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Neptune: The Eighth Planet of the Solar System

Updated on January 2, 2023
Larry Slawson profile image

Larry Slawson received his Master's Degree from UNC Charlotte in 2018. He has a keen interest in Astronomy and Physics.

Neptune: The eighth planet of the solar system, and most distant planetary object from the Sun.
Neptune: The eighth planet of the solar system, and most distant planetary object from the Sun.


Our Solar System occupies a unique place in the cosmos due to its composition and overall structure. Within this realm, a particular area that deserves attention is the planet Neptune. As the eighth planet from our Sun, this Ice Giant plays a special role in the overall function of our Solar System. This article provides an in-depth analysis of Neptune that focuses on its origins, properties, and composition. After reading the material that follows, it is this author’s hope that a deeper understanding (and appreciation) of this remarkable planet will be gained by his readers.

Scientific Properties of Neptune

  • Planet Name: Neptune
  • Orbital Semimajor Axis: 30.07 Astronomical Units (4,498 Million Kilometers)
  • Orbital Eccentricity: 0.009
  • Perihelion: 29.81 Astronomical Units (4,460 Million Kilometers)
  • Aphelion: 30.33 Astronomical Units (4,537 Million Kilometers)
  • Mean/Average Orbital Speed: 5.43 Kilometers Per Second
  • Sidereal Orbital Period: 163.7 Years (Tropical)
  • Synodic Orbital Period: 367.49 Days (Solar)
  • Orbital Inclination (To the Ecliptic): 1.77 Degrees
  • Greatest Angular Diameter (As Viewed From Earth): 2.4”
  • Overall Mass: 1.02 x 1026 Kilograms; Approximately 17.15 Times the Mass of Earth (If Earth = 1)
  • Equatorial Radius: 24,766 Kilometers; Approximately 3.88 Times the Radius of Earth (If Earth = 1)
  • Mean/Average Density: 1,638 Kilograms Per Meter Cubed; Approximately 0.297 Times the Density of Earth (If Earth = 1)
  • Overall Surface Gravity: 11.14 Meters Per Second Squared; Approximately 1.14 Times the Surface Gravity of Earth (If Earth = 1)
  • Escape Speed/Velocity: 23.5 Kilometers Per Second
  • Sidereal Rotation Period: 0.67 Days (Solar)
  • Axial Tilt: 29.6 Degrees
  • Surface Magnetic Field: 0.43 of Earth’s Overall Magnetic Field Strength (Earth = 1)
  • Magnetic Axis Tilt (Relative to the Rotation Axis): 46.0 Degrees
  • Mean/Average Surface Temperature: 59 Kelvins (-353.47 Degrees Fahrenheit)
  • Number of Natural Satellites (Moons): 14 Total (Discovered so far)

Urbain Le Verrier: the astronomer credited with helping to discover the planet Neptune.
Urbain Le Verrier: the astronomer credited with helping to discover the planet Neptune.

The Discovery of Neptune

Neptune is the eighth planet in our solar system, and the most distant Gas Giant from the Sun. Neptune was first discovered in modern times (1846) due to the fact that it is invisible to the naked eye; making it unknown to ancient civilizations. The planet was first observed from an observatory in Berlin during the year 1846 by Johann Gottfried Galle. However, astronomers were aware of Neptune a year earlier when mathematical models indicated the presence of an object beyond Uranus that was having a gravitational impact on the planet’s orbital patterns.

Using mathematical formulas to calculate the unknown object’s trajectory, English and French mathematicians, John Couch Adams and Urbain Le Verrier, respectively, were able to pinpoint the exact region of the sky that the new planet would appear in. When Galle actually discovered the planet a year later, each of the astronomers debated who first discovered Neptune (a debate that still hasn’t been settled today). Despite this early controversy, astronomers dubbed the new discovery “Neptune” in honor of the Roman God of the Sea.

The planet Neptune.
The planet Neptune.

Orbital Properties and Rotation

Similar to Uranus’s rotational rate, Neptune’s atmosphere completes a full rotation every 17.3 hours. This is a unique trait to the Jovian planets, however, as Neptune’s interior performs a full rotation every 16.1 hours; thus, indicating a slower atmospheric rotation than its interior.

Due to its tremendous distance from the Sun (approximately 4.5 billion kilometers away), one year for Neptune is equivalent to nearly 165 Earth years. Despite travelling at speeds of approximately 5.43 kilometers per second, the tremendous orbit requires approximately 89,666 days to complete a full cycle. As of 2020, Neptune has completed only one orbital period since its discovery in 1846.

Orbital Resonance

As a result of Neptune’s proximity to the Kuiper Belt, the planet is believed to have a profound gravitational effect on the various asteroids, meteors, and comet-like objects that occupy this portion of the solar system. For this reason, large gaps in the Kuiper Belt have been observed by the scientific community in recent years; particularly in areas close to Neptune’s present orbit. This is problematic, as the effects of gravity occasionally cause large celestial bodies to be hurled towards the inner solar system (in the proximity of Earth). Many of the current comets that inhabit our system are believed to have originated from Neptune’s effect on the Kuiper Belt. However, more evidence is needed to substantiate such claims.

The interior of Neptune. 1.) Upper Atmosphere, 2.) Lower Atmosphere, 3.) Mantle, 4.) Inner Core.
The interior of Neptune. 1.) Upper Atmosphere, 2.) Lower Atmosphere, 3.) Mantle, 4.) Inner Core. | Source

Neptune's Atmosphere and Composition

Scientists believe that Neptune is composed primarily of gas (both helium and hydrogen) and ice, with a solid, rocky core. Despite the fact that the planet is incredibly large, its surface gravity is actually close to Earth’s (only 17-percent stronger). It is hypothesized that the gravitational pull is relatively low since Neptune’s mass is spread out over a larger surface area than smaller planets. The abundance of ammonia and other gaseous elements also help to explain this discrepancy as well.


Neptune’s internal composition is comparable to Uranus, with an atmosphere that forms approximately 5 to 10-percent of its overall mass. Scientists believe that Neptune’s atmosphere is comprised of hydrogen, methane, and helium gases that surround a watery mantle consisting of ammonia, methane, ice, and water. In total, it is estimated that Neptune’s atmosphere reaches an internal pressure of 10 GPa; making it nearly 100,000 times the pressure of Earth’s atmosphere.

In the planet’s higher altitudes, scientists estimate that Neptune’s atmosphere is comprised of 80-percent hydrogen, 19-percent hydrogen, and approximately 1-percent methane. The presence of methane plays a critical role in the planet’s coloration, as methane helps to absorb red light; thus, giving Neptune its dark, oceanic blue hue. The planet Uranus maintains a similar composition, with slightly less methane gas. This helps to explain why Neptune’s shade of blue is deeper than that of Uranus (which is more greenish-blue in comparison).


Neptune’s mantle is relatively large, and is equivalent to nearly 15 Earth masses. Comprised of water, methane, and ammonia, scientists often describe Neptune’s mantle as a water-ammonia ocean due to its fluidity and soup-like composition. It is estimated that this vast “ocean” is nearly 7,000 kilometers deep, and possesses a high-level of electrical conductivity. Due to the extreme conditions (and pressure) within the mantle, scientists have speculated that diamonds may be abundant in this region, with diamond-like crystals falling like rain in the lower atmosphere.

Despite its distance from the Sun, this portion of Neptune reaches extraordinarily high temperatures that range between 2,000 to 5,000 Kelvins (3,140 to 8,540 Degrees Fahrenheit). Neptune’s mantle shows surprising similarities to that of Uranus; however, it widely believed that Neptune’s mantle is far larger with a higher volume of oceanic materials.


Scientists believe that Neptune’s core is comprised primarily of nickel, iron, and silicates that provide an overall mass that exceeds 1.2 times that of Earth. With an internal pressure of nearly 7 Mbar (or 700 GPa), it is estimated that Neptune’s internal temperature reaches an astounding 5,400 Kelvins. Similar to the planet’s mantle, it is theorized that large concentrations of diamonds may also be present within this region due to the extreme temperature and pressure.

The Great Dark Spot of Neptune; a massive storm system known to generate tremendous wind speeds on the planet's surface.
The Great Dark Spot of Neptune; a massive storm system known to generate tremendous wind speeds on the planet's surface.

Weather Patterns and Systems


Out of all the planets in our solar system, Neptune has the strongest winds. At approximately 2,100 Kilometers/Hour (1,304.9 M.P.H.), the planet’s winds make Earth’s most powerful hurricanes look weak, in comparison. Scientists remain uncertain as to why the planet’s winds reach such speeds. However, it is hypothesized that Neptune’s cold temperatures allow a flow of gases throughout its atmosphere that reduces friction and, in turn, allows winds to move uninhibited.

At approximately 2.793 billion miles from the Sun, Neptune is also the coldest planet in our solar system. In its upper atmosphere, temperatures are estimated to reach 51.7 Kelvin (or -366.6 Degrees Fahrenheit). These temperatures are three times the lowest recorded temperature on Earth (which hit a record -129 Degrees Fahrenheit).

The Great Dark Spot

Neptune’s “Great Dark Spot” is an anti-cyclonic storm system first discovered by NASA’s Voyager 2 in 1989. Similar to Jupiter’s “Great Red Spot,” scientists estimate that the storm is approximately 13,000 kilometers wide.

Additional dark spots dot Neptune’s atmosphere as well. These smaller storm systems (considered large by Earth standards) are thought to form in Neptune’s troposphere. Capable of sustaining themselves for several months at a time, these large cyclones often migrate across Neptune’s surface before finally dissolving. It is unclear what causes these large storm systems to suddenly dissipate; however, scientists speculate that the planet’s equatorial regions may play a role in dissolving the storms due to the sudden change in temperatures around these areas.

During its flyby of the planet, Voyager 2 also discovered two smaller storm systems (dark spots) moving at extraordinary speeds across Neptune’s surface. These spots were affectionately dubbed “Scooter” and the “Small Dark Spot.” Similar to the Great Dark Spot, these cloud groups are thought to be powerful storm systems that originated in Neptune’s southern hemispheres. Little is currently known about the specifics of these storms; however, future missions to the planet may provide scientists with unprecedented insight into their movement and power in the years to come.

Clouds spotted by Voyager 2 in Neptune's upper atmosphere.
Clouds spotted by Voyager 2 in Neptune's upper atmosphere.

Neptune's Magnetosphere

Similar to Uranus, Neptune possesses an unusual magnetosphere that is tilted away from its rotational axis. Reaching nearly 100 times the strength of Earth’s magnetosphere, it is believed that the planet’s magnetic field is “populated largely by electrons and protons” captured by solar wind or from ionized hydrogen gas from Neptune itself (McMillan, 344).

Also like Uranus, it is believed that Neptune’s unusual magnetosphere is powered by its slush-like interior that is composed primarily of ammonia. Combined with molecular hydrogen, this solution is believed to provide a “thick, electrically conducting ionic layer” that accounts for both the misalignment of the magnetic field and its source of energy (McMillan, 346). This feature is unique in the realm of Jovian planets, as most magnetic fields are generated from within their core. For Uranus and Neptune, however, it would appear that these fields generate in the depths of their water-ammonia “oceans” that cover the planets.

Image from the Hubble Space Telescope that showcases Neptune's rings and natural satellites (moons).
Image from the Hubble Space Telescope that showcases Neptune's rings and natural satellites (moons).

Planetary Rings of Neptune

Like the other Jovian planets (Jupiter, Saturn, and Uranus), Neptune also possesses an intricate ring-system; albeit, one that is quite faint and difficult to see. In contrast to Saturn’s bright and well-developed rings, Neptune’s five rings are not well-organized and are relatively dark due to the presence of organic compounds and the effects of radiation darkening.

Scientists believe that the rings are composed of twenty-percent dust, and eighty-percent rock (small). The planet’s rings are named for astronomers who made crucial discoveries concerning the planet such as Galle, Le Verrier, Adams, Arago, and Lassell. Scientists are uncertain as to what created Neptune’s rings, however, it is hypothesized that they are relatively young and may have formed from the collision of moons.

Three of Neptune’s rings are relatively narrow, with the other two showing similarities to Jupiter’s “broad and diffuse” rings (McMillan, 354). Neptune’s outermost ring, known as Adams, is probably the most unique in that it appears “clumped” in various places. It is unclear what causes this phenomenon; however, astronomers believe that Neptune’s large array of “Shepherd Moons” may be partially responsible.

Triton; Neptune's largest moon.
Triton; Neptune's largest moon.

Neptune's Moons

In total, Neptune possesses fourteen known moons. These are Triton, Nereid, Proteus, Galatea, Despina, Thalassa, Hippocamp, Halimede, Laomedeia, Neso, Psamathe, Naiad, Larissa, and Sao. Triton, Neptune’s largest moon, was first discovered in 1846, with Nereid being located by Gerard Kuiper in 1949. Six additional moons were uncovered by Voyager 2 in its 1980s flyby, with an additional six moons being uncovered by “ground-based surveys” in the early 2000s (McMillan, 348).

Neptune’s moons are unique in the solar system due to their unusual orbits. While most moons that orbit the Jovian planets maintain stable orbits, the moons of Neptune follow a “circular, equatorial, and prograde” orbit around their host. These elongated trajectories made the tracking of these natural satellites difficult in years past.


Triton is Neptune’s largest (and perhaps most interesting) moon, and is composed primarily of nitrogen, ice, and dust. The moon was first discovered by William Lassell only 17 days after Neptune’s discovery. Scientists believe that Triton was once a KBO (Kuiper-Belt Object) and was captured (recently) by Neptune’s gravitational pull. Astronomers base this hypothesis on the fact that Triton orbits in a retrograde manner, indicating that its formation likely occurred elsewhere.

Triton follows a synchronous rotation (similar to Earth’s Moon) around Neptune due to its close proximity to the planet. Unfortunately, it is believed that the moon is slowly accelerating towards Neptune and will eventually tear itself apart in approximately 3.6 billion years (upon reaching the Roche limit).

Triton is extremely cold, reaching temperatures of only 37 Kelvins (-393 Degrees Fahrenheit). This is due, in part, to Triton’s tremendous distance from the Sun (approximately 4.5 billion kilometers), as well as its thin atmosphere that is composed mostly of nitrogen. Scientists estimate that Triton’s atmosphere is nearly “a hundred thousand times thinner than Earth’s,” with a surface composed primarily of water and ice.

Size comparison of Neptune and Earth.
Size comparison of Neptune and Earth.

Formation of Neptune

Similar to the other gas giants, it has been extremely difficult for scientists to provide a model on how Neptune formed within our solar system. Current scientific models indicate that Neptune likely formed closer to the Sun, due to the lack of matter on the outer regions of the solar system during its formative years. Astronomers hypothesize that Neptune probably developed in close proximity to the Sun, where matter density was far higher.

Nice Model

Following its development, it is theorized that Neptune’s mass caused the planet to slowly drift away from the center of our solar system. After migrating towards the outer fringes of the solar system, however, scientists believe that Neptune was later captured by the Sun’s gravitational pull; locking the planet into an orbital cycle. This theory, known as the “Nice Model” remains the most accepted theory for Neptune’s formation, to date, until additional data pertaining to the planet can be uncovered by the scientific community.

Exploration of Neptune

To date, Voyager 2 remains the only spacecraft to have visited Neptune (1989). Although the images (and measurements) provided by the spacecraft have provided scientists with invaluable information, many questions remain about the planet’s fundamental properties and composition. Currently, there are no plans to visit Neptune; however, numerous proposals (including a flyby from the Argo spacecraft which was launched in 2019) have been put forward by the scientific community for future projects. Current estimates place new missions to Neptune in the late 2020s or 2030s.

Fun Facts About Neptune

  • Fun Fact #1: Neptune’s upper atmosphere is composed primarily of 80% Hydrogen, 17% Helium, and less than 3% Methane gas. It is believed that Neptune’s deep blue coloration is the result of methane absorbing light with wavelengths that correspond to the color red.
  • Fun Fact #2: Similar to Jupiter’s “Great Red Spot,” Neptune also possesses dark spots which are believed to be giant storms. In contrast to Jupiter’s Great Red Spot, however, these storms are often short-lived. The first spot was discovered by Voyager-2 during its flyby decades prior. The second spot, also observed by Voyager-2 was nicknamed “Scooter,” and is also believed to be a fast-moving storm system that will eventually dissipate.
  • Fun Fact #3: One year on Neptune lasts approximately 165 years on Earth due to its tremendous distance from the Sun. To put its distance in perspective, it takes light approximately four hours and forty minutes to reach Neptune from the Sun. Despite these long years, one day on Neptune is remarkably short, due to its rapid rotational speed. Neptune has the third shortest day in the solar system, at only sixteen hours.
  • Fun Fact #4: Only one spacecraft has visited Neptune, known as Voyager-2. The spacecraft swept by the planet in 1989 and provided the first in-depth photos of the planet for NASA and the world at large. Although Neptune has been studied extensively by the Hubble Space Telescope, along with a number of ground-based observatories, there are currently no future missions planned for further exploration of the planet.
  • Fun Fact #5: Since its discovery in 1846, Neptune has only completed one orbit around the Sun; a feat that was accomplished in 2011.
  • Fun Fact #6: Triton is the only known moon in the solar system that orbits Neptune in a retrograde motion, making it a very unique object in our solar system.

“No one planet can tell us everything about the universe, but Neptune seems to hold more than its share of information about the formation of our own solar system – as well as the solar systems beyond.”

— Heidi Hammel

Quotes About Neptune

  • Quote #1: “No one planet can tell us everything about the universe, but Neptune seems to hold more than its share of information about the formation of our own solar system – as well as the solar systems beyond.” – Heidi Hammel
  • Quote #2: “If you put Earth out beyond Neptune, you wouldn’t be able to call it a planet because it couldn’t clear its zone.” – Alan Stern
  • Quote #3: “Neptune’s unusual behavior is showing us that though we can make great models of planetary atmospheric circulation, there may be key pieces missing.” – Heidi Hammel
  • Quote #4: “I grew up believing my sister was from the planet Neptune and had been sent down to Earth to kill me. I believed this because my sister Emily convinced me of it when I was a toddler.” – Zooey Deschanel

Concluding Thoughts

In closing, Neptune remains one of the most fascinating objects in our solar system. With its beautiful dark blue coloration, and mysterious allure, Neptune will continue to capture the hearts and minds of the scientific community for decades to come. What secrets does this fascinating planet hold about the formation of our solar system? What can the scientific community learn about Neptune’s composition and design? Finally, and perhaps most importantly, what can the planet tell us about the solar system and universe at large?

As NASA and the scientific community continue to delay plans for future space missions to Neptune, it is unlikely that we will be able to learn the answers to these questions in the near future. However, with the rise of private enterprises (and their keen interest in space exploration), additional missions, such as Voyager-2 may become a reality sooner than we expect. Only time will tell.

Works Cited

Choi, Charles Q. "Planet Neptune: Facts About Its Orbit, Moons & Rings." May 12, 2017. Accessed April 26, 2019.

McMillan, Steve and Eric Chaisson, Astronomy Today. San Francisco, California: Pearson Education, 2008.

Slawson, Larry. "Uranus: The Seventh Planet of the Solar System." HubPages. 2020.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Larry Slawson


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