Neptune (planet): characteristics, composition, orbit, movement, structure - science - 2023



Neptune It is the eighth planet in the solar system in terms of orbital distance, an ice giant and the outermost of all. This is the case since Pluto ceased to be considered a planet in 2006, becoming adwarf planet which is part of the Kuiper belt.

In the night sky Neptune looks like a small bluish dot about which very little was known, until space missions of the late 1980s, such as Voyager 2, provided data about the planet and its satellites.

Voyager 2 images showed for the first time a planet with a blue-green surface, with strong storms and fast wind currents, producing dark anticyclonic patches. They are very similar to those of Jupiter, although not as permanent in time as these.

Neptune's atmosphere is rich in methane and has a very faint ring system. The planet has a magnetosphere, which is why it is presumed to have a metallic core.

So far 15 satellites of Neptune have been counted, among which Triton and Nereida are the main satellites.

Discovery and history

The discovery of Neptune was the result of a mathematical prediction, based on observations of disturbances in the orbits of the planets Uranus and Saturn. Previously in 1610, Galileo had already sighted Neptune with the same telescope that he used to discover the moons of Jupiter, but he mistook it for a star.

Much later, in 1846, the French mathematician specialized in celestial mechanics Urbain Le Verrier, studied certain perturbations in the orbits of Saturn and Uranus. The best explanation was to propose the existence of a new planet, of which he predicted the orbit and position in the sky. The next step was to find the planet, so Le Verrier convinced the German astronomer Johann Gottfried Galle to look for it.

During the night of September 23, 1846, Galle confirmed, from his observatory in Berlin, the existence of the new planet, and a few days later Triton, its largest satellite, appeared.

Almost simultaneously in Cambridge, England, the young mathematician John Couch Adams, who had also been working on the problem for some time, made similar predictions.

Neptune owes its name to the god of the sea in Roman mythology (equivalent to the Greek god Poseidon), following the tradition of naming the planets after the deities of the Roman pantheon.

General characteristics

The diameter of Neptune is almost 4 times the diameter of Earth, but about a third of the gigantic Jupiter.

Its mass is 17 times that of the Earth and its volume is 57 times greater. In terms of mass, it ranks third among the planets in the solar system and fourth in size.

Summary of the main physical characteristics of Neptune

-Dough: 1,024 × 1026 kg (17,147 times that of Earth)

-Medium radius: 24,622 km, equivalent to 3.87 times the radius of the Earth.

-Shape: flattened at the poles by a factor 0.983.

-Mean radius of the orbit: 4,498 x 109 km equivalent to 30.07 U.A.

Rotation axis inclination: 30º with respect to the orbital plane.

-Temperature: -220ºC (clouds)

-Gravity: 11.15 m / s2 (1.14g)

-Own magnetic field: Yes, 14 microtesla at the equator.

-Atmosphere: Hydrogen 84%, Helium 12%, Methane 2%, Ammonia 0.01%.

-Density: 1,640 kg / m3

-Satellites: 15 known to date.

-Rings: Yes, they are thin and composed of ice and silicate particles.

Translation movement

Neptune, the eighth planet in the solar system, is a gas giant whose orbit around the Sun has a mean radius of 30 AU. One astronomical unit AU equals 150 million kilometers and is the average distance between the Sun and the Earth.

This means that the radius of Neptune's path is 30 times that of Earth, so it takes 165 years to complete one revolution around the Sun.

Fun facts about Neptune

-It is the planet furthest from the Sun, since Pluto, which is after the orbit of Neptune, is now a dwarf planet.

-Neptune is the smallest of the four giant planets (Jupiter, Saturn, Uranus, Neptune).

-The gravity of Neptune is very similar to that of Earth.

-It is the coldest planet in the solar system, with an average temperature of -221.4ºC.

-It has a system of rings, but unlike those of Saturn, they are not continuous, but instead form arcs along their orbital path.

-Neptune is the densest of the giant planets.

-It has hurricanes with the fastest winds in the solar system, which can reach an astonishing 2,100 km / h.

-Neptune has a Great Dark Spot, a whirlpool the size of planet Earth. This spot, photographed in 1989, disappeared in 1994, but gave rise to a new Dark Spot.

-Triton, the largest satellite of Neptune, rotates in the opposite direction to its other satellites, which is why it is thought that it was trapped by the planet and did not form at the same time as it.

-Triton (the largest satellite of Neptune) has volcanoes and nitrogen geysers, however it is one of the coldest stars in the solar system (-235ºC).

-The Voyager 2 mission passed 3,000 kilometers from the north pole of the planet Neptune in 1989.

-On July 12, 2011, Neptune completed its first full orbit since its discovery on September 23, 1846.

Rotatory motion

Neptune's rotation is 15 hours, 57 minutes, and 59 seconds, according to the most accurate measurement to date.

It is not an easy task to determine the rotation speed of a planet that only shows the surface of its atmosphere and that also moves. It is much easier to determine the rotational speed of rocky planets.

