Neptune

## Atmosphere

### **Composition**

-Hydrogen (80%)-Helium (19%)-Methane (1.5%)-Hydrogen deuteride (0.019%)-Ethane (0.0002%)

## General Information

Neptune is the Solar System's farthest planet as recognized by the IAU, and it is ranked ninth from the Sun. By diameter, it is the third most massive, the densest big planet, and the fourth largest planet in the Solar System. It is little more massive than its near-twin Uranus, with 17 times the mass of Earth. Because of its larger mass and the increased gravitational compression of its atmosphere, Neptune is both physically smaller and denser than Uranus. Its principal constituents are gasses and liquids, so it lacks a definite solid surface. The planet has an orbital distance of 30.1 astronomical units around the Sun, which it circles once every 164.8 years.

Being invisible to the unassisted eye, Neptune is the one planet in the Solar System discovered by mathematical calculations rather than direct sight. Alexis Bouvard conjectured that an unidentified planet was causing gravitational disturbances to Uranus' orbit after observing unexpected alterations in its trajectory. Following Bouvard's demise, John Couch Adams and Urbain Le Verrier independently calculated Neptune's location based on his observations. Johann Galle later used a telescope to examine Neptune on September 23, 1846, and found it to be within one degree of Le Verrier's anticipated location. Shortly after, the planet's biggest moon, Triton, was found; but, until the 20th century, none of the planet's remaining 14 known moons could be found by telescope. Because of its great distance from Earth, the planet appears incredibly tiny, which makes it difficult to observe with Earth-based telescopes. Voyager 2 is still the sole spacecraft to have visited Neptune; it did so on August 25, 1989, during a flyby of the planet. Further in-depth views from a distance have been made possible by the development of the Hubble Space Telescope and big ground-based observatories with adaptive optics.

Similar to the gas giants, Jupiter and Saturn, Neptune's atmosphere is mainly made up of hydrogen and helium, with smaller amounts of hydrocarbons and maybe nitrogen. However, it also has a larger amount of ices, including water, ammonia, and methane. Its interior is mostly made of ice and rock, much like Uranus's; to differentiate them, both planets are sometimes referred to as "ice giants". The planet's blue hue is partially explained by methane traces in its outermost regions, in addition to Rayleigh scattering. Because of Neptune's smaller atmosphere, which is more active than Uranus', the blue color is more saturated.

Neptune's atmosphere contains dynamic and discernible weather patterns, in contrast to Uranus's foggy and mostly featureless atmosphere. For instance, the planet's southern hemisphere featured a Great Dark Spot akin to Jupiter's Great Red Spot during the Voyager 2 encounter in 1989. 2018 saw the identification and investigation of a smaller and more recent major dark spot. With documented wind speeds reaching 2,100 km/h, these weather patterns are propelled by the highest persistent winds of any planet in the Solar System. Neptune's outer atmosphere is among the coldest in the Solar System due to its immense distance from the Sun; at its cloud tops, temperatures are over 55 K (−218 °C; −361 °F).

### History

#### Discovery

Plotted spots in Galileo Galilei's drawings from December 28, 1612, and January 27, 1613, some of the first known telescope observations ever made, correspond to Neptune's known positions on those dates. Galileo appears to have mistook Neptune for a fixed star on both times when it seemed near. As a result, he is not given credit for discovering Neptune. Neptune had recently turned retrograde on the day of his first sighting in December 1612, thus it was essentially motionless in the sky. According to a 2009 research, Galileo may have known that the "star" he had seen had shifted in relation to fixed stars.

The orbit of Uranus, Neptune's neighbor, was documented in astronomical tables by Alexis Bouvard in 1821. When significant discrepancies from the tables were found in later observations, Bouvard postulated that an unidentified body was causing gravitational perturbations in the orbit. Using the information at his disposal, John Couch Adams started working on Uranus' orbit in 1843. In February 1844, he received further data that he had sought from the Astronomer Royal, Sir George Airy. Adams worked on his project during 1845 and 1846, coming up with many estimations for a new planet.

Independent of Adams, Urbain Le Verrier produced his own calculations between 1845 and 1846, but his fellow countrymen were unenthusiastic. James Challis was convinced to hunt for the planet by Airy in June 1846 when he saw Le Verrier's initially released estimate of the planet's longitude and how well it matched Adams's estimate. Challis searched the skies in vain during the months of August and September. In actuality, Challis had seen Neptune twice, on August 4 and 12, 1845, a year ahead of Johann Gottfried Galle, the planet's ultimate discoverer. But until he performed more research, he was unable to identify the findings as such due to his antiquated star maps and subpar observational methods. Despite his deep regret, Challis attributed his carelessness to his maps.

Le Verrier, in the meantime, wrote to Galle, the astronomer at Berlin Observatory, requesting that he conduct a search using the observatory's refractor. A student at the observatory named Heinrich d'Arrest advised Galle that in order to find the displacement typical of a planet rather than a fixed star, they might compare a newly created map of the sky in the area of Le Verrier's probable location with the present sky. Galle found Neptune on September 23, 1846, the evening of the letter's receipt. It was located just northeast of Iota Aquarii, 1° from Le Verrier's "five degrees east of Delta Capricorn" position, about 12° from Adams's prediction, and on the modern IAU constellation boundaries bordering Aquarius and Capricornus.

Following the discovery, the French and the British engaged in a fierce patriotic competition over who should receive credit for the finding. International opinion eventually came to the conclusion that Le Verrier and Adams should share credit. Dennis Rawlins has questioned the veracity of Adams's claim to co-discovery since 1966, and historians reexamined the matter in 1998 when the historical records known as the "Neptune papers" were returned to the Royal Observatory, Greenwich.

#### Status

Neptune was the furthest known planet from when it was discovered in 1846 until Pluto's discovery in 1930. Except for a 20-year period between 1979 and 1999, when Pluto's elongated orbit placed it closer to the Sun than Neptune, making Neptune the ninth planet from the Sun, Pluto was thought to be a planet when it was found, making Neptune the second-farthest known planet. Following the discovery of the Kuiper belt in 1992, and with Pluto's mass estimated to be between ten times that of Earth and much smaller than that of the Moon, scientists have been debating whether Pluto belongs in the Kuiper belt or should be classified as a planet. In 2006, the IAU defined the term planet, and made Pluto a dwarf planet, and made Neptune the farthest planet once again.

### Properties

With a mass of 1.0243 x 10^26 kg, Neptune lies in the middle of the bigger gas giants, 17 times that of Earth and only 1/19th that of Jupiter. Only Jupiter has a greater gravity than it at 1 bar, which is 11.15 m/s2, or 1.14 times that of Earth's surface. With an equatorial radius of 24,764 kilometers, Neptune is almost four times larger than Earth. Neptune and Uranus are both smaller than Jupiter and Saturn and have higher volatile contents, making them both ice giants, a subclass of giant planets. Neptune has been used as a metonym in the hunt for exoplanets: things of comparable mass found are commonly called "Neptunes," much as scientists call other extrasolar entities "Jupiters."

#### Structure

The interior structure of Neptune is similar to that of Uranus. Its atmosphere, which makes up between 5 and 10% of its mass, may reach the core by 10% to 20%. The atmosphere's pressure can reach up to 10 GPa, or 100,000 times that of Earth's atmosphere. The lower parts of the atmosphere include increasing amounts of water, ammonia, and methane.

Methane, ammonia, and water abound in the mantle, which has the mass of ten to fifteen Earth masses. This combination is a hot, dense fluid (supercritical fluid), although it is referred to be frosty as is typical in planetary research. This fluid, known for its tremendous electrical conductivity, is also referred to as an ocean of water and ammonia. A stratum of ionic water, where the water molecules disintegrate into a soup of hydrogen and oxygen ions, and a layer of superionic water, where the oxygen crystallizes but the hydrogen ions are free to move inside the oxygen lattice, might make up the mantle. Conditions might be so bad at 7,000 km down that methane breaks down into diamond crystals that fall to the surface like hailstones.

According to one internal model, Neptune's mass is approximately 1.2 times that of Earth, and its core is most likely made of iron, nickel, and silicates. There may be a temperature of 5,400 K and a pressure of 7 Mbar (700 GPa), which is almost twice as high as the Earth's center.

#### Atmosphere

Neptune's atmosphere is composed of 80% hydrogen and 19% helium at high altitudes. Methane is also found in trace amounts. At wavelengths over 600 nm, methane exhibits prominent absorption bands in the red and infrared regions of the spectrum. Similar to Uranus, Neptune is blue due in part to the capture of red light by atmospheric methane; however, Neptune's blue is not the same as Uranus's, which is a paler light blue.

There are two primary sections of Neptune's atmosphere: the lower troposphere, where height causes temperature to fall, and the stratosphere, where altitude causes temperature to increase. The tropopause, which separates the two, is located at 0.1 bars of pressure. At pressures lower than 10−5 to 10−4 bars, the thermosphere eventually replaces the stratosphere. The exosphere progressively replaces the thermosphere.

Models indicate that clouds with different compositions at different altitudes ring Neptune's troposphere. The pressure below one bar, where the temperature is conducive to methane condensation, is where the upper-level clouds are located. Ammonia and hydrogen sulfide clouds are predicted to occur for pressures between one and five bars (100 and 500 kPa). The clouds may include water, hydrogen sulfide, ammonia, and ammonium sulfide over five bars of pressure. At pressures of around 50 bars (5.0 MPa), where the temperature exceeds 273 K (0 °C), deeper clouds of water ice should be encountered. There can be ammonia and hydrogen sulfide clouds underneath.

On Neptune, shadows from high-altitude clouds have been seen on the opaque cloud deck below. Additionally, there are bands of high-altitude clouds that circle the earth at fixed latitudes. These circumferential bands are located 50–110 km above the cloud deck and have widths of 50–150 km. These elevations are found in the troposphere, the stratum that produces weather. The thermosphere and upper stratosphere are not where weather occurs. Neptune's clouds disappeared in August 2023, maybe as a result of a "solar flare". Neptune's cloud activity is correlated with solar cycles rather than the planet's seasons, as demonstrated by thirty years of studies conducted by the Hubble Space Telescope and ground-based observatories.

Based on its spectra, Neptune appears to have hazy lower stratosphere caused by condensation of ethane and ethyne, which are byproducts of UV photolysis of methane. Traces of hydrogen cyanide and carbon monoxide are also found in the stratosphere. Because Neptune has a higher hydrocarbon content than Uranus, its stratosphere is warmer.

With a magnetic field that is significantly skewed toward its rotational axis at 47° and offset by at least 0.55 radius, or around 13,500 km, from the planet's physical center, Neptune's magnetosphere is similar to Uranus'. Uranus's sideways rotation was thought to be the cause of its slanted magnetosphere until Voyager 2 arrived at Neptune. Based on a comparison of the two planets' magnetic fields, scientists now believe that the unusual orientation might be indicative of flows within the planets. A dynamo action might be produced by convective fluid movements in a thin, spherical shell of electrically conducting liquids.

At Neptune's magnetic equator, the dipole component of the magnetic field is around 14 microteslas. Neptune has a dipole magnetic moment of around 2.2 × 1017 T·m3. The intricate structure of Neptune's magnetic field contains sizable contributions from non-dipolar elements, such as a potent quadrupole moment that could be stronger than the dipole moment. In comparison, the quadrupole moments of Earth, Jupiter, and Saturn are comparatively tiny, and their fields exhibit less tilt with respect to the polar axis. Neptune's enormous quadrupole moment might be caused by a geometric limitation on the dynamo generator in the field as well as an offset from the planet's center.

