Uranus is the seventh planet from the Sun and is the third largest in the solar system. It was discovered by William Herschel in 1781. It has an equatorial diameter of 51,800 kilometers (1.9 Earth radii) and orbits the Sun once every 84.01 Earth years.

Uranus has a mean distance from the Sun of 19.1 A.U.'s. The length of a day on Uranus is 17 hours 14 minutes. Uranus masses 14.5 Earth masses.

Uranus is distinguished by the fact that it is tipped on its side with a rotational axis that is inclined to the orbit by 98 degrees.

Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history (also note the moons show evidence of a violent event in the past).

Note that this unusual axial tilt leads to a peculiar seasonal and diurnal motion as viewed from the ``surface''. For example, during summer in the northern hemisphere an observer would see the Sun making circles in the sky every 17 hours. As summer wanes, the Sun would gradually move south. Eventually, the Sun would rise and set to the autumnal equinox of equal day and night 21 years after the summer solstice. Then the nights would grow longer until one day the Sun would not rise and a long 21 year night would begin.

Uranus' atmosphere:

Since Uranus lies more than 19 AU from the Sun, it receives 360 times less light and heat from the Sun than Earth. As a result, its atmosphere is extremely cold, with a temperature of about -214C at the 1 bar pressure level (equivalent to the average air pressure at sea level on Earth).

The atmosphere of Uranus is composed of 83% hydrogen, 15% helium, 2% methane and small amounts of acetylene and other hydrocarbons. Methane in the upper atmosphere absorbs red light, giving Uranus its blue-green color. In the inner Jovian worlds (Jupiter and Saturn), ammonium hydrosulfide dominates the coloration of atmospheres with its red's and yellows. But as the temperature drops below 70 K, ammonia gas freezes into ice crystals and drops out of the atmosphere. Methane becomes more dominate and, being a blue gas, the outer Jovian worlds (Uranus and Neptune) go from blue-green to deep blue in their coloration. Note also that methane, CH4 is a greenhouse gas.

Other minor constituents include hydrogen sulfide (H2S), deuterated hydrogen (HD), ethane (C2H6) and acetylene (C2H2). The last two are created by photochemistry the action of sunlight on methane gas. Despite their scarcity, these minor ingredients do have a subtle influence on their surroundings: they condense to form high level hazes which absorb and reflect sunlight, raising the temperature of the upper atmosphere while lowering the temperature of the deeper atmosphere.

The atmosphere of Uranus is arranged into clouds running at constant latitudes, similar to the orientation of the more vivid latitudinal bands seen on Jupiter and Saturn, although these clouds are only visible in the infrared. Winds at mid-latitudes on Uranus move in the direction of the planet's rotation. These winds blow at velocities between 90 and 360 miles per hour.

Uranus lacks an internal energy source such as Jupiter and Saturn, and thus its atmosphere energy system is much less active, resulting in fewer features (i.e. storms, eddies, etc.). Cloud patterns are only seen at the warmer, lower levels deep below the atmospheric haze. In addition, the tilted axis of Uranus produces uneven warming in the two hemisphere which produce long-term North-South flows across the latitude zones. The combination of these effects means that the atmospheric features are washed out much like Saturn.


Neptune is the outermost planet of the gas giants. It 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 16 hours and 6.7 minutes long. 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.

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. Recent 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.

Uranus/Neptune interior:

The interiors of Uranus and Neptune are almost identical, due to the fact they are similar in mass and size. Both 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 interiors of Uranus and Neptune. Instead, a large mantle of icy water and ammonia forms about 20,000 km below the surface.

Uranus/Neptune magnetic field:

The magnetic fields for both Uranus and Neptune are unusual and are 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).

Uranus and 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 Uranus and Neptune's high rotation rates.


Pluto/Charon was discovered on February 18, 1930, making it the last planet found in our Solar System. Technically, Pluto is not an independent planet, but rather the largest member of an outer asteroid field called the Kuiper Belt. However, for historical reasons, we still call it a planet, although it is now classified as a dwarf planet. Originally thought to be a single planet, it has since been shown that Pluto and Charon are a binary planet system, two worlds in orbit around each other.

Pluto/Charon is usually farther from the Sun then any of the eight planets with a mean distance of 39.4 A.U.'s; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249-year orbit. Pluto/Charon made its closest approach during 1989 and will remain within the orbit of Neptune until March 14, 1999. Pluto/Charon'orbit is also highly inclined by 17 degrees to the orbital plane of the other planets. Observations also show that Pluto/Charon's spin axis is tipped by 122 degrees.

Pluto/Charon is an enigma for its orbital irregularites and how it could have formed as a binary planet. But mostly for the fact that it is a Terrestrial world out beyond the Jovian worlds, difficult to explain in the context of the protoplanet hypothesis for the origin of the solar system.

Because Pluto has not yet been visited by any spacecraft, it remains a mysterious planet. Due to its great distance from the sun, Pluto's surface is believed to reach temperatures as low as -240C (-400F). From Pluto's surface, the Sun appears as only a very bright star. Ground-based observations indicate that Pluto's surface is covered with methane ice and that there is a thin atmosphere that might freeze and fall to the surface as the planet moves away from the Sun.

Charon (in the upper left) is bluer than Pluto and appears to be covered with water-ice rather than methane ice. Its orbit is gravitationally locked with Pluto, so both bodies always keep the same hemisphere facing each other. Pluto's and Charon's rotational periods and Charon's orbital period are all 6.4 Earth days.

The above Hubble Space Telescope picture, which was made in blue light, show that Pluto is an unusually complex object, with more large-scale contrast than any planet, except Earth. Pluto probably shows even more contrast and perhaps sharper boundaries between light and dark areas than is shown here, but Hubble's resolution (just like early telescopic views of Mars) tends to blur edges and blend together small features sitting inside larger ones.

Pluto/Charon Surface Features:

The brightness variations in this movie may be due to topographic features such as basins and fresh impact craters. However, most of the surface features are likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles and chemical byproducts deposited out of Pluto's nitrogen-methane atmosphere.

The surface temperature of Pluto/Charon is near the freezing point of methane. As Pluto/Charon approaches perihelion a haze forms from evaporating methane. The Pluto/Charon moves away from the Sun, the methane haze ``snows'' onto the surface. The surface of Pluto/Charon should be dark due to radiation darkening where cosmic ray (high energy particle from outside the Solar System) break down molecules which leads to hydrocarbon growth (i.e. soot). However, the ``yearly'' methane snow regenerates the surface and produces the geography we see today.

Pluto/Charon Interiors:

The interiors of Pluto and Charon are mostly a mystery with models based on their mean densities. Pluto is so cold that there should be no liquid mantle of any element. A rocky core surrounded by ice is the most likely configuration. Charon is a mix of rocky and ice similar to the outer moons of Jupiter.

Kuiper Belt:

Starting in 1992, astronomers have become aware of a vast population of small bodies orbiting the sun beyond Neptune. There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU. Observations show that the trans-Neptunians are mostly confined within a thick band around the ecliptic, leading to the realization that they occupy a ring or belt surrounding the sun. This ring is generally referred to as the Kuiper Belt.

The Kuiper Belt holds significance for the study of the planetary system on at least two levels. First, it is likely that the Kuiper Belt objects are extremely primitive remnants from the early accretional phases of the solar system. The inner, dense parts of the pre-planetary disk condensed into the major planets, probably within a few millions to tens of millions of years.

The outer parts were less dense, and accretion progressed slowly. Evidently, a great many small objects were formed. Second, it is widely believed that the Kuiper Belt is the source of the short-period comets. It acts as a reservoir for these bodies in the same way that the Oort Cloud acts as a reservoir for the long-period comets.

Unfortunately, the KBOs are difficult astronomical targets, so that even such basic physical properties as the sizes and albedos remain unknown. Here we report the first simultaneous thermal and optical measurements of a bright KBO and use them to solve separately for the albedo and size. Varuna has equivalent circular diameter D = 900 km and an albedo 0.070. The surface is darker than Pluto, suggesting a composition largely devoid of fresh ice, but higher than the canonical albedo of 0.04 previously assumed for these bodies.