When Voyager 2 reached Neptune in 1989 a rotation period of 16 hours 6.5 seconds was estimated. Today this measurement is known to be inaccurate, thanks to the painstaking measurements of planetary scientist Erich Karkoschka of the University of Arizona.

Rotation speed and magnetic field

The speed of rotation of the other giant planets is measured by the pulses emitted by the magnetic field. However, this method does not apply to Neptune, since neither the axis nor the center of the magnetic dipole coincide with the axis of rotation of the planet, as we see in the following comparative image:

The image shows the model of the magnetic field produced by a dipole (a magnet), located more or less in the center of the planet. This model is also suitable for the Earth's magnetic field.

But the Neptune field is anomalous, in the sense that the quadrupole and higher-order inputs may be greater than the dipole field. And as we see in the figure, the dipole is displaced from the center.

So Karkoschka devised a different method, using more than five hundred images from the Hubble telescope. He found two characteristic features of the planet that he called:South Polar Feature Y South Polar Wave.

These have rotated at the same speed since the 1990s, corroborating that this is the true speed of the planet.

The image in Figure 5 (above) shows colors and contrasts modified by filters to emphasize the atmospheric characteristics of the planet.

As we have said, winds in Neptune's atmosphere frequently exceed the speed of sound.

Thus, the Great Dark Spot of Neptune varies its relative position over time, while Dark Spot 2 and the South Polar Feature maintain their relative positions. This suggests that they are tied to the rotation of the planet, which allowed Karkoschka to accurately determine the length of a day on Neptune.


Elements such as hydrogen (84%), helium (12%), methane (2%), and other gases such as ammonia, ethane, and acetylene are found in Neptune's atmosphere. Under this atmosphere there is a mixture of water, liquid ammonia, methane and molten rock, containing silica, iron and nickel.

Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere. Unlike Uranus, the twin planet, Neptune's composition has a larger volume of ocean.


The planet has a rocky core surrounded by an icy shell, all under a dense and thick atmosphere, occupying a third of its radius. It is similar to that of the twin planet Uranus.

The following figure shows the structure of Neptune in more detail.

Neptune has a structure with well differentiated layers:

Top layer: it is made up of clouds that are mostly hydrogen and helium, and to a lesser extent methane and other gases. It corresponds to approximately 5-10% of the mass of the planet.

Atmosphere: hydrogen, helium and methane.

Mantle: Beneath the atmosphere is the great mantle of the planet, a liquid region where temperatures can reach between 1,727 and 4,727 ° C. It is composed of water, methane and ammonia in a fluid state.

The mantle ranges from 10 to 15 land masses and is rich in water, ammonia, and methane. This mixture is called “ice”, despite being a hot and dense fluid, and it is also called the ocean of water and ammonia.

The mantle itself has very high temperatures, between 1,700ºC and 4,700ºC, and its electrical conductivity is also high.

Nucleus:composed of rock of silica, iron and nickel, similar to Uranus, the other giant of ice and gas. The mass of the nucleus is 1.2 times that of the Earth. The pressure in the center is estimated at 700 GPa, approximately twice that in the center of the Earth, with temperatures of up to 5,670 ºC.


The atmosphere of Neptune is very interesting and deserves a special section. To begin with, it is extremely cold, since it is the most distant planet from the Sun and receives very little solar radiation. Due to this, the temperature in the upper part of the atmosphere is in the order of -220 ºC.

But Neptune has an internal heat source, probably due to the collisions of the conduction electrons in the fluid mantle and also to the remaining heat during its formation.

Due to this huge temperature gradient, tremendous convection currents occur, making the planet's climate system very extreme.

And so the largest storms and hurricanes in the solar system are produced, as evidenced by the formation of huge patches of anticyclonic currents, due to opposing winds at different latitudes.

Among all the anticyclonic systems of Neptune, the Great Dark Spot stands out, photographed for the first time by the Voyager 2 probe in 1989, when it passed 3,000 kilometers from the planet.

In terms of color, Neptune's is even more bluish than Uranus, precisely because of its higher concentration of methane, which absorbs the red wavelength and reflects the blue wavelength. But there are also other molecules that contribute to its color.

In the lower region of the atmosphere (troposphere) the temperature decreases with height, but in the upper region (stratosphere) the opposite occurs. Between these layers the pressure is 10 thousand pascals (Pa).

Above the stratosphere is the thermosphere, which gradually transforms into the exosphere, where the pressure decreases from 10 Pa to 1 Pa.

Neptune satellites

To date, 15 natural satellites of the planet have been counted. The largest of its satellites and the first to be discovered, in 1846, is Triton. In 1949 a second satellite was discovered, named Nereida.

In 1989 the Voyager 2 mission discovered six more satellites: Naiad, Thalassa, Despina, Galatea, Larisa and Proteus.

Later in 2003 Halimedes, Sao, Laomedeia, Psámate and Neso are discovered. The small satellite 14 was discovered in 2013 by the SETI institute, its orbital period being 23 hours.

