. Even more typical winds in the banded equatorial region can possess speeds of around 1,200 km/h (750 mph).

## Storms

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[*citation needed*]. The **Scooter** is another storm, a white cloud group further south than the Great Dark Spot. Its nickname was bestowed when it was first detected in the months leading up to the Voyager encounter in 1989: it moved faster than the Great Dark Spot. Subsequent images showed clouds that moved even faster than Scooter. The Wizard's eye/*Dark Spot 2* is a southern cyclonic storm, the second most intensive storm during the 1989 encounter. It initially was completely dark, but as Voyager approached the planet, a bright core developed and is seen in most of the highest resolution images.

The 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 or 49 K) 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 kilometers per hour. Rather than seeing slower winds, the scientists found faster winds (over 1600 km/h) on more distant Neptune.

One suggested cause for this apparent anomaly is that if enough energy is produced, turbulence is created, which slows the winds down (like those of Jupiter). At Neptune however, there is so little solar energy that once winds are started they meet very little resistance, and are able to maintain extremely high velocities. Nonetheless, Neptune radiates more energy than it receives from the Sun, and the internal energy source of these winds remains undetermined.

## 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.

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.

## 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 −235 °C (38 K, −392 °F).

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. 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, having a brightness between magnitudes +7.7 and +8.0, which can be outshone 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 resolve Neptune as a small blue disk, similar in appearance to Uranus; the blue color comes from the methane in its atmosphere. Its small apparent size has made it challenging to study visually; most telescopic data was fairly limited until the advent of Hubble Space Telescope and large ground-based telescopes with adaptive optics.

With an orbital period (sidereal period) of 164.88 Julian years, Neptune will soon return (for the first time since its discovery) to the same position in the sky where it was discovered in 1846. This will happen three different times, along with a fourth in which it will come very close to being at that position. These are April 11, 2009, when it will be in prograde motion; July 17 2009, when it will be in retrograde motion; and February 7 2010, when it will be in prograde motion. It will also come very close to being at the point of the 1846 discovery in late October through early-mid November 2010, when Neptune will switch from retrograde to direct motion on the exact degree of Neptune's discovery and will then be stationary along the ecliptic within 2 arc minutes at that point (closest on November 7, 2010). This will be the last time for approximately the next 165 years that Neptune will be at its point of discovery.

This is explained by the concept of retrogradation. Like all planets and asteroids 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.

## Voyage 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.