Neptune

Neptune (planet)

I

INTRODUCTION

Neptune (planet), eighth planet in distance from the Sun, fourth largest planet in diameter, and third largest in mass in the solar system. Neptune’s gravity has a major influence on the Kuiper Belt, a region of icy bodies in the outer solar system that is a source of comets and includes the dwarf planet Pluto, formerly counted as the ninth planet.

Because of its great size and mass, scientists classify Neptune as one of the giant or Jovian (Jupiter-like) planets—along with Jupiter, Saturn, and Uranus. Like Uranus, Neptune is also classified as an ice giant planet, mainly made of the ice-forming molecules water, ammonia, and methane as a liquid mixture above what is thought to be a rocky core. Its atmosphere is mainly hydrogen and helium, along with methane gas that gives the planet a blue-green color.

Neptune orbits the Sun at an average distance of about 4,490 million km (about 2,790 million mi) in a period of 165 Earth years and only receives about 1/900th the amount of sunlight that Earth does. Neptune’s diameter at the equator is about 49,520 km (about 30,767 mi). Even though Neptune’s volume is 72 times Earth’s volume, its mass is only 17.15 times Earth’s mass.

Neptune has four rings and 13 known moons. The planet is named after the sea god Neptune in Roman mythology.

II

OBSERVATION FROM EARTH AND SPACE

Neptune was the second major planet, after Uranus, to be detected using a telescope. Mathematical theories of astronomical orbits led to the discovery of Neptune. To account for wobbles in the orbit of the planet Uranus, British astronomer John Couch Adams and French astronomer Urbain Jean Joseph Leverrier independently calculated the existence and position of a new planet in 1845 and 1846, respectively. They theorized that the gravitational attraction of this planet for Uranus was causing the wobbles in Uranus’s orbit. Using information from Leverrier, German astronomer Johann Gottfried Galle first observed the planet by telescope in 1846.

After its discovery, Leverrier proposed that the new planet be named after the sea god Neptune from Greek and Roman mythology. The appropriateness of this name was confirmed in the 20th century when astronomers learned about Neptune’s watery interior.

Neptune is barely visible to the naked eye and is so faint that even through binoculars it appears as a dim star. Through a large telescope, the planet appears from Earth as a small greenish disk with a diameter of about 2.3 arc seconds. Astronomers use the unit arc second to describe the size of objects in the night sky. Arc seconds give the angle an object blocks out in the sky (a quarter held at arm’s length is approximately 7,000 arc seconds).

Because Neptune is so far from Earth (about 4.49 billion km or 2.79 billion mi), only one spacecraft has visited the planet. During a rare alignment of the four giant planets, the spacecraft Voyager 2, launched on August 20, 1977, was able to pass by Jupiter (in 1979), Saturn (in 1981), Uranus (in 1986), and Neptune (in 1989). Scientists launched Voyager 2 with just enough energy to pass Jupiter. However, the strong gravitational pull of Jupiter accelerated the spacecraft as it passed by the planet so that Voyager 2 had enough energy to reach Saturn. As Voyager 2 successively passed each of the four giant planets, the gravitational pull of the planet accelerated the spacecraft enough to help it reach the next planet, until it reached Neptune more than ten years after its launch.

As Voyager 2 passed by Neptune, it recorded and transmitted images of the planet, its rings, and its moons. Astronomers studying these images discovered four rings and five previously undiscovered moons. Four of these newly discovered moons are the innermost moons of Neptune, the largest of which measures only 180 km (112 mi) in diameter—small enough to fit in a large crater of Earth’s Moon.

III

MOTION OF NEPTUNE

Neptune orbits about 4,490 million km (about 2,790 million mi), or 30 astronomical units (AU) from the Sun, beyond Uranus. An AU is equal to the average distance between the Earth and the Sun, or about 150 million km (93 million mi). Neptune takes 164.79 years to complete a single revolution around the Sun, so a year on Neptune is 164.79 times longer than a year on Earth. The orbit of Neptune traces out a flat region of space called the planet’s orbital plane. The orbital plane of Neptune lies close to Earth’s orbital plane. As a result, Neptune always crosses the same region of Earth’s sky.