The magnetosphere starts to slow down the solar wind near Neptune's bow shock, which is 34.9 times the planet's radius away. The magnetopause is located 23–26.5 times the radius of Neptune, where the solar wind is counterbalanced by the pressure of the magnetosphere. The magnetosphere's tail reaches at least 72 times Neptune's radius, and probably much beyond.

#### Climate

Storm storms that are incredibly dynamic and have winds that can surpass supersonic velocity at over 600 m/s are what define Neptune's weather. More usually, wind speeds have been shown to vary from 20 m/s in an easterly direction to 325 m/s in a westward direction by following the movements of persistent clouds. The dominant winds at the tops of the clouds travel at speeds between 400 m/s around the equator and 250 m/s in the poles. The majority of Neptune's winds blow against the planet's rotation. Prograde rotation was observed in the overall wind pattern at higher latitudes, while retrograde rotation was observed at lower latitudes. It is believed that the variation in flow direction is the result of a "skin effect" rather than deeper atmospheric processes.

Neptune's average degree of meteorological activity is different than Uranus's. During its 1989 approach of Neptune, Voyager 2 saw weather events; but, during its 1986 flyby of Uranus, it did not see any such phenomena.

Methane, ethane, and acetylene are 10–100 times more abundant near Neptune's equator than they are at the poles. Since photochemistry cannot explain the pattern without meridional circulation, this is regarded as evidence for upwelling at the equator and sinking toward the poles.

The upper troposphere of Neptune's south pole was found to be around 10 K warmer than the remainder of its atmosphere, which has an average temperature of 73 K, in 2007. Methane, which is frozen in the troposphere elsewhere, can escape into the stratosphere close to the pole due to a significant temperature disparity. Because of Neptune's axial tilt, which has exposed the south pole to the Sun during the final quarter of Neptune's year, or around 40 Earth years, there is a relative "hot spot". The methane emission will shift to the north pole as Neptune progressively travels towards the other side of the Sun, darkening the south pole and illuminating the north.

##### Storms

NASA's Voyager 2 spacecraft detected the Great Dark Spot, an anticyclonic storm region measuring 13,000 km × 6,600 km, in 1989. The storm had similarities to Jupiter's Great Red Spot. The Great Dark Spot on the earth was not visible to the Hubble Space Telescope on November 2, 1994, which was approximately five years later. Rather, in Neptune's northern hemisphere, a new storm akin to the Great Dark Spot was discovered.

Another storm, the Scooter, is a collection of white clouds located further south than the Great Dark Spot. When they were seen traveling faster than the Great Dark Spot in the months preceding the 1989 encounter with Voyager 2, this moniker was originally used. The storm that was seen to be the second most powerful during the encounter in 1989 is a southern cyclonic storm known as the Small Dark Spot. The planet was initially entirely dark, but when Voyager 2 got closer, a brilliant center formed, which is visible in the majority of the highest-resolution photos. 2018 saw the identification and investigation of a smaller and more recent major dark spot. The first ground-based detection of a Neptune dark spot was reported in 2023.

Neptune's black patches look as holes in the top cloud decks because it is believed that they exist in the troposphere at lower altitudes than the brighter cloud features. They are believed to be vortex formations since they are stable characteristics that can last for several months. Brighter, longer-lasting methane clouds that grow around the tropopause layer are frequently linked to black patches. It is possible that certain past black patches, which are no longer apparent as dark features, are still cyclones due to the presence of associated clouds. When dark spots travel too close to the equator or perhaps by some other, unidentified cause, they may disappear.

Part of the reason Neptune's weather is more variable than Uranus' is because of its greater interior temperature. The temperature of the upper troposphere on Neptune drops to 51.8 K. The temperature is 72.00 K at a depth when air pressure is equivalent to 1 bar (100 kPa). The temperature steadily climbs further into the gas layers. The cause of this heating is the same as that of Uranus, but the difference is greater: Neptune radiates around 2.61 times as much energy as it gets from the Sun, whereas Uranus only radiates 1.1 times that much.

