Neptune is the eighth planet in the solar system with respect to its average distance from the Sun. It is the outermost of the giant gas planets (along with Jupiter, Saturn, and Uranus). The discovery of Neptune was one of the triumphs of mathematical astronomy. To understand the perturbations in the orbit of the planet Uranus, astronomers John Couch Adams and Urbain Jean Joseph Leverrier independently calculated the existence and position of the new planet. Then, Johann Gottfried Galle, of the Berlin Observatory, discovered Neptune on September 23, 1846, based on the theoretical mathematical calculations sent to him by Leverrier.
The diameter of Neptune is approximately 3.88 times the size of Earth's diameter, with an equatorial diameter of 30,763 mi (49,528 km) and a polar diameter of 30,236 mi (48,680 km). Its mass is about 17.2 times that of Earth's mass, with a value of 7.019 x 1024 slug (1.024 x 1026 kg). Neptune's volume is equivalent to about 60 earth-volumes. Its mean density is 3.18 slug/ft3 (1.64 g/cm3 ). Neptune is the coldest planet in the solar system, with an average temperature of -318°C (-218°F).
As determined by the space probe Voyager 2, Neptune's rotation about its axis (or length of day) is 16 hours, 6.7 minutes. The planet's period of revolution about the Sun is 164.79 earth-years. Neptune is about 30 times further from the Sun than Earth. The Voyager 2 flyby of the planet resulted in the calculation of its minimum distance from the Sun to be 2.7748 billion mi (4.4656 billion km) and its maximum distance from the Sun to be 2.8240 billion mi (4.5461 billion km). Neptune's minimum distance from the Earth is 2.680 billion miles (4.313 billion km) and maximum distance is 2.910 billion miles (4.683 billion km). The tilt of Neptune's equator relative to its orbit is 29.6 degrees, somewhat larger than Earth's 23.4 degrees.
The inside 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. Neptune's magnetic field traps solar wind and galactic cosmic-ray particles in a belt around the planet, similar to Earth's Van Allen radiation belts. It is theorized that motions in the interior of Neptune help form its magnetosphere. The magnetosphere of Neptune is smaller than Earth's and highly tilted at 47 degrees from the rotation axis. Scientists believe the extreme orientation may be characteristic of liquid flows in the interior of the planet. The magnetic material of Neptune is thought to be within its icy core. Heat generated within Neptune's core helps form the unusual winds of the atmosphere.
Neptune's low mean density (at 1.8 times that of water) and its high proportion of light that is reflected off its surface indicate that Neptune has a thick layer of atmosphere. The atmosphere of Neptune is believed to be composed 98% of hydrogen and helium that are nearly invisible. The remaining molecules (mostly in the upper part of the planet's atmosphere) consist of methane gas that absorbs red light. This effect is responsible for Neptune's striking greenish-blue outward appearance. Because the atmosphere of Neptune is so dramatic, changing in a matter of weeks, or even days, a large recurrent disturbance called the great dark spot, seems to disappear and then reappear at frequent intervals. These dark spots are being created by strong upward motions of pockets of methane gas at the higher, colder altitudes of 31-62 mi (50-100 km) of the atmosphere.
The Voyager 2 probe also confirmed the existence of an orbiting ring system with at least four bands that are thin and very faint. The rings are made up of dust particles thought to have been made by tiny meteorites smashing into Neptune's moons. Mathematical theory suggests that the rings of Neptune affect the motion of particles in its magnetosphere.
Neptune has eight known natural satellites. Triton (discovered in 1846) is the largest and brightest of the moons, being only slightly smaller than Earth's moon. It has a diameter of 1,700 mi (2,700 km), travels in an unusual retrograde orbit, and has several active geysers of nitrogen ice and gas. Nereid (discovered in 1949) has an estimated diameter of 145-290 mi (235-470 km). The Voyager 2 space probe encountered Neptune in 1989, and discovered six smaller, dark moons (Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus).
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