The planet Neptune is the eighth and farthest planet from the sun. It is also the smallest of the four giant planets in the solar system. Neptune has a rich blue color and is named after the Roman god of the sea.
Neptune was first observed in 1846. But very little was known about the planet until recently. Observations were limited because the planet is so far away. Then in 1989 the planetary probe Voyager 2 flew past Neptune.
The probe revealed some remarkable facts about the planet and its moons. Among the things discovered is that this planet has a thick atmosphere that grows denser with depth, gradually merging with a vast, strange ocean.
Neptune’s revolution and rotation
Neptune revolves around the sun once every 165 years. As it revolves, Neptune traces an ellipse. (An ellipse is roughly oval-shaped.) The planet travels a huge distance to complete just one orbit.
Neptune’s average orbital distance from the sun is enormous. In fact, this distance is about 30 times greater than the corresponding value for Earth.
Neptune is the eighth and farthest planet from the sun. Beyond Neptune’s orbit is a region that is traversed by many trans-Neptunian objects. Of these trans-Neptunian objects, the most familiar is Pluto. It sometimes ventures closer to the sun than Neptune.
Neptune’s magnetic field rotates once in about 16 hours. This is the length of the planet’s “day.” The rate of rotation of the visible features in Neptune’s atmosphere varies greatly depending on the features’ latitudes.
A dim and distant world
Neptune’s diameter is about four times greater than Earth’s. Although Neptune is large, it cannot be seen from Earth unless a telescope is used. Neptune is very far away. And so it appears small and dim. Neptune is about 2.8 billion miles (4.5 billion kilometers) from the sun. Because Neptune is so far from the sun, it receives very little sunlight. In fact, the sunlight that reaches Neptune is 900 times less intense than the sunlight that reaches Earth. The less sunlight reflected off a planet, the dimmer the planet appears from Earth.
Great dark spot. © NASA
Neptune’s great distance from Earth makes it appear very tiny even when viewed through large telescopes. It is no wonder no one discovered the planet until Adams and Leverrier told astronomers where to look.
Also there are huge anticyclonic storms on Neptune, do you remember Jupiter's Great Red Spot? Something familiar can be seen on Neptune, it is called Great Dark Spot and it has about the same size as Earth. Unlike other planets, Neptune’s Great Dark Spot has winds with a speed of 2,400 kilometers (1,500 mile) per hour, the fastest in the Solar System.
The blue gas giant
Neptune, Jupiter, Saturn, and Uranus are the solar system’s gas giants. They are very different from Earth, which has a rocky crust with an interior of molten rock. Neptune has a deep, thick atmosphere of hydrogen and helium, along with traces of methane gas. The atmosphere becomes increasingly dense with depth. It gradually becomes less and less gaseous and transitions into a strange kind of fluid. This fluid is called a supercritical fluid. It has both gas-like and liquid-like properties. The supercritical fluid on Neptune consists of water, methane, and ammonia.
Although it has no distinct surface, it amounts to a kind of ocean.
Neptune’s internal structure. © NASA
Neptune is different from the other gas giant planets. For example, Neptune and Jupiter have very different compositions. About 10 percent of Neptune’s mass is made up of hydrogen and helium gases. In contrast, Jupiter‘s mass is about 96 percent hydrogen and helium. The other 90 percent of Neptune’s mass consists mostly of heavier elements. These include carbon, nitrogen, and oxygen. On Neptune, these elements often occur in compounds such as methane, ammonia, and water.
Neptune appears blue because of the methane gas in its atmosphere. Sunlight is made up of a spectrum of colors that are absorbed by some materials and reflected by others. Methane absorbs the reddish colors in the spectrum. When sunlight shines through methane gas, the reddish colors are absorbed and the bluish colors are reflected. Neptune’s methane makes the planet appear blue. Thin, wispy clouds floating in the upper atmosphere are above most of the methane, so they appear white.
The temperature in the upper atmosphere of Neptune is a frigid -355°F (-215°C). From there the atmosphere extends thousands of miles toward the planet’s interior. The deeper you go, the warmer it gets. When the pressure of its atmosphere is about the same as the atmospheric pressure at the Earth’s surface, the temperature on Neptune is high enough for methane to form clouds.
Neptune, picture from Hubble Space Telescope. © L. Sromovsky & NASA
Below the clouds the atmosphere extends downward for thousands of miles. And the pressure and temperature of the atmosphere gradually rise. The atmosphere never reaches a solid surface or even a liquid surface. Instead, it gradually merges with a strange ocean. This ocean is not quite gas and not quite liquid. It is located at a depth of about 6,000 miles (10,000 kilometers). It is made up of water, methane, and ammonia. And it may have a temperature as high as 4500°F (2500°C). Neptune may have a rocky core about the size of Earth. The temperature at the core may reach 15,000°F (8000°C).
Triton, a world of ice volcanoes
Triton is Neptune’s largest moon, or satellite. It is slightly smaller than the Earth’s moon, about 1,700 miles (2,700 kilometers) in diameter. It circles Neptune once every 5 days and 20 hours.
Triton. © NASA
Despite these dry facts, Triton is very unusual. Most satellites orbit their planet in the same direction as the planet’s rotation. But Triton circles Neptune in the opposite direction. Some scientists think that Triton’s orbit is different because it was once a separate planet that was captured by Neptune’s gravity.
Among Triton’s other unusual features are ice volcanoes on its surface. Images show eruptions blasting material as much as 5 miles (8 kilometers) into space.
These materials include frozen nitrogen, dust particles, and other materials from Triton’s interior, which is quite warm. To explain this warmth, scientists cite a phenomenon called tidal friction. Tidal friction occurs when a body’s shape is changed during gravitational interactions. More precisely, tidal friction is the resistance to changes in shape.
Neptune’s gravitational pull stretches Triton. And the degree of stretching constantly changes because of Triton’s slightly elliptical orbit around Neptune. (The stretching is increased when Triton is closer to Neptune, and it is decreased when Triton is farther away.) Frictional resistance to changes in the degree of stretching heats Triton’s interior.
Despite the warmth of Triton’s interior, the surface of Triton is bitterly cold. The temperature is about -391°F (-235°C). The surface of Triton may be even colder than the surface of Pluto. (Triton is usually closer to the sun than is Pluto. But Triton also has a highly reflective surface.)
Parts of Triton’s surface are wrinkled. Other parts appear to be covered by frozen lakes. In these places, water may have erupted from inside Triton and then frozen as hard as rock. Much of Triton’s surface is covered with fresh white snow. This snow cannot be ice formed from water because Triton’s surface is too cold. Instead, scientists think the snow consists of crystals of nitrogen that have frozen out of the atmosphere. Sunlight reflecting off Triton’s icy surface makes it one of the whitest objects in the solar system.
Triton is also unusual because it has a thin atmosphere consisting mostly of nitrogen gas. Most satellites are too small to hold an atmosphere. But Triton is so cold that even its weak gravitational force can hold its nitrogen atmosphere for billions of years.
Neptune’s other satellites and its rings
In addition to Triton, Neptune has twelve other moons. The moons that orbit more closely than Triton are: Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus. The moons that orbit more distantly are: Nereid, Halimede, Sao, Laomedeia, Psamathe, and Neso.
Neptune’s rings. © based on NASA images
Psamathe, the smallest moon, was discovered by Mauna Kea Observatory in 2003. It is only about 24 miles (38 kilometers) in diameter. It orbits about 30 million miles (48 million kilometers) away from the planet. Only Neso orbits Neptune at a greater distance.
There are four thin rings around the planet. The rings are probably composed of dust, bits of sand, and small particles of ice. They may also contain rocks and boulders. The rings may have originated from debris blasted off the small innermost satellites by meteorites. Or they may have formed from bits and pieces of a satellite that broke up long ago.
Unlike most planetary rings, which appear smooth and uniform, Neptune’s outermost ring has three bright arcs. These arcs may be caused by the moon Galatea, which orbits just inside the ring. A complex gravitational interaction between Galatea and the ring may cause light-reflecting ice in the ring to clump together in certain parts. This interaction also affects the moon’s orbit.
The one, brief exploration of Neptune by Voyager 2 certainly helped to expand our knowledge about the planet. But there is much about Neptune that remains a mystery. The next visit - perhaps around the year 2030 or later - will no doubt reveal much more about the beautiful blue planet near the edge of the solar system.