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The relatively large natural bodies that revolve in orbits around the sun, and presumably around other stars as well, are called planets. The term does not include such smaller bodies as comets, meteors, and asteroids, many of which are little more than pieces of ice or rock. (See also Comet; Meteor and Meteorite; Asteroid.)
The sun, the nine planets, their satellites, and all the smaller bodies, particles, and dust that circle the sun form the solar system. The sun, near the center of the solar system, governs the planets' orbital motions by gravitational attraction and provides the planets with light and heat. In order of increasing mean distance from the sun, the nine planets of the solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. (See also Gravitation; Solar System; Sun.)
Mercury, Venus, Mars, Jupiter, and Saturn can be seen without a telescope. The ancient Greeks called them planetes, or "wanderers," because the objects appeared to move across the background of the apparently fixed stars. Although Uranus is also sometimes visible without a telescope, ancient astronomers were unable to distinguish it from the true stars.
The planets may be divided into groups in several ways. In one scheme Mercury and Venus, the planets that revolve around the sun in orbits smaller in diameter than that of the Earth, are classified as inferior planets. The so-called superior planets are those that revolve around the sun in orbits larger in diameter than the Earth's orbit.
The planets may also be classified into two groups according to their gross physical characteristics. The terrestrial, or Earth-like, planets are close to the sun and are composed primarily of rock and metal. They include Mercury, Venus, Earth, and Mars. The terrestrial planets are also called the inner planets.
The Jovian, or Jupiter-like, planets are very large compared to the terrestrial planets and are much farther from the sun. They are also called the outer planets. They include Jupiter, Saturn, Uranus, and Neptune. These planets are composed mostly of hydrogen and helium in gaseous and liquid form. Pluto, the outermost planet, is usually considered neither a terrestrial nor a Jovian planet. It is composed of ice and rock and is much smaller than the other planets.
Formation and Evolution
Although the origin of the solar system is uncertain, most scientists believe that it began to develop about 4 1/2 billion years ago from a large cloud of gas and dust. The cloud began to contract. As the material within the cloud became compressed, it grew hot. Most of this mass was drawn toward the center of the cloud, eventually forming the sun. The remaining material, less than 1 percent of the original, formed a spinning disk, called the solar nebula, around the center. The planets and satellites evolved from the nebula as it cooled. (See also Solar System, "Past and Future of the Solar System.")
Close to the center, the material in the disk condensed into small particles of rock and metal that collided and stuck together, gradually growing into larger bodies called planetesimals. As they traveled around the center, the largest planetesimals swept up smaller material in their paths, a process known as accretion. Eventually these accreting bodies evolved into the terrestrial planets. The numerous impact craters still evident on the oldest surfaces of some planets are believed to have been created during this phase, when the nascent planets collided with other bodies.
Farther from the center of the disk, cooler temperatures allowed not only rock and metal but also ice and gas to develop. These materials formed small eddies in the spinning disk that evolved into the Jovian planets. Each young planet had its own, relatively cool nebula from which its satellites formed.
As the planets and satellites accreted, their interiors grew hot and melted. In a process known as differentiation, heavier materials sank to the centers, generating more heat in the process and gradually forming cores. In the case of the terrestