The "Last Gas" Planet:

Neptune is the outermost planet of the gas giants.In many ways Neptune and Uranus seem similar, but when we study them in some detail, we will find that they have some significant differences.


Neptune was discovered on September 23, 1846 by Johann Gottfried Galle, of the Berlin Observatory, and Louis d'Arrest, an astronomy student, through mathematical predictions made by Urbain Jean Joseph Le Verrier.

More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.

The Planet:

Neptune has an equatorial radius of 24,900 kilometers (about 1.4 Earth radii). If Neptune were hollow, it could contain nearly 60 Earths. Neptune orbits the Sun every 165 years at a distance of 30 A.U.'s. It masses 17 Earth masses and has a mean density of 1.7 gm/cc.

Neptune has eight moons, six of which were found by Voyager. A day on Neptune is 15hr:57min:59sec. long.

Neptune's atmosphere:

Unlike Uranus with its lack of atmospheric features, Neptune is a dynamic planet with several large, dark spots reminiscent of Jupiter's hurricane-like storms. The largest spot, known as the Great Dark Spot, is about the size of the earth and is similar to the Great Red Spot on Jupiter.

Other dark spots display cyclone-like structure in their centers.

Just like the storms on Jupiter, the dark spots on Neptune "tumble" along the zones absorbing smaller storms to power themselves. The most surprising thing about these storms is that, unlike Jupiter, they are short-lived. Some HST images do not show the Great Dark Spot.

Long bright clouds, similar to cirrus clouds on Earth, were seen high in Neptune's atmosphere. At low northern latitudes, Voyager captured images of cloud streaks casting their shadows on cloud decks below.

The strongest winds on any planet were measured on Neptune. Most of the winds there blow westward, opposite to the rotation of the planet. Near the Great Dark Spot, winds blow up to 1,200 miles an hour.

Neptune emits 2.7 times more energy than it receives from the Sun. This access energy powers the atmosphere to produce the storms that are not seen on its twin planet Uranus. The source of internal energy cannot be due solely to leftover energy from formation (i.e. Jupiter) since Neptune is smaller and would have radiated away the energy long ago. Nor is it due to an unusual chemical change, such as the helium rain for Saturn. Rather, it appears that Neptune is more efficient at trapping leftover formation heat due to the fact that methane is highly abundant in Neptune's atmosphere, and methane is an excellent insulator of heat (i.e. the greenhouse effect). Neptune has a sub-zero type greenhouse effect that is trapping formation heat that should have been radiated billions of years ago like Uranus.

Neptune's interior:

The interior of Neptune have rocky cores like Jupiter and Saturn. But at that point the similarity ends. The pressures are never sufficient to convert molecular hydrogen to metallic hydrogen in the interior of Neptune. Instead, a large mantle of icy water and ammonia forms about 20,000 km below the surface.

Neptune's magnetic field:

The magnetic fields for Neptune is unusual and is not well understood at this time. As the diagram below shows, the magnetic fields of the strongest three worlds, Jupiter, Saturn and the Earth, are all roughly aligned with the rotational axis of the planets. The generation of these magnetic fields occurs in liquid mantles around solid cores (liquid rock for the Earth, metallic hydrogen for Jupiter and Saturn).

Neptune, on the other hand, have radically different magnetic fields. Not only are they not aligned with the rotational axis of the planet, but neither are they located at the center of the planet either. The magnetic fields are probably be generated by local events in the icy mantles of both planets and may be unstable.

The best theory for the origin of these magnetic fields involves the high concentration of ammonia, NH3 in the planet's interiors. Ammonia, in solution, is high electrically conductive. This is due to a high amount of free ions (atoms missing electrons so that they have net positive charge). These free ions could form a conducting ionic layer in the mantle which would then produce a magnetic field with Neptune high rotation rates.


The principal Neptune's characteristcs are:

Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989.

Of Neptune’s 13 moons, Triton is by far the largest, and, at about 1,700 miles (2,700 km) in diameter (or three quarters of the diameter of the Earth’s moon), is the seventh largest moon in the whole solar system. It was discovered in 1846 by William Lassell just 17 days after the discovery of Neptune itself. In 1855, he built a 48-inch (1,200 mm) telescope, which he installed in Malta because of the better observing conditions compared to England. It was probably the biggest telescope at that time.

Triton has fascinated astronomers because of its geologic activity, the many different types of surface ices, such as frozen nitrogen as well as water and dry ice (frozen carbon dioxide), and its unique retrograde motion (which means it rotates around its planet in the opposite direction of its planet’s rotation).

Twin Planets:

Uranus and Neptune, are extremely similar.

In the above image, Uranus (left) and Neptune (right), with Earth, for comparison. Image credit: NASA