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.

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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.