Neptune is the eighth planet from the Sun, except when Pluto's eccentric orbit takes it inside Neptune's orbit (for example, in the period 1979 to 1999). Neptune is named after the ancient Roman god of the sea, identified as Poseidon in Greek mythology. Its existence was predicted independently by two mathematicians in 1845: J. C. Adams in England and U. J. Le Verrier in France, who calculated that the deviations observed in the orbit of Uranus from theoretical predictions could only be explained by the gravitational pull of an unknown, more distant planet. The position of the new planet was calculated and a telescopic search conducted in September of 1846 by J. Galle found Neptune within 1� of its predicted position, on the first night of searching!

Interestingly, calculations show that Neptune would have been very close to Jupiter in the sky of January 1613. Galileo's journals mention that he observed an object very close to Jupiter in December 1612 and again in January 1613, and that he noticed a motion of this object with respect to a nearby star. Unfortunately he did not follow up his observations and failed to recognize the existence of a new planet.

As you will see from the sections below, Neptune is very similar to Uranus in almost all respects. In fact, Uranus and Neptune are more similar to each other than any of the other planets.

Neptune has eight satellites that can be divided into two groups. One group consists of six moons that are close to the planet (48,000 to 118,000 km away) and have regular orbits (i.e., almost circular orbits that lie close to the plane of the planet's equator); these were discovered during the Voyager flyby in 1989. The largest of the six has a diameter of 420 km, the others range in size between 60 and 190 km. They are all dark, with an albedo of 0.06, and are probably made up of carbonaceous material. Voyager photographs of two of these satellites show that their surfaces are cratered. Two of the satellites lie close to the outer and middle rings and may act as shepherd moons.

Triton. NASA Voyager 2.

The stretching and flexing of Triton in the past contributed to erase some of the old surface features like craters, and indeed the surface is observed to lack large impact craters and densely cratered areas. Frozen methane, frozen nitrogen, water ice, carbon monoxide ice and carbon dioxide ice are present on Triton's surface. The presence of argon ice is inferred but is difficult to detect directly. Different types of terrain are present on the surface: a large area looks like the surface of a cantaloupe, another region has splotches of dark material, and yet another region looks smoother and more uniform. The ices on the surface make Triton a highly reflecting satellite, with an albedo of 0.8. Triton has a thin atmosphere consisting mostly of nitrogen and traces of methane. In such an atmosphere, organic compounds could form by chemical reactions between the methane and the nitrogen.

Nereid has the most eccentric orbit of any known satellite (eccentricity =0.75). Nereid is 340 km across, but it is still unknown whether its shape is spherical or irregular. It has a reflectivity of 15%.

These data tell us how Neptune rotates about its own axis. The rotation period of Neptune was a lot easier to measure than Uranus's rotation period, because cloud patterns can be seen in Neptune. As with the other Jovian planets, the real rotation period is determined from measurements of the magnetic field, because if it is measured from the cloud motions, the rotation contributed by winds would be superimposed to the internal rotation, thus yielding a wrong measurement of the true rotation period.

The seasons in Neptune are similar to those on Earth because the inclination of both rotation axes are similar, but since Neptune takes about 164 years to circle the Sun, its seasons last 41 years instead of 3 months.

Compare these numbers to those of Uranus and you'll find out how similar they both are. The most important differences are the density and the heat ratio. The different densities indicate differences in the compositions and/or structure of the interior. The heat ratio is the ratio of heat emitted by the planet to the solar energy absorbed. While Uranus does not emit more than it receives, Jupiter, Saturn and Neptune emits about twice as much heat as they receive from the Sun. It is thought that Neptune is still radiating heat left over from the time of its formation. It has been cooling down very slowly because of the large proportion of rock and ice compared to its total mass.

Small telescopes on Earth reveal a featureless greenish-bluish Neptune. Like Uranus, methane in the atmosphere absorbs red light and makes it look blue. But the Voyager approach to Neptune in August 1989 revealed white cloud features and a dark blue spot named the Great Dark Spot, similar to the Great Red Spot in Jupiter. By 1994, observations with the Hubble Space Telescope showed that the Great Dark Spot had disappeared. It was a high pressure storm system 30,000 km across, and it looked darker because we were looking deeper into Neptune's atmosphere. Since then another dark spot has developed.

The white clouds can change in size and shape from one rotation to another. These are probably clouds of methane ice crystals, although no one is sure of the composition yet. A lower cloud deck is observed and again its composition is still uncertain, it may be made up of methane droplets or hydrogen sulfide ice crystals. Neptune's atmosphere has differential rotation and is banded, but the bands are much fainter and wider than in Saturn. Winds in Neptune reach speeds of 1700 km/h.

The atmosphere of Neptune is composed of 84% molecular hydrogen gas (H₂), 14% helium gas (He) and 2% methane (CH4), although these amounts are still uncertain. Trace amounts of carbon monoxide (CO) and hydrogen cyanide (HCN) have been measured, and also acetylene (C₂H₂) and ethane (C₂H₆) in lesser amounts.

Scientists think that the interior of Neptune is similar to that of Uranus: a massive rocky core surrounded by a core consisting of a mixture of water ice and rock. Relative to its size, Neptune's core is larger than that of Uranus; this would explain the higher density of Neptune. Around the core is an envelope of liquid hydrogen, and then a layer of gaseous hydrogen with helium, methane, ammonia and water.

We encounter yet another similarity between the magnetic fields of Uranus and Neptune: Neptune's magnetic field also does not pass through its center, and it is inclined 47� to its rotation axis. It is off center by about half of the radius toward the south pole. The strength of the magnetic field is about one half that of Uranus and, like Uranus, the strength varies with latitude on the planet. The mechanism for the generation of such fields is unknown, but it is believed that they are generated within the icy cores that surround the rocky central cores. It is possible that at the large pressures in the interior, the elements that make up the icy core are ionized and become conducting. This would explain why the magnetic fields are offset from the center of these two planets.

By using an occulting bar that blocks the bright planet, the thin, well-separated rings of Neptune become visible. NASA Voyager 2.

Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.
Astrophysical Data: Planets and Stars, 1992, K. R. Lang, Springer-Verlag New York, Inc.
The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc.

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