Neptune Full Disk

Neptune Full Disk: This photograph was created using the Voyager 2's narrow-angle camera: during August 16 and 17, 1989, it photographed Neptune almost continuously, recording approximately two and one-half rotations of the planet.
Source: Courtesy NASA/JPL-Caltech.

Average Distance from the Sun: 4.496 billion km (30.109 AU)

Size (Equatorial Diameter): 49 500 km (3.88 x that of Earth)

Mass: 1.02 x 1026 kg (17.15 x that of Earth)

Length of Day (Solar Rotation Period): 16.11 Earth hours

Length of Year (Sidereal Orbital Period): 164.8 Earth years

Temperature: -220°C (clouds)

Gravity:  11 m/s² (1.12 x that of Earth)

Neptune was discovered in 1846 by John Adams, from England, and Urbain John Joseph Le Verrier, of France, working independently. There was some controversy at the time over who was the first to discover the planet, though now they are recognised as co-discoverers. Like Uranus, it cannot be seen with the naked eye from Earth. Moving away from the Sun, it is the last of the Gas Giants.

Surface and Atmosphere

Neptune's rocky core is covered by a thick, cold atmosphere of hydrogen, helium and methane. This atmosphere appears blue due to the methane gas. It is believed that the temperature increases the closer you get to the core of Neptune.

Whirling through Neptune's atmosphere are giant storms like those found on Jupiter. Each storm moves at different speeds around the planet. One of these storms is nearly as large as the Earth and takes just over 18 hours to travel once around the planet. Swirling around the upper atmosphere are white cirrus clouds of methane ice. Winds have been clocked at 2400 km/h, the fastest in the Solar System.


Neptune has 13 moons. The largest are:
Triton (diameter: 2620 km; discovered: 1846)
Proteus (diameter: 200 km; discovered: 1989)
Nereid (diameter: 170 km; discovered: 1949)

Proteus plus five other small moons were discovered by Voyager 2 in 1989. Included in this group are shepherding moons which help keep the particles of Neptune's ring in formation.

Five additional small moons (just 30-40 km across) were discovered in 2002/2003 by a team of astronomers from Canada and the USA using telescopes in Chile and Hawaii. This new discovery suggests that the moons of each of the gas giants represents a family formed by the collision of a passing comet or asteroid and an original moon of the system.

Triton, the largest moon, has a very thin atmosphere of nitrogen and methane and, at -235°C, is the coldest body yet discovered in the Solar System. Despite this it is quite active and has volcanoes that emit pink snow instead of molten rock. This material is thrown several kilometres into the atmosphere, carried for long distances, then deposited as dark streaks on the moon's surface. The pink and reddish hues on Triton's surface are possibly caused by hydrocarbons, formed by the effects of cosmic rays, giving it a unique appearance. Near the equator fault valleys criss-cross the surface.

Triton has a retrograde orbit and is slowly spiralling in towards the planet. In 10 to 100 million years it will get so close that Neptune's gravity will tear it apart, forming another ring.


Neptune has 3 main rings and a broad sheet of rocky particles. The inner ring is less than 18,000 km from Neptune's surface.

Special Features

Neptune's magnetic field is tilted at 52° degrees from the poles. This means that when Neptune has aurora they occur near its equator.

In Mythology

Neptune was the principal god of the seas and waters. Associated with the Greek god Poseidon, the son of Cronos and elder brother of Zeus. By coincidence, the planet turned out to be a sea-blue colour.

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