Neptune - ice giant

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Vital Stats

Neptune is usually the eighth planet from the Sun, but is occasionally further away than Pluto, which has a highly elliptical orbit. Neptune is the smaller of the four gas giants, just smaller than Uranus, but much more massive.

Predicted Discovery

Neptune was found when astronomers Adams and Leverrier separately calculated the position of a new planet which they thought was altering the orbit of Uranus. Neptune was first observed by Galle in 1846. Even after its discovery, the orbits of Uranus and Neptune were still different from what astronomers thought they should be. This led to the search for yet another planet...

Rings or Ringlets?

ringletsFaint dark rings have been detected around the blue planet, but are much less substantial than those of Saturn or Uranus. When first discovered from Earth, it was thought they may not be complete. This Voyager 2 image shows bead-like features, but the rings are complete.

The planet has two main moons: Triton with a diameter of 2,720km, and Nereid only a few hundred km across, and in a highly elliptical orbit. Triton is a very dense moon and very cold, with a surface temperature of -236°C. Its surface is shrouded in nitrogen and methane ice, and has a polar cap covered with pink 'snow'. Six more moons were discovered to lie close to the planet within its rings, by Voyager 2.


Despite being so far away from the Sun, Neptune has considerable internal heating due to its mass. This provides enough energy to drive 1,100km/h winds in the planet's atmosphere, which is mainly composed of hydrogen (85%), helium (13%) and methane which is visible as white 'cirrus-like' clouds high up in the atmosphere.

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Surprising Features

Other features can be seen such as a large dark spot rotating in the southern hemisphere. This feature is similar in nature to the Great Red Spot on Jupiter. Like Uranus, Neptune has a magnetic field inclined to the rotational axis by a surprising amount, in this case 50°, and also offset from the planet's centre by some 10,000km.

Stormclouds Gathering!

From 1996-2002, the Hubble Space Telescope took a series of observations of Neptune. It discovered that it was getting cloudier! Neptune's orbit is quite elliptical, and it is getting closer to the sun. The clouds are likely to be the result of increased atmospheric temperature.
Credit: NASA, Sromovsky & Fry
Neptune's clouds in 19961996
Neptune's clouds in 19981998
Neptune's clouds in 20022002

Sole Visitor

The only spacecraft to visit Neptune so far has been Voyager 2, in 1989, when it made its last encounter with a planet in our solar system. Despite a journey lasting 12 years, Voyager returned data and high-resolution colour images of excellent quality, revealing Neptune and its moons as magnificent sights at the boundary of our place in the Galaxy.

See Voyager 2 Schematics.


Map of Neptune | Mythical Icons

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