The planet spins around its axis once every 16 hours in a counterclockwise direction, just as Earth spins once every 24 hours. The axis of rotation on Neptune tilts 29.6° into its orbital plane (the plane created by Neptune’s orbit around the Sun). This tilt gives Neptune almost Earthlike seasons. (Seasons on Earth result from our planet’s 23.5° tilt into its orbital plane.)

IV

COMPOSITION AND STRUCTURE

A

Interior of Neptune

Neptune contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Neptune formed from frozen water and rock supplied by icy comet-like material found in the outer regions of the solar system. As the planet grew in size, pressures and temperatures in the planet’s interior increased, heating the planet’s frozen water into a hot, dense liquid sometimes described as slushy “ice.”

Although Neptune is one of the giant planets, it is smaller and has a different chemical composition than those of Saturn and Jupiter. While Saturn and Jupiter are made of mostly hydrogen and helium, Neptune captured a much smaller amount of these elements as the solar system formed. Instead, Neptune captured mostly water. Because water is more dense than hydrogen or helium, Neptune is more compact than either Jupiter or Saturn. Jupiter, for example, has a radius of 71,355 km (44,338 mi), while Neptune has a radius of about 24,760 km (about 15,383 mi). Neptune is also more massive and compact than Uranus, which has a radius of 25,560 km (15,882 mi).

Neptune likely has a solid core no larger than Earth (Earth’s diameter is 12,756 km/7,926 mi); this core could be composed primarily of iron and magnesium silicates. Neptune’s core may be small because most of the rock composing the planet remains mixed with the vast ocean that extends upward from the core to the atmosphere.

Neptune’s vast body of liquid accounts for most of its volume. Scientists think this pressurized ocean or mantle of slushy ice is composed mostly of water as well as molecules of methane and ammonia. Neptune’s ocean is extremely hot (about 4700°C/about 8500°F). The ocean remains liquid at this temperature instead of evaporating because the pressure deep in Neptune is several million times higher than the atmospheric pressure on Earth. Higher pressure holds molecules in liquid closer together and prevents them from spreading apart to form vapor.

B

Atmosphere

The gaseous atmosphere of Neptune contains hydrogen, helium, and about 3 percent methane. It extends about 5,000 km (about 3,000 mi) above the planet’s ocean. Light reflected from Neptune’s deep atmosphere is blue, because the atmospheric methane absorbs red and orange light but scatters blue light. In 1998 astronomers also identified molecules of methyl in Neptune’s atmosphere. Methyl molecules each contain one carbon atom and three hydrogen atoms. Methyl molecules are known as hydrocarbon radicals because they are short-lived and highly reactive. They combine with each other to form ethane (C₂H₆), a flammable, colorless gas. The discovery of methyl in Neptune’s atmosphere marked the first observation of a hydrocarbon radical in the atmosphere of the outer planets. Astronomers hypothesize that great storm systems on Neptune eject methane into the upper atmosphere. Once in the upper atmosphere, the Sun’s energy breaks the methane down into methyl molecules. Below Neptune's methane clouds, at levels where the pressure rises to more than four times the atmospheric pressure at sea level on Earth, there may be a dense cloud layer composed of hydrogen sulfide particles.

Neptune emits about 2.7 times the amount of heat it absorbs from the Sun. Astronomers believe the excess heat that Neptune radiates comes from comet-like material that crashed into Neptune as the planet formed. Due to the force of gravity in the planet’s interior, the material in Neptune’s core is continually being pulled inward. As the material compacts, the molecules strike each other more frequently and with more force, releasing energy in the form of heat. Neptune’s core, which reaches temperatures of 5149°C (9300°F), is hotter than the Sun’s surface.

Neptune has an active atmosphere, with winds and massive storms that may be caused by heat escaping the planet’s interior. Neptune’s winds, which blow in a latitude direction, are faster in the planet’s polar regions than they are at Neptune’s equator. Neptune has the fastest winds in the solar system, reaching speeds of 2,000 km/h (1,200 mph).