Despite being the planet with the greatest distance from the Sun—more than 50% further than Uranus—and receiving just 40% of the sun's energy, Neptune's own energy is enough to propel the Solar System's fastest planetary winds. The heat from Neptune's origin may be enough to explain its present heat flow, depending on the thermal parameters of its interior; Uranus's lack of internal heat is more difficult to explain concurrently while maintaining the apparent resemblance between the two planets.

#### Orbit

Neptune's orbital period, with a variation of around ±0.1 year, is 164.79 years, with an average distance of 4.5 billion kilometers from the Sun. 29.81 AU is the perihelion distance, while 30.33 AU is the aphelion distance.

Neptune's first complete barycentric orbit since its discovery in 1846 was completed on July 11, 2011, albeit it did not appear in the sky at the same location of its discovery since Earth was at a different point in its 365.26-day orbit. Neptune was not at its precise discovery location in respect to the Sun on July 11 due to the Sun's motion around the Solar System's barycentre; if the more widely used heliocentric coordinate system is employed, the discovery longitude was achieved on July 12, 2011.

Neptune's elliptical orbit has an angle of 1.77° with respect to Earth's.

Neptune has an axial tilt of 28.32°, which is comparable to the tilts of Mars (25°) and Earth (23°). Neptune thus goes through seasonal variations that are comparable to those on Earth. The seasons span forty Earth years due to Neptune's extended orbital period. Its period of sidereal revolution is approximately 16.11 hours. The change in day length during its long year is not so great, because its axial tilt is similar to that of Earth.

Differential rotation occurs in Neptune's atmosphere because it is not a solid body. The broad equatorial zone rotates more slowly than the planet's magnetic field, which rotates every 16.1 hours, with a period of almost 18 hours. In contrast, the polar areas, where the rotation time is 12 hours, experience the opposite situation. There is significant latitudinal wind shear as a result of this asymmetrical rotation, which is the most noticeable of any planet in the Solar System.

The Kuiper belt, which is the area directly beyond Neptune's orbit, is significantly impacted by it. The Kuiper belt, which stretches from Neptune's orbit at 30 AU out to around 55 AU from the Sun, is a ring of tiny frozen planets that resembles the asteroid belt but is significantly bigger. Neptune's gravity dominates the Kuiper belt in a manner similar to how Jupiter's gravity shapes the asteroid belt's formation. The gravitational pull of Neptune caused some areas of the Kuiper belt to become unstable throughout the solar system's lifetime, resulting in structural voids. One example is the area between 40 and 42 AU.

It is true that there are orbits within these voids where things can endure for the duration of the Solar System. When Neptune's orbital period is precisely a fraction of the object's, like 1:2 or 3:4, these resonances take place. An object will only have completed half an orbit by the time Neptune returns to its initial location if, for example, it circles the Sun once every two orbits of Neptune. With more than 200 confirmed objects, the 2:3 resonance is the most densely inhabited resonance in the Kuiper belt. The biggest known object in the Kuiper belt, Pluto, is among the objects in this resonance, which completes two orbits for every three of Neptune. These objects are referred to as plutinos. Pluto often travels around Neptune.

Numerous trojan objects are known to occupy the Sun–Neptune L4 and L5 Lagrangian points, which are the gravitationally stable areas that lead and lag Neptune in its orbit, respectively. One way to think of Neptune trojans is as a 1:1 resonance with Neptune. Remarkably stable in their orbits, certain Neptune trojans most likely originated with Neptune instead of being captured. 2008 LC18 was the first object found to be connected to Neptune's trailing L5 Lagrangian point. There is also a transient quasi-satellite on Neptune called (309239) 2007 RW10. For over 12,500 years, the object has been a quasi-satellite of Neptune, and it will continue to stay in that dynamical condition for a further 12,500 years.

