Neptune (planet)

Neptune is the eighth planet from the sun, and the outermost gas giant in our solar system. Neptune is named after the Roman god of the sea.

Physical characteristics

Orbiting so far from the sun, Neptune receives very little heat. Its surface temperature is − 218 °C. However, the planet seems to have an internal source of heat. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space. Neptune's atmosphere has the highest wind speeds in the solar system, up to 2000 km/h, thought to be powered by this flow of internal heat.

The internal structure resembles that of Uranus - a rocky core covered by an icy crust, buried deep under its thick atmosphere. The inner two thirds of Neptune is composed of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a mixture of heated gases comprised of hydrogen, helium, water and methane. Like Uranus, and unlike the uniform composition of Jupiter and Saturn, Neptune's internal structure is thought to consist of distinct layers. Like Uranus, Neptune's magnetic field is strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometers) from the planet's physical center. Comparing the magnetic fields of the two planets, scientists think the extreme orientation may be characteristic of flows in the interior of the planet and not the result of Uranus' sideways orientation.

Exploration of Neptune

Galileo's astronomical drawings show that he had observed Neptune in January 1613, when it appeared close to Jupiter. Believing it to be a star, he cannot be credited with its discovery.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesise some perturbing body. In 1843, John Couch Adams, calculated the orbit of an eighth planet that would account for Uranus' motion. He sent his calculations to Sir George Airy who dismissed them with some coolness, leading Adams to drop the subject.

In 1846, Urbain Le Verrier, independently of Adams, reproduced his calculations but also experienced difficulties in encouraging any enthusiasm in his compatriots. However, in the same year, John Herschel started to champion the mathematical approach and persuaded James Challis to search for the planet.

After much procrastination, Challis began his reluctant search in July 1846. However, in the mean time, Le Verrier had convinced Johann Gottfried Galle to search for the planet. Though still a student at the Berlin Observatory, Heinrich d'Arrest suggested that a recently drawn chart of the sky, in the region of Le Verrier's predicted location, could be compared with the current sky to seek the displacement characteristic of a planet, as opposed to a stationary star. Neptune was discovered that very night, September 23, 1846, within 1° of where Adams and Le Verrier had predicted it to be. Challis later realised that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.

With an orbital period of 165 years, Neptune will first return to the point in its orbit where Galle discovered it in 2011. Due to Pluto's eccentric orbit, Neptune is sometimes the farthest known planet from the Sun.

Neptune is never visible with the naked eye. With the use of a telescope it appears as a blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

The Great Dark Spot, a distinguishing feature of Neptune, disappeared in 1994 but another reappeared later.

The rings of Neptune

Neptune has a faint planetary ring system of unknown composition. The rings have a peculiar "clumpy" structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.

Evidence that the rings are incomplete first arose in the mid-1980s, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings, the outermost of which, Adams, contains three prominent arcs now named Liberté, Egalité, and Fraternité (Liberty, Equality, and Fraternity). The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales.

The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs. Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the center of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.

The moons of Neptune

Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself.

^* Awaiting confirmation and naming.
^** Negative orbital periods indicate a retrograde orbit around Neptune (opposite to the planet's rotation)

Some asteroids share the same names as moons of Neptune: 74 Galatea, 1162 Larissa.