Using the Hubble Space Telescope, astronomers have observed storms thousands of kilometers across in Neptune’s atmosphere. These storms are often visible as dark spots that appear and disappear in Neptune’s atmosphere over many months. The largest storm, known as the Great Dark Spot, appeared in the planet’s southern hemisphere and was photographed extensively in 1989 by the Voyager 2 spacecraft. Scientists estimated that the Great Dark Spot was as large in diameter as Earth is. By 1994 images transmitted to Earth by the Hubble Space Telescope showed that the Great Dark Spot had disappeared. Scientists believe this dark spot was an immense storm that either dissipated or was covered by other atmospheric features. From 1994 through 1998, astronomers used the Hubble Space Telescope to observe the emergence of additional large dark spots in Neptune’s northern hemisphere, indicating that the planet’s atmosphere changes rapidly. The chemical makeup of the cloud particles that form Neptune's Great Dark Spots is not known. Some scientists believe that the bright clouds rimming the poleward edges of the dark spots are composed of condensed methane particles.

C

Magnetic Field

Neptune, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material. Scientists believe that the slow escape of heat from the planet’s core circulates currents of electrically charged particles in Neptune’s deep ocean, generating a magnetic field. Neptune’s magnetic axis, the line indicating the direction of the force the planet’s magnetic field exerts, is aligned at an angle of 47° to Neptune’s axis of rotation. The influence of Neptune’s magnetic field extends for several hundred thousand kilometers above the planet.

V

RINGS AND MOONS

From Voyager 2 spacecraft images, astronomers identified four rings of debris encircling Neptune’s equator. These rings range in width from 15 km (9.3 mi) to 5,800 km (3,600 mi). All of these rings completely encircle the planet, but the outermost ring includes three or more arcs of concentrated debris, some of which had been detected from Earth before the Voyager 2 encounter. In 1998 a new infrared camera on the Hubble Space Telescope (HST) obtained the first new images of Neptune's mysterious ring-arcs since the 1989 Voyager 2 encounter. Astronomers had speculated that the gravitational pull from nearby moons caused smaller particles to form the concentrated debris arcs, but the new images showed that this theory is incorrect. Observations by the Earth-based Keck telescope, announced in 2005, showed that some of the ring-arcs have faded since the Voyager 2 encounter, indicating that the rings are features that change noticeably over time.

Thirteen moons are known to orbit Neptune. Only two of these moons—Triton and Nereid—were directly observed from Earth prior to the 1990s. Triton was discovered in 1846 by British astronomer William Lassell, and Nereid was discovered in 1949 by Dutch-born American astronomer Gerard Kuiper. Scientists discovered another moon, Larissa, in 1981 when the moon occulted (moved in front of) a star, and they discovered five more moons of Neptune from images transmitted to Earth by the Voyager 2 spacecraft. Searches carried out with large Earth-based telescopes led to the discovery of 3 more moons in 2003 and 2 more in 2004, bringing the total to 13. These five additional moons are the smallest and most distant from Neptune of all the planet’s moons, and astronomers know little else about them. They may be asteroids captured by the pull of Neptune’s gravity.

The four innermost moons of Neptune are quite small, ranging in diameter from 58 km (36 mi) to 180 km (110 mi). From the closest to Neptune outward, these moons are Naiad, Thalassa, Despina, and Galatea. Larissa is the fifth moon in distance from Neptune. It is heavily cratered and irregular in shape. Its density, chemical composition, and internal structure are unknown. Proteus is the sixth moon out from Neptune. This satellite is the largest irregularly shaped moon in the solar system, measuring 436 km (262 mi) through its widest diameter and 402 km (241 mi) through its narrowest diameter.

Triton is the seventh moon from Neptune and is the largest of the planet’s moons, measuring 2,700 km (1,700 mi) in diameter, larger than the dwarf planet Pluto. Triton, which consists of about one-quarter ice and three-quarters rock, has few craters on its surface, which suggests that this moon has undergone recent geological changes. It has a thin nitrogen atmosphere and a polar cap. Voyager 2 recorded geyser-like plumes erupting from its icy crust.