#### Formation

Precise modeling of the genesis of Neptune and Uranus, the ice giants, has proven challenging. Many theories have been put out to explain the origin of these massive entities, but current models indicate that the matter density in the outer parts of the Solar System was too low to explain how they formed using the widely recognized process of core accretion. One is that the ice giants were created by instability inside the first protoplanetary disk rather than by core accretion, and that radiation from a nearby big OB star eventually destroyed their atmospheres.

Alternatively, they might have originated in a region with a higher matter density nearer the Sun, and then moved to their present orbits when the gaseous protoplanetary disk was extracted. Since it provides a more satisfactory explanation for the occupation of the populations of tiny objects seen in the trans-Neptunian zone, the concept of migration after formation is supported. A migratory Neptune and the other large planets' effects on the Kuiper belt's structure are the subject of the Nice model, now the most commonly accepted explanation for the specifics of this idea.

#### Moons

There are 14 known moons of Neptune. As the only moon huge enough to be spheroidal, Triton is the largest Neptunian moon, making up about 99.5% of the mass in orbit around Neptune. William Lassell found Triton barely seventeen days after discovering Neptune. Triton was most likely previously a dwarf planet in the Kuiper belt since it has a retrograde orbit, unlike any other massive planetary moon in the Solar System, which suggests that it was captured rather than developing in place. It is spiraling inward slowly due to tidal acceleration and is close enough to Neptune to be locked into a synchronous revolution. When it hits the Roche limit, which will happen in around 3.6 billion years, it will finally be destroyed.

The erratic moon Nereid, Neptune's second known satellite, has one of the solar system's most eccentric orbits. With an eccentricity of 0.7512, its apoapsis distance from Neptune is seven times greater than its periapsis distance.

Six moons of Neptune were found by Voyager 2 between July and September 1989. Among them, the asymmetric Proteus stands out for being the largest possible body for a body with its density without being forced into a spherical form by its own gravity. Despite being the second-massive Neptunian moon, its mass is just 0.25% that of Triton. The four innermost moons of Neptune, Thalassa, Despina, and Galatea, orbit in such a way as to be contained inside Neptune's rings. Larissa, the star that is the furthest away, was first found in 1981 when it occulted a star.

* Triton
* Hippocamp
* Proteus
* Nereid
* Thalassa
* Despina
* Galatea
* Neso
* Psamathe
* Halimede
* Laomedeia
* Naiad
* Larissa
* Sao

#### Rings

A planetary ring system exists on Neptune, although it is far less extensive than Saturn's. The rings' reddish color is most likely due to the possibility that they are made of ice particles covered in silicates or carbon-based substances. The three primary rings are the larger, fainter Galle Ring at 42,000 km, the narrow Adams Ring at 63,000 km from Neptune's center, and the Le Verrier Ring at 53,000 km. Lassell is the name given to a thin outward extension of the Le Verrier Ring that is 57,000 km in length and has the Arago Ring as its outer boundary.

A group under the direction of Edward Guinan discovered the first of these planetary rings in 1968. Analysis of these data, together with more recent findings, suggested in the early 1980s that this ring may not be complete. When the rings veiled a star during a stellar occultation in 1984, but not during emersion, there was initial evidence that the rings could include gaps. Voyager 2 images from 1989 resolved the dispute by revealing many faint rings.

The five notable arcs that make up the outermost ring, Adams, are currently called Courage, Liberté, Egalité 1, Egalité 2, and Fraternité. Since the laws of motion would have predicted that arcs would spread out into a uniform ring over short timeframes, the existence of arcs proved puzzling. Today, astronomers believe that Galatea, a moon that is somewhat inward of the ring, is responsible for corralling the arcs into their present shape.

Announced in 2005, Earth-based data seemed to indicate that Neptune's rings were far more unstable than previously believed. Comparing photos from Voyager 2 with those from the W. M. Keck Observatory in 2002 and 2003, significant ring loss is visible. Specifically, it appears that the Liberté arc may vanish in as little as a century.