Neptune in the outermost of the eight planets. Like Uranus, methane gives it a bright blue color. It is 30 AU from the sun, and, at that distance, only makes one complete orbit (one Neptune year) in 165 Earth years. Its diameter is 31,000 miles. Four Earth diameters side by side would be about 32,000 miles, so Neptune could almost line up four Earth's at its equator. It rotates faster than Earth, so its day is only 16 hours long. Its axis tilt is close to Earth's, at 30 degrees.

axis tilts


Like all of the gas giants, we can only see the top of Neptune's clouds, but Neptune has some very high 'streaks' of clouds which cast shadows on the lower clouds.

Neptune cloud shadows


Like Uranus, its cloud-top temperature averages about -360 F. It should be colder, being so much farther out, so this means there is internal heating going on. The white streaky clouds are mainly crystals of methane ice. Below that, the higher clouds appear to be about 79% hydrogen and about 18% helium, with enough methane (about 3%) to give the planet its blue color.

Wikipedia has an excellent article on Neptune:

Neptune is ... the fourth-largest planet by diameter and the third-largest by mass. Among the gaseous planets in the Solar System, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth.

...Neptune was the first and only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 moons were located telescopically until the 20th century. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989.

... Astronomers sometimes categorise Uranus and Neptune as "ice giants"...The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. Perhaps the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing.

In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns. For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (1,300 mph). ... Neptune has a faint and fragmented ring system (labelled "arcs"), which may have been detected during the 1960s but was indisputably confirmed only in 1989 by Voyager 2.

Neptune's dark spot can be clearly seen by spacecraft. It appears to be a giant storm system, the same way Jupiter's giant red spot is. Neptune's winds are the fastest known in the solar system.


We are not sure of the composition of the northern dark rings, but it is suspected that these are up-currents in the atmosphere. The methane ice crystal clouds can be seen near the dark spot.

We assume its internal structure is similare to Uranus'.

Neptune has rings that are very faint. You can see that on the left photograph below. For the most part the particles in them are quite dark. That is because they are methane ice converted to dark carbon compounds on their surfaces by the bombardment of electrons trapped in the atmosphere. However, there are sections of the rings that look brighter than the others, and this is why the rings are sometimes referred to as arcs, as the Wikipedia article mentioned. You can see that on the right hand picture.

Neptune's faint rings Neptune ring arcs

The rings are all within 2.5 times the radius of the planet, so, like the other gas giants, they are close (except, again, for Saturn, whose rings start close in and then extend out quite far). This distance of 2.5 times the radius of the planet, where all the ring systems start, is called the Roche Limit.

Neptune's moons: there are fourteen of them -- at least fourteen that we have found. Most are small, icy bodies.

Neptune moons


The largest is Triton. Between Triton and the planet are seven regular moons, orbiting close to Neptune's equatorial plane, with some in its rings. The largest of those seven moons is Proteus, an irregularly-shaped rock 262 miles across. It is heavily cratered, and orbits Neptune about every 27 hours.



The largest moon, outside those seven, is Triton. Triton has a retrograde orbit around Neptune, which means it is a captured moon -- and it is the largest captured moon in the solar system. It orbits at an angle of 23 degrees to Neptune's equator.

Triton's orbit

Triton is about 2000 miles across (smaller than our moon). The absence of large craters shows that it has a relatively young surface. As it approached Neptune, the tidal forces would have been strong and may have melted the interior. This would have given way to volcanic action, but the lavas which outpoured would have been of liquid ices rather than rock. The surface temperature is less than -390 F, at he moment this is the lowest measured temperature in the solar system. Measurements of objects in the Kuiper Belt, such as Pluto, may show colder temperatures. The ice volcanoes are still active -- plumes of dark material (carbon and nitrogen) have been seen shooting to a height of five miles up. The activity may be coming from a hot spot far below the surface. this volcanic activity has given this mooon a thin nitrogen atmosphere. The pinkish hue around the poles is nitrogen ice.


The photograph above is of Triton's south pole. the terrain that looks blurry to the north of it is called the "cantaloupe terrain." The south pole itself shows dark wind-blown deposits. The outer moons, as shown above, are all captured, too.

Like Uranus, the magnetosphere of Neptune has a tilt, almost 50 degrees. Because Neptune itself is not tilted however, the magnetosphere of Neptune has a more standard, but still completely unique structure.

Uranus and Neptune

To see what happens with Neptune's magnetosphere (plasma sphere), look at it compared to Earth:

Neptune magnetic poles

So, during its orbit around the sun, here is what happens to the magnetosphere:

Neptune's changing magnetosphere

Neptune's plasmasphere