Neptune is the fourth largest and the farthest planet of the Solar System with the most powerful wind speeds out of all the planets. It is the smallest of the gas giants and is the first planet to be discovered by mathematical predictions in 1846.
Key Facts & Summary
- It is the only planet that is invisible to the unaided eye, thus it eluded ancient astronomers until it was officially discovered in 1846 by Le Verrier and Johann Galle.
- The greatest factor of the discovery was the planet Uranus, whom astronomers were calculating its orbit and observed that Uranus wasn’t following their models.
- The perturbed orbit of Uranus forced astronomer Urbain Le Verrier to conclude that another undiscovered planet must be at fault. He predicted where it should be, and astronomer Johann Galle found it one degree away from the predicted point thus making Neptune the first planet to be discovered by mathematical calculations and predictions.
- Neptune was given the name of the roman god of the sea due to its bluish-ocean like color. Its satellites also received names of water deities.
- Neptune’s color is believed to be influenced by the presence of methane in its atmosphere and also an unknown factor.
- Neptune has an average distance of 2.8 billion miles/4.5 billion kilometers or 30 AU away from the Sun, being the furthest of the eight planets. Currently it is 29.4 AU away from Earth with its light taking up to 4 hours to arrive to us. Neptune’s actual position can be checked online as the planet is constantly tracked.
- As a result from its distance, it also has the longest orbital duration completing a trip around the Sun in about 165 years.
- However, one rotation or day on Neptune is completed in 16 hours.
- Until Pluto’s eccentric orbit was understood and its status dropped from that of a planet to a dwarf planet, Neptune was considered the second furthest planet from the Sun.
- Neptune has a radius of 15.387 miles or 24.764 kilometers, about four times wider than Earth, and a diameter of 49.244 km or 30.598 mi being the fourth largest planet of the solar system.
- Neptune and Uranus are termed as ice giants because they are smaller and have different compositionality from the gas giants Jupiter and Saturn. Neptune being made out of layers of helium 29% and 80% hydrogen with traces of methane.
- It has a core about 1.5 times the size of Earth with twice its pressure: 7 Mbar or 700 GPa, making up about 45% of the planet’s mass, but it does not have a surface.
- This is a characteristic of the ice giants, their “rocky”, icy cores who are proportionally larger than the amount of gas they contain, unlike the gas giants.
- Neptune is the smallest ice giant with a mass of 1.024 × 10^26 kg but has the greatest density out of all the gas giants, about 1.64 g/cm³.
- It has an average temperature of -214 degrees Celsius;-353 degrees Fahrenheit, however Uranus is the coldest ice giant, and for that fact the coldest of all planets in the solar system.
- Neptune however, excels from something else. It has the fastest wind speed of any planet. Wind speeds blowing westward on the equator reach up to 2,160 kilometers or 1.324 miles per hour, nearly a supersonic flow. Most winds travel retrograde to the rotation of the planet they are 5 times stronger than the strongest winds recorded on Earth, breaking the sound barrier.
- Neptune has a total of 6 known rings with some containing ring arcs or clusters of dust particles in a ring.
- Neptune also has 14 known moons, the largest is called Triton, and it is the seventh largest known moon of any planet, also being the only one in the solar system that orbits in retrograde or in opposition to the planet’s rotation. This means that it is a captured object by Neptune’s gravity. Due to its size it is even believed it is a captured dwarf planet.
Neptune was observed in 1613 by Galileo Galilei however, he taught that it was just a star, and when he wanted to further research it, the motion of the planet began to be far too slight to be detected again. The greatest factor of the discovery was the planet Uranus, whom astronomers were calculating its orbit and observed that Uranus wasn’t following their models.
The perturbed orbit of Uranus forced astronomers to conclude that another undiscovered planet must be at fault. They predicted where it should be, and astronomer Urbain Le Verrier asked the help of Johann Galle, an astronomer at the Berlin Observatory. Galle found the planet one degree away from the predicted point thus making Neptune the first planet to be discovered by mathematical calculations and predictions.
Galle proposed the name Janus, the Roman god with two faces and La Verrier proposed the name Neptune, however he did tried to name the planet after him but he didn’t have popular support outside of France. The astronomer Struve, came out in favor of the name Neptune in December 29th 1846. Soon after, it became wildly accepted. Neptune was the Roman god of the sea, identified with the Greek Poseidon as the planet appeared water-like in color.
Through simulations after the Nice model, it has been suggested that both Neptune and Uranus formed closer to the sun and later drifted away. It is hypothesized that the Solar System formed from a giant rotating ball of gas and dust known as the pre-solar nebula.
Much of it formed the Sun while more of its dust went on and merged to create the first proto-planets. As they grew, some accreted enough matter for their gravity to hold unto the nebula’s leftover gas. Estimates suggest the creation to have taken place about 4.5 billion years ago, and the drifting about 4 billion.
Distance, Size and Mass
From its discovery until 1930 when Pluto was discovered, Neptune was considered the farthest planet. After the discovery of Pluto, Neptune was thought to be the second furthest planet. When Pluto’s eccentric orbit was understood and its status dropped from that of a planet to a dwarf planet in 2006, Neptune regained the title of the farthest planet in Solar System.
Neptune has an average distance of 2.8 billion miles/4.5 billion kilometers or 30.1 AU away from the Sun, and its currently 29.4 AU away from Earth with its light taking up to 4 hours to arrive to us.
The mass of Jupiter is about 17 times that of Earth or 1.0243×1026 kg, but when compared to the largest gas giant Jupiter, it has only 1/19th of Jupiter’s mass. It has an equatorial radius of 15.387 miles or 24.764 kilometers, about four times wider than Earth, and a diameter of 49.244 km or 30.598 mi, being the fourth largest planet of the solar system.
Orbit and Rotation
Since it is the farthest planet from the Sun it has the longest orbital duration, completing a trip around the Sun in about 165 years. However, one sidereal rotation or day on Neptune is completed in 16.11 hours.
The average distance from the Sun is about 30.1 AU, while at its perihelion 29.81 AU, and its aphelion 30.33 AU. The elliptical orbit of Neptune is inclined 1.77° compared to that of Earth.
The axial tilt of Neptune is 28.32°. It is similar to the tilts of Earth 23° and Mars 25°. As a result, Neptune experiences similar seasonal changes to Earth but due to its long orbital period, the seasons last for 40 Earth years. Due to the fact that it lacks a solid body, its atmosphere undergoes differential rotations.
In the wide equatorial zone the rotation period is completed within 18 hours, much slower than the 16.11 – hour rotation of the planet’s magnetic field.
The polar regions complete a rotation in 12 hours being the most pronounced differential rotation of any planet in the Solar System, resulting in strong latitudinal wind shear.
Neptune's orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of small icy worlds, similar to the asteroid belt but far larger, extending from Neptune's orbit at 30 AU out to about 55 AU from the Sun. Many dwarf planets are located here.
Jupiter’s gravity dominates the asteroid belt, shaping its structure and dominating the Kuiper Belt. Many regions in the Kuiper belt became destabilized due to Jupiter’s gravity, creating gaps in the belt’s structure. This can be observed in the 40 – 42 AU region. The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance.
Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutinos because the largest of the known Kuiper belt objects, Pluto, is among them. Although Pluto crosses Neptune's orbit regularly, the 2:3 resonance ensures they can never collide. The 3:4, 3:5, 4:7 and 2:5 resonances are less populated.
The internal structure is very similar to that of the planet Uranus. Its atmospheres forms about 5% to 10% of its mass and extends about 10% to 20% of the way towards the core reaching pressures of about 10 GPa or 100.000 times that of Earth’s atmosphere. Concentrations of methane, ammonia and water are found in the lower regions of the atmosphere with the mantle equivalent to 10-15 Earth masses.
This mixture is referred to as “icy” even though it is a hot, dense fluid sometimes called a water-ammonia ocean. Going even deeper the conditions may be such that even methane decomposes into diamond crystals that rain downwards like hailstones. It is thought that this kind of diamond rain occurs also on Jupiter, Saturn and Uranus. The core is likely composed of iron, nickel and silicates. The core is estimated to be about 1.5 times the mass of Earth. The pressure at the center is 7 Mbar or 700 Gpa, twice as high as that at the center of Earth with temperatures of around 5.400 K.
