Neptune is similar to Uranus: it is an ice giant, is made up of gas, ice and water with a rocky core; and is surrounded by ring arcs. Why the rings are incomplete is not fully understood. It is also about the same size as Uranus: 49 532 kilometres in diameter at the equator as against 51 118 kilometres for Uranus.Neptune

Voyager 2 (the only spacecraft to visit the planet) showed that Neptune radiates more than twice the energy it receives from the sun—another mystery. In part, this may be due to planetary material releasing energy as it is continually compacted by gravity.

Neptune was discovered in 1846, following predictions of its existence by two astronomers. Its discovery was based on the behaviour of Uranus. Considering the astronomical tables of the orbits of Uranus mathematicians determined that there must be an eighth planet. Its discovery is a triumph of the theory of gravity. It is so far away that astronomers are unable to detect any features. And indeed it is not visible to the naked eye.

The planet is very similar in size and structure to Uranus. It has a core of solid rock, and possibly ice. Surrounding the core is a layer of water, methane and ammonia ices. Like Uranus, the planet has a magnetic field offset from its rotational axis: in Neptune’s case by about 47°. It is posited that the magnetic field is generated by the convective flow in the main layer of conductive liquids.


The atmosphere is quite thin with only 15% of the planet’s mass accounted for by hydrogen. Neptune’s atmosphere is also fairly shallow and is composed of 80% hydrogen, 19% helium and 1.5% methane, with traces of hydrogen deuteride, and ethane. Like Uranus methane is responsible for its blue colour. It is possible Neptune is a darker blue simply for the fact that it is much further away from the sun than Uranus: 4.5 billion kilometres, as opposed to 2.87 billion kilometres for Uranus. In view of the deep blue colour, it is perhaps appropriate it should be named after the Roman god of the oceans.

Neptune has a hyperactive atmosphere with winds speeds reaching an astonishing 2 160 kilometres an hour along the equatorial band. It is certainly the windiest place in the solar system. It is thought the superfast winds are driven, not by the sun’s heat—the planet is too far away from the sun for that—but by the planet’s internal heat source.

Overlying the deep blue colour are white clouds in the upper atmosphere. Even some of these clouds are travelling at sonic speeds, creating raging storms around the planet. One violent area, called the Great Dark Spot, is as big as Earth. It was oscillating in size over a period of eight days when it was seen by Voyager in 1989. But by 1996, as determined by the Hubble Space Telescope, the super gigantic storm had completely disappeared. So the planet’s weather system must be very dynamic.

The temperature at cloud top level is -218°C

Orbit and rings

The period of rotation for Neptune is 16 hours 6 minutes and is responsible for an equatorial bulge with a diameter of 49 532 kilometres. Its diameter at the poles is 48 686 kilometres. Neptune orbits every 164.8 years. Its average distance from the earth is 4.31 billion kilometres. Neptune has five sparse but distinct rings. Four of Neptune’s moons are within the ring system, and it is thought the material forming the rings comes from them.

The moons of Neptune

Neptune has a complex system of moons – with 14 found so far. The largest is Triton. It is even bigger than the dwarf planet Pluto. Triton orbits opposite to the planet’s rotation—a retrograde orbit—so it is likely a captured body, and not formed from debris around Neptune. Otherwise, it might have survived as a planet in its own right. Triton orbits at about 354 000 kilometres distant, with an orbital period of 5.88 days. It is actually getting closer to Neptune and will eventually be destroyed. It may even form a ring system around Neptune one day.

The moons of Neptune 
Inner moons
Larissa216 kms73 458 kms
Proteus440 kms117 647 kms
Major moon
Triton2 707 kms354 760 kms
Outer moon
Nereid340 kms5.5 million kms

Until the discovery of Xena (since renamed Eris), Triton was known as the coldest place in the solar system (-235°C). Even its volcanoes are frozen! Liquid nitrogen, methane and ammonia are spewed out by geysers and fall back onto the surface as snow. Unlike Enceladus or Io, this moon is not thought to be powered by tidal heating from the planet, but merely from sunlight concentrated under the solid nitrogen of the moon’s polar cap. As the nitrogen turns to gas it punches a hole through cracks in the crust to escape under pressure.

The rest of Neptune’s moons are probably captured asteroids. The inner moons are Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp and Proteus. The outer moons are Nereid, Halimede, Sao, Laomedeia, Psamathe and Neso.

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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Friday, 14th February 2020