planet Neptune

In our solar system, Neptune is the eighth furthest planet from the Sun. In fact, it is so far away that we can't see it without the aid of a telescope. Through a telescope Neptune appears as a small blue disk.

Neptune is 4504.4 million km (2799 million miles) from the Sun. For comparison, Uranus, the next closest planet inward from Neptune, is 2875 million km (1787 million miles) from the Sun (Moche, 2004).

It takes Neptune 164.79 Earth years to orbit the Sun and about 16.1 hours to rotate once on its axis.

Neptune is only slightly smaller than Uranus. Neptune has an equatorial diameter of 49,530 km (30,780 miles), whereas Uranus has a diameter of 51,120 km (31,770 million miles). Neptune's density is higher than that of Uranus and all of the other gas planets (i.e., Jupiter, Saturn) in our solar system.

Neptune has a similar atmospheric composition to the other gas planets as well. Neptune's atmosphere consist of hydrogen (84%), helium (14%), and methane (3%)(Chaisson and McMillan, 2005). It is the presence of methane that gives Neptune its blue color. Methane absorbs red wavelengths and reflects blue. Because Neptune has a slightly higher concentration of methane than Uranus does (2%), Neptune appears more blue in color than Uranus does (Chaisson and McMillan, 2005).

In addition, Jupiter and Saturn have high concentrations of ammonia, but ammonia is either not found on Neptune or Uranus, or the concentration of ammonia is very low.

Neptune's dark spot

Neptune's Dark Spot

Neptune has a dark spot on its surface, which can be seen in both images of the planet Neptune. Scientists believe that this is a giant storm (it is a planet sized storm) that is similar to the red spot on Jupiter. The flow of the storm is counterclockwise and was discovered by the Voyager 2 spacecraft in 1989.

However, when the Hubble telescope viewed Neptune about a decade later, the dark spot had disappeared, indicating that the storm may have subsided. In contrast, the red spot on Jupiter is a storm that has been ongoing for several hundred years. The Hubble telescope found evidence of new storms on Neptune in the mid 1990s. Evidence shows that Neptune's atmosphere is quite stormy, with rapidly changing weather conditions. Winds on Neptune often exceed 932 mph (1500 km per hour)(Chaisson and McMillan, 2005).

Neptunes clouds consist mainly of frozen methane.

Neptune's rings

Neptune's Rings

Like all of the gas planets, Neptune also has a ring system. Neptune's ring system is similar to that of Uranus, with dark rings that don't reflect light well. The image of Neptune's rings was taken by Voyager 2 in 1989.

Neptune's Moons

Neptune has 13 moons, the largest of which is Triton. Triton is an unusual moon, in that it has a retrograde orbit. Most satellites orbit their planets in a prograde orbit, meaning that the moon moves in a West to East orbit around its planet. Retrograde is the opposite, and means that the moon travels in an East to West direction.

Neptune's Internal Structure

Scientists belive that Neptune consists of a rocky core that may be surrounded by water. This layer is surrounded by molecular hydrogen. Scientists believe that the composition of Uranus is very similar to that of Neptune. Neptune also has a strong magnetic field.

Unlike Uranus, and similar to Jupiter and Saturn, Neptune radiates more heat internally than it absorbs from the Sun.


Chaisson, E. and McMillan, S. (2005). Astronomy Today. Pearson Prentice Hall: Upper Saddle River, NJ.

Moche, D. L. (2004). Astronomy. Wiley: Hoboken, NJ.

Images of Neptune and Neptune's rings: Courtesy of NASA/JPL-Caltech.