Atmosphere and Climate
The atmosphere is made up mostly of hydrogen, helium and methane. Very similar to Uranus, its vivid blue color is influenced by the presence of methane and some unknown factor causes the more intense color. Neptune may not be as cold as Uranus but it has the most powerful winds out of all the planets in the Solar System despite being the furthest planet from the Sun and receiving the lowest energy input from it. Having the wildest and strangest weather in the entire Solar System, it is not understood how it gets this much energy in order to produce such weather.
Neptune has an average temperature of -214 degrees Celsius;-353 degrees. Wind speeds blowing westward on the equator reach up to 2,160 kilometers or 1.324 miles per hour, nearly a supersonic flow. Most winds travel retrograde to the rotation of the planet they are 5 times stronger than the strongest winds recorded on Earth, breaking the sound barrier. Bands and colossal storms also form on the planet.
In 1989 when Voyager 2 observed the planet, a Great Dark Spot was present among other spots. It was a storm about the size of Earth passing through Neptune’s atmosphere. When Hubble was launched, astronomers wanted to see if these spots were a permanent feature like Jupiter’s Great Red Spot but the storms were no longer there. Further observations led to discovery of other storms, each appearing and disappearing. The more active weather on Neptune might be due, in part, to its higher internal heat compared to that of Uranus.
Neptune’s magnetic field is offset 47° relative to its rotational axis. It is believed that this is caused by the magnetic field not being generated in the core but rather by an electrically conducting liquid mantle, or that the mantle deflects the magnetic field from the core which gives it a weird offset in relation to the rotational axis.
No planet in the solar system has a perfectly aligned magnetic field. Even Earth’s magnetic north is different from where the North Pole is. However, only Uranus and Neptune have such a tilted magnetosphere. Auroras exist on Neptune but they are very faint due to particles not getting charged so much by the sun, and because of the direction of the magnetosphere, they are mainly type B aurora, or SAR arcs. SAR arcs are not around the poles, but rather around the mid latitudes of the planet.
A common feature of the gas giants are their ring systems, which Neptune also has. It is very faint due to its low denseness and extremely dark color, a reddish hue. Neptune has 5 ring systems named after the people involved in the discovery and research of Neptune. The innermost is the Galle Ring, which is faint and wide at 2.000 km or 1.242 miles. The second is the first bright ring, named Le Verrier, it is only 113 km wide or 70.2 mi.
The third is the Lassell Ring, a very faint band 4.000km or 2.485 mi across. On the edge of this ring is the Arago Ring, slightly brighter and less than 100 km or 62 mi wide.
The last known and outermost ring is named the Adams Ring. It is about 35 km or 21.7 mi wide but it is one of the brightest rings. It has a special feature. It is slightly inclined with bright arcs in it. The three main rings are narrow and extend up to 63.000 km or 39.146 mi from Neptune. The Le Verrier Ring, at 53.000 km or 32.932 mi, and the broader, fainter Galle Ring, at 42.000 km or 26.097 mi.
A total of 14 known moons surround Neptune. They have all been named after water deities in Greek mythology. The first moon discovered was Triton, just 17 days after Neptune itself was discovered. It is also the largest of the 14 moons. The 14 moons of Neptune can be divided into regular, irregular and unusual irregular moons.
Out of the 14 moons, only 7 are inner regular moons, meaning they orbit along Neptune’s ecliptic with very circular orbits or orbits with very low eccentricity.
In order of their distance from Neptune: Naiad, Thalassa, Despina, Larissa, Hippocamp and Proteus.
The smallest is Hippocamp 35km or 21.7 mi followed by Naiad 58 km or 36 mi, Thalassa 80 km or 49.7 mi, Despina 148 km or 91.9 mi, Galatea 158 km or 98.1 mi, Larissa 196 km or 121.7 mi and the second largest moon Proteus 438 km or 272 mi.
Naiad is the closest regular moon and the second smallest of the inner moons.
All of these regular moons have an average temperature between -221 degrees Celsius up to -224 degrees Celsius.
The rest of the 14 Neptunian moons are irregular moons. Irregular moons follow an inclined, eccentric and often retrograde orbit.
In order of their distance: Triton, Nereid, Halimede, Sao, Laomedeia, Psamanthe, and Neso.
The smallest of the irregular moons is Psamanthe with a diameter of 40 km or 24.8 mi followed up by Laomedeia 42 km or 26 mi, Sao 44 km or 27.3 mi, Neso 60 km or 37.2 mi, Halimede 62 km or 38.5 mi, Nereid 340 km or 211 mi and the biggest of the moons Triton 2.706 km or 1.681.
All of these irregular moons have an average temperature between – 222 degrees Celsius up to -233 degrees Celsius.
Unusual Irregular Moons
Nereid is the third largest of the Neptunian moons, both it and Triton the largest moon - are considered unusual irregular moons and are two of the largest irregular moons known in the solar system. They both have atypically small semi-major axes and unusual orbital eccentricities
Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton's orbit is a nearly perfect circle. Nereid also has the lowest inclination of any known irregular satellite.
Triton orbits clockwise around Neptune as Neptune rotates counterclockwise. Triton orbits at a 130 degree angle to the ecliptic of the planet though its orbital eccentricity is close to zero, being almost perfectly circular. The biggest moons in the solar system are usually regular moons, making Triton a bit special. Tritons size is even bigger than Pluto’s a dwarf planet. It is strongly believed that Triton is actually a captured dwarf planet. It comprises 99.5% the mass found in Neptune’s orbit.
It is the seventh largest moon in the solar system. Although Triton is now a captured object of Neptune’s gravity, it is considered the biggest and most massive Kuiper Belt Object. Triton and Pluto share a identical composition, which supports the idea that they share a common origin.
Both Pluto and Triton have nitrogen ice surface, with other ices mixed in like water and carbon dioxide. It has a flat terrain with its topography never varying more than a kilometer. Ridges, plateaus and ice plains are present.
The small amounts of craters imply that its surface is very young and constantly renewed. It also has reddish patches like Pluto which is thought to be methane ice that reacted to UV light from the Sun, producing what is known as tholins, an organic compound.
However, even if primordial elements needed for life are present, it is far too cold on its surface -235 degrees Celsius. It is thought that underneath Triton’s surface there is a silicate mantle and metallic core, this could explain its relatively high density for a moon of 2 g/cm3.
The radioactive decay from the core could be enough to heat and power convection in a subsurface ocean of water, similar to what is thought to be under the surfaces of Europa, Enceladus and some other large moons. Just as in the case of Europa and Enceladus, cryovolcanism is an active process on Triton.
Liquid water in the mantle erupts onto the surface like lave on Earth thus the main reason why Triton’s surface appears so young. It is actively renewed by liquid water erupting and then freezing on the surface.
If these eruptions are the reason why minerals, the source of the tholins and organic matter are present on the surface, then it means that organic compounds are found in the subsurface ocean thus creating a possibility for life to have been able to form there.
Since it doesn’t have a true surface but rather swirling fluids it is not conducive to life as we know it. Neptune’s moon Triton however, appears more likely to develop life.
Future plans for Neptune
With new technologies coming up, Neptune is definitely a target for future studies by the use of new telescopes. Planned missions for Neptune are pretty vague since it would take well over 10 years to get there since it is the farthest planet.
Did you know?
- The name Neptune was first given to the seventh discovered planet of the Solar System, Uranus. However, the name was discarded but Uranus ultimately led to the discovery of the eight planet, that would bear the name Neptune.
- Uranus is Neptune’s near twin in size and composition.
- In 2011 Neptune completed its first 165-year orbit since its discovery in 1846, meaning we have witnessed just 1 Neptunian year.
- Between 2013 - 2049, the closest approach of Neptune to Earth will happen in 2041 at a distance of 28.8 AU.
- Because of the dwarf planet Pluto’s elliptical orbit, Pluto is sometimes closer to the Sun and Earth than Neptune is.
- Neptune is 30 times farther than Earth’s orbit from the sun.
- The Sun is 900 times dimmer on Neptune than on Earth.
- Most languages use some variant of the name “Neptune” for the planet. In Chinese, Vietnamese, Japanese and Korean, the planet’s name was translated as “sea king star” while in Mongolian it is called Dalain Van. In modern Greek the name is Poseidon while in Māori the planet is called Tangaroa, the name of the Māori god of the sea.
- Neptune has gravity at 1 bar of 11.15 m/s2, 1.14 times the surface gravity of Earth, and surpassed only by Jupiter who has a gravity of 24.79 m/s².
- A total of 58 Earths can fit in Neptune.
- The Voyager 2 is the only spacecraft to have reached Neptune to this day.
- Neptune has a very strong magnetic field. It is around 27 times stronger than the one on Earth.