Unlike any other large moon in the solar system, Triton has a retrograde (clockwise) orbit, meaning that it revolves around Neptune in the opposite direction that the planet rotates (counterclockwise). Its circular orbit is also tilted out of the plane of Neptune's equator by 157°. Triton’s odd orbit is a puzzle. The main moons of every other planet orbit nearly in the plane of the planet’s equator and in the same direction as the planet spins, retaining the motion of a rotating disk of debris around the planet from which the moons would have formed. One current theory is that Triton is actually a Kuiper Belt Object that was captured by Neptune’s gravity. The stress of the capture may have heated Triton, partially melting its interior and surface, explaining its relatively young appearance. The event may also have disrupted the orbits of other moons around Neptune, including Nereid.

Nereid is the eighth moon from Neptune and has an extremely elliptical orbit, varying in distance around Neptune from 1.4 million km (870,000 mi) to 9.6 million km (6 million mi). Voyager 2 did not pass near enough to Nereid to obtain detailed information.

Astronomers have detected swarms of asteroids that share Neptune’s orbit, similar to the Trojan asteroids that share Jupiter’s orbit. In both cases, the asteroids are found at points in the planet’s orbital path where the gravitational pull from the Sun and from the planet are balanced. These stable regions occur at points 60 degrees ahead and behind the planet as it orbits the Sun.

VI

INFLUENCE ON PLUTO AND THE KUIPER BELT

Neptune’s orbit lies near the inner edge of the Kuiper Belt, a disk of icy and rocky bodies that orbit in the outer solar system at 35 to 55 astronomical units (AU) from the Sun. Neptune itself likely formed out of similar planetesimal material. These bodies, called Kuiper Belt Objects (KBOs), range in size from small clumps of rock and ice to planetary bodies called dwarf planets. Dwarf planets such as Pluto and Eris have rounded shapes from effects of their own gravity but are not massive enough to clear their regions of space of other orbiting bodies the way major planets do.

As Neptune moves around the Sun, its gravity tugs on these KBOs, shifting their orbits. Sometimes the objects are disturbed enough to leave the Kuiper Belt and enter the inner solar system as short-period comets that give off gas and dust. In other cases, KBOs may be thrown into eccentric orbits that are tilted out of the main plane of the solar system, and may become Scattered Disk Objects (SDOs) such as Eris. Neptune also shapes the outer edge of the belt in a more stable relationship. For every two orbits made by Neptune, objects at 50 AU orbit the Sun once, a pattern called a 2:1 orbital resonance.

Another large group of KBOs also have established stable orbits that interact with Neptune. The best known and largest of these objects is the dwarf planet Pluto, formerly classified as the ninth planet in the solar system—its status was changed by the International Astronomical Union in 2006. Neptune orbits the Sun three times for every two orbits made by Pluto. Astronomers have discovered many other KBOs that also orbit in this 3:2 orbital resonance pattern with Neptune. Dubbed plutinos (meaning “little Plutos”), the largest of these bodies include Orcus, Ixion, Rhadamanthus, and Huya. In some cases these objects have orbits that cross Neptune’s orbit at their nearest points to the Sun. Every 248 years, Pluto’s elliptical orbit brings it inside Neptune’s nearly circular orbit for about 20 years. The last time Pluto’s orbit brought it inside Neptune’s orbit was in 1979. In 1999 Pluto’s orbit carried it back outside Neptune’s orbit.

Neptune’s moon Triton may have been a KBO that was captured by Neptune's gravity. Scientists are still not certain how such a capture happened. One theory is that Triton was originally a dwarf planet that orbited the Sun and had a large moon, similar to Pluto and its moon Charon. Neptune’s gravity may have shifted Triton’s original orbit over time and brought Triton and its moon so close to Neptune that Triton’s moon was pulled away and propelled into a completely different orbit away from Neptune. The gravitational energy exchanged in the encounter slowed Triton enough for it to be captured in an orbit around Neptune.

Contributed By:
William B. Hubbard

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