Let's see some details about the main moons of Neptune:


It is the largest of Neptune's satellites, with a diameter of 2,700 km, about 18 times smaller than its host planet and almost 5 times smaller than Earth.

Its orbital period is almost 6 days, but curiously it rotates in the opposite direction to the rotation of Neptune and its other satellites. In addition, its orbit is inclined 30 degrees with respect to the orbital plane of the planet.

It is the coldest object in the solar system, with an average temperature of -235 ° C and is made up of three-quarters of rock and a quarter of ice. On its surface there are geysers, with dark emanations towards the atmosphere, while the surface presents plains and some volcanoes with craters of 200 km.


It was discovered by Gerard Kuiper in 1949, thanks to the fact that it reflects 14% of the sunlight it receives.

Its size is one eighth of Triton and it has a very eccentric orbit, the closest closest distance to Neptune is 1,354,000 km and the furthest distance 9,624,000 km, taking 360 days to complete.


This irregular satellite was discovered in 1989 by the Voyager 2 mission and in size, it ranks second among the satellites of Neptune. It is a very porous, low-density satellite.

Despite its large size, it could not be discovered by telescopes because it is very dark and reflects little sunlight. Its average diameter is of the order of 400 km, being 6.7 times smaller than Triton.

Neptune Rings

Neptune has five thin and faint rings, composed mainly of dust and ice particles. It is believed that its origin is in the debris left by collisions between meteors and the planet's natural satellites.

The rings are named after the last names of the scientists who have contributed the most to their discovery and study. From the innermost to the outermost they are Galle, Le Verrier, Lassell, Arago and Adams.

There is also a ring whose orbit it shares with the Galatea satellite, which we can see in the following image:

When and how to observe Neptune

Neptune cannot be seen with the naked eye, even with an amateur telescope it looks so small that it can be mistaken for a star.

For this it is best to use a computer program or application that works as a planetarium. For the Android operating system, the Sky Maps application stands out, which allows you to quickly locate planets and other celestial objects with considerable precision.

The best time to observe is when the planet is in opposition, that is, the Earth is between the line that joins the Sun with Neptune.

This phenomenon occurs every 368 days and by 2020 it will occur on September 11. It is certainly not the only occasion to observe Neptune, which is also visible at other times of the year.

With a good telescope, Neptune can be distinguished from the background stars, as it looks like a blue-green disk.

Neptune's magnetosphere

Earlier it was commented on the peculiarities of Neptune's magnetic field. The planet's magnetic poles are inclined 47º with respect to the axis of rotation.

The magnetic field is generated by the movement of conductive fluids that form a thin spherical layer inside the planet. But on Neptune, the magnetic poles are displaced from the center by about 0.5 radii from the planet.

The intensity of the field at the magnetic equator is of the order of 15 microtesla, being 27 times more intense than that of the Earth.

The geometry of the field is complex, since the quadrupole contributions can exceed the dipole contribution, unlike the Earth in which the most relevant contribution is the dipole.

The magnetosphere Neptune's radius extends up to 35 times its radius at the shock front and 72 radii at the tail.

The magnetopause, which is the place where the magnetic pressure equals the pressure of the charged particles from the Sun, is between 23 to 27 radii from the planet.

Missions to Neptune

Voyager 2

The only space mission to orbit the planet Neptune was Voyager 2, which arrived on the planet in 1982.

At this time only two satellites were known: Triton and Nereida. But thanks to the Voyager 2 mission, six more were discovered: Naiad, Thalassa, Despina, Galatea, Larisa and Proteus. These satellites are quite smaller than Triton, with irregular shapes and smaller radius orbits.

These six satellites are suspected of being the remnants of a collision with an ancient satellite that collided with Triton when the latter was captured by Neptune's gravitational pull.

Voyager 2 also discovered new rings on Neptune. Although the first of the rings was discovered in 1968, its existence and the discovery of new ones was not possible until the arrival of said probe in 1989.

The spacecraft's closest approach to the planet occurred on August 25, 1989, which took place at a distance of 4,800 km above Neptune's north pole.

Because this was the last major planet the spacecraft could visit, it was decided to do a close flyby of the moon Triton, much like Voyager 1, which flew by Saturn and its moon Titan.

On August 25, 1989, the spacecraft headed for a close encounter with the moon Nereid before reaching 4,400 km from Neptune's atmosphere and on the same day passed near Triton, the planet's largest moon.

The spacecraft verified the existence of the magnetic field surrounding Neptune and found that the field was offset from the center and tilted, similar to the field around Uranus.


  1. N + 1. 200 kilogram diamonds are raining down on Uranus and Neptune. Recovered from:
  2. Powell, M. The Naked Eye Planets in the Night Sky (and how to identify them). Recovered from:
  3. Seeds, M. 2011.The Solar System. Seventh Edition. Cengage Learning.
  4. Wikipedia. Planetary ring. Recovered from:
  5. Wikipedia. Anneaux d’Neptune. Recovered from:
  6. Wikipedia. Exploration of Neptune. Recovered from:
  7. Wikipedia. Neptune (planet). Recovered from: