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The eighth planet from the Sun. It appears distinctly blue, due to absorption of red light by methane in its atmosphere. Its mean opposition magnitude is +7.8, too faint to be seen with the naked eye. It was discovered in 1846 by J. G. Galle, after its position had been predicted mathematically by U. J. J. Le Verrier. Its shape is distinctly ellipsoidal (equatorial diameter 49 528 km, polar diameter 48 682 km). The rotation period of the visible surface varies between about 12 hours near the poles and 20 hours near the equator, but radio bursts indicate that the core rotates in 16h 07m.

Neptune has a thick atmosphere composed of 80% hydrogen, 19% helium, and 1.5% methane (molecular percentages). The temperature near the top of the atmosphere is around −220°C. Internally there is thought to be a small rocky core at a high temperature, probably surrounded by a layer of icy materials, and above that a layer of hydrogen and helium. The internal heat is thought to have been released by differentiation, when denser material separated out and sank to the core. Neptune has a magnetic field slightly weaker than the Earth's, with an average strength at the equator of around 10−5 tesla. As with Uranus, the magnetic axis does not run through the planet's core but is offset half-way to the surface, and is tilted at 47° to the planet's axis of rotation.

Neptune's atmosphere has dark belts or bands with bright zones between, similar to those on Jupiter and Saturn but fewer in number and less prominent. Dark and bright spots also occur, the most prominent being the Great Dark Spot discovered in 1989 by Voyager 2, similar to Jupiter's Great Red Spot but not as long-lived. There are also bright wispy clouds of methane ice resembling cirrus clouds on Earth, 50–100 km higher than Neptune's main cloud tops. One of them in the southern hemisphere, photographed by Voyager 2, had a much higher velocity than other clouds and was dubbed the Scooter. Wind speeds reach 2000 km/h, faster than on any other planet.

Neptune has six rings. The Galle, Le Verrier, Lassell, Arago, and Adams rings are named after those involved in Neptune's prediction and discovery. A sixth ring, as yet unnamed, lies along the orbit of the moon Galatea. The outer (Adams) ring has most of its matter in four bright concentrations, known as ring arcs. Neptune currently has thirteen known natural satellites.



Physical data

Diameter (equatorial)


Inclination of equator to orbit

Axial rotation period (sidereal)

49 528 km



16.11 hours

Mean density

Mass (Earth = 1)

Volume (Earth = 1)

Mean albedo (geometric)

Escape velocity

1.64 g/cm3




23.6 km/s

Orbital data

Mean distance from Sun



Eccentricity of orbit

Inclination of orbit to ecliptic

Orbital period (sidereal)





164.79 years

Subjects: Astronomy and Astrophysics.

Reference entries

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