Neptune was first discovered as a result of detailed studies made to Uranus's orbit around the Sun. Astronomers discovered discrepancies in Uranus's orbit and hypothesized that there had to be another planet exerting a gravitational force on Uranus. Calculations based on the discrepancies were made, and John Galle found Neptune in 1940, 1 to 2 degrees off the predicted position. However, it was Adam and Leverrier, the two mathematicians who calculated Neptune's position, that were jointly credited with Neptune's discovery.
Neptune cannot be seen with the naked eye from Earth. Seen through a large telescope, it appears as a bluish disc. Neptune orbits the Sun with a semi-major axis of 30.1 A.U. (4.5 billion km) and has an eccentricity of 0.01. Its sidereal orbit of 164.8 years means that Neptune has yet to complete one revolution around the Sun since its discovery.
Neptune has a diameter of 49500 km (3.9 times that of Earth's). It has a mass of 1.0 x 10^26 kg (17.1 Earth masses) and an average density of 1700 kg/m^3 (1.7 g/cm^3).
Neptune's interior has a rotation period of 16.1 hours. However, its atmosphere rotates more slowly, completing a rotation once every 17.3 hours. Neptune's rotation axis is inclined at an angle of 29.6 degrees to its orbital plane.
Spectroscopic studies of light reflected from Neptune indicates an atmosphere similar to the other Jovian planets (84% molecular hydrogen, 14% helium, 3% methane). The higher percentage of methane in Neptune's atmosphere gives Neptune its blue colour. This is because methane absorbs red light.
Neptune's atmosphere has clouds and band structure that is similar to Uranus but more distinct. Neptune also has an internal heating source, with an origin most likely similar to those of Jupiter and Saturn (energy left over from formation of Neptune).
An atmospheric storm similar to the Great Red Spot (found in Jupiter's atmosphere) is also present in Neptune's atmosphere. Named the Great Dark Spot, it was discovered by Voyager 2 in 1989. Just like the Great Red Spot, the Great Dark Spot is found near the equator. It is about the size of the Earth, rotates counter-clockwise, and is in many ways similar to the Great Red Spot. However astronomers were unable to study the Great Dark Spot for long, as it had vanished by the time the Hubble Space Telescope turned to focus on Neptune in 1994.
INTERNAL STRUCTURE AND MAGNETOSPHERE
Neptune has a relatively strong magnetic field, at least 100 times stronger than the Earth's. However, just as in the case of Uranus, Neptune's magnetic field is spread out (due to a larger volume than Earth), resulting in a magnetic field similar in strength to the Earth's.
Neptune has been theorized to have a rocky core about the size of the Earth but 10 times more massive. Hydrogen remains as a liquid all the way down to Neptune's core, because its internal pressure is not high enough to change liquid hydrogen into its solid, metallic state. Ammonia is suspected to be dissolved in the liquid hydrogen surrounding Neptune's core, and acts as an electrically-conducting layer (similar in function to the molten inner mantle of Earth) which explains for Neptune's strong magnetic field.
Neptune has 8 moons, of which 2 were found from Earth-based observations – Triton (William Lassell in 1846) and Nereid (Gerard Kuiper in 1949). The remaining 6 were found by Voyager 2 – Naiad, Thalassic, Designate, Galatea, Larissa and Proteus.
The largest moon is Triton. It has a diameter of 2800 km and orbits Neptune in a retrograde, circular fashion. It is the only moon in the entire system to have a retrograde orbit. Triton has a very thin atmosphere made up entirely of nitrogen. Its surface is lined with fissures and faults. Due to its retrograde motion, Triton is slowly spiraling towards Neptune and in a 100 million years or so, would break up under the gravitational influence of Neptune.
Nereid is only 200 km across and orbits Neptune in the prograde sense but has an elliptical orbital path.
Neptune has 4 dark rings – 3 are narrow and 1 is rather broad and diffused. Seen from Earth, Neptune's rings appear nothing more than partial arcs. Just as in the other jovian planets, astronomers believe that the rings are kept in place by shepherding moons.