Neptune is the eighth and last planet of our Solar System. It is similar in size and composition with the other ice giant of the Solar System Uranus. Its name comes from Neptune, the Roman god of the sea. Unlike his twin Uranus, Neptune seems to be more popular. Let’s see below why is that, and what may classify as fun facts, Neptune.
Neptune is the third most massive planet in the Solar System, with a mass of ~ 1.025×1026 kg (~ 17.1 of Earth’s mass). Neptune has a mean radius of 24,622 km (~9.45 greater than Earth) and a mean density of ~ 1.638 g/cm3. All these correspond to a surface gravity of ~1.14g (11.17 m/s2).
Neptune orbits the Sun at a mean distance of 4.5 billion km (this is 30.1 times greater than the mean distance between the Earth and the Sun). To complete one orbit around the Sun, Neptune needs 164.8 years. Despite the long year, a Neptunian day is short, since the planet needs 16 hours, 6 minutes, and 36 seconds to rotate around its spin axis.
The temperature on Neptune increases as you move towards the interior of the planet. Thus, at a pressure of 0.1 bar, the temperature is -218°C (-360°F) while at a pressure of 1 bar the temperature is -201°C (-330°F). Neptune has a faint and fragmented ring system, and it has 14 satellites.
Discovery of Neptune
Neptune is very dim, and it is not visible with the naked eye. The earliest recordings of observing Neptune were made by Galileo. In 1612 and 1613, he made plots with points that we now know correspond to Neptune, but he recorded Neptune as a star.
The discovery of Uranus led many astronomers of the Era to study its orbit extensively. The data analysis left the astronomers puzzled, since they found evidence of interaction of Uranus with an unknown object. Thus through mathematical calculations, the orbit of Neptune was estimated.
On the 23rd of September 1846, Urbain Jean Joseph Le Verrier reported the discovery of the new planet. It is interesting that apart from the Le Verrier, two other people were working independently, searching for Neptune. These were John Couch Adams and Johann Gottfried Galle. Interestingly, this led to a dispute on who discovered Neptune.
Structure of Neptune
Neptune’s structure is similar to that of Uranus. The core of the planet is most likely composed of iron, nickel, and silicates. Its mass is around 1.2 times that of Earth. The temperature at the core could be as high as 5,100°C (9,200°F), while the pressure may reach values of 7 million bars (two times of Earth’s core).
The mantle contains most of the planet (10-15 Earth masses) and it is rich in water, ammonia, and methane. Note that even though such a mixture is usually referred as icy, it is actually a hot dense fluid and it is called water-ammonia ocean. This layer has also high electrical conductivity, and it is responsible for the magnetic field of Neptune. Based on theoretical models of the planet’s interior structure, the conditions in the mantle could be such that methane decomposes into diamond crystals. Thus, it is possible that in Neptune there is an ocean of liquid carbon with floating diamonds that fall in the form of rain.
The gaseous envelope that surrounds the mantle contains around 10% of the planet’s mass, and it is 10-20% of it. Its composition is ~80% hydrogen, 19% helium, around 1% methane, and ices in the form of ammonia, and water. Similarly to Uranus, the blue color of the planet is due to the presence of methane.
Satellites of Neptune
The moons of Neptune that we currently know, are 14. All their names come from sea deities of Greek mythology. Triton is largest, and it comprises 99.5% of the mass that is in orbit around Neptune. Additionally, it is the only one that is massive enough to have a spheroidal shape. William Lassell discovered Triton 17 days after the announcement of Neptune’s discovery.
Triton has a radius of 1,353 km, a mass of 2.14×1022 kg, and a mean density of 2.061 g/cm3. These correspond to surface gravity: 0.08g (~0.78m/s2). It orbits its host (in a tidally locked orbit like our Moon) at a mean distance of 35,4759 km, and it needs almost 6 days to orbit Neptune. The mean temperature on Triton is -235°C (-391°F). Observations have shown the presence of a very thin atmosphere on Triton that is composed of nitrogen and methane.
Images from Voyager 2 have shown that Triton’s surface is sparsely cratered. This means that the surface Triton’s surface is young, and it is renewed through volcanic activity. In such conditions, the only possible scenario is icy lava flow. Triton’s high density suggests that it has a core of rocky material that is surrounded by an icy mantle. So Triton has a similar structure with the large satellites of Jupiter and Saturn (i.e,. core, mantle, crust).
The other 13 satellites of Neptune have irregular shapes and they are small, ranging from a radius of 17 km (Hippocamp) to 210 km (Proteus).
Rings of Neptune
The first evidence for a ring system around Neptune came from ground-based observations in 1984. When Voyager 2 arrived in Neptune in 1989, it confirmed their existence. Most likely the rings are a mixture of ice and silicates or carbon. Their names come from astronomers that had significant contribution to the research of Neptune. Closest to Neptune is the faint Galle ring, located at a distance around 41,000 km, with a width of 2,000 km. The Le Verrier ring is second at a distance ~53,200 km with a width of 115 km. The third ring is the Lassell ring, and it is the widest one. It extends for 4,000 km at a distance of 52,300 km. At ~ 57,200 km from the planet is the Arago ring, which is only 100 km wide. The last ring is the Adams ring, which is only ~35 km wide at a distance of 63,000 km.
Neptune Fun facts
- The naming of Neptune was controversial. Before the adoption of its name, the suggestions were: Le Verrier’s planet, Janus, and Oceanus. In France, they adopted Herschel’s planet for Uranus, to persuade the British to name Neptune after his discoverer.
- Neptune’s rotational axis is tilted by 28.32° to its orbital plane. This means that Neptune has seasons just like Earth.
- Neptune is usually referred as the “other blue planet”. The original blue planet is the one that you are stepping on.
- Voyager 2 discovered a dark spot on Neptune. This is known as the Great Dark Spot, and just like Jupiter’s Great Red Spot, is an anticyclonic storm.
- The Great Dark Spot disappeared in 1994, but in 2015 a similar one was found in the northern hemisphere. This is known as the Northern Great Dark Spot. We currently do not know if the Northern Great Dark Spot exists, since Hubble is devoted in “more important” projects.
- The fastest recorded wind speeds in the Solar System are 2,100 km/s. These have been measured on the edge of the Great Dark Spot of Neptune.
- Due to the high eccentricities in the orbits of Neptune and Pluto, there are periods that Pluto is closer to Earth than Neptune. This was the case for example between 1979 and 1999.
- Voyager 2 is the only probe that visited Neptune. There is no plan for another one.
- Triton is the only major satellite in the Solar System with a retrograde orbit. This suggests that it was captured by Neptune’s gravity, rather than forming in place. Thus most likely it was a dwarf planet in the Kuiper belt just like Pluto.
- Triton’s future is not bright. Triton is close enough to Neptune to be locked into a synchronous rotation, and it is slowly spiraling inward because of tidal acceleration. Thus in about 3.6 billion years, it will be torn apart due to the immense tidal forces it will experience. Perhaps Neptune will have an amazing ring system in the future.
- William Lassell was able to build his own telescopes and make Astronomy his way of life, with the money he earned from his brewery.
- Hippocamp may have originated from debris ejected from Proteus, by a cometary impact that formed its largest crater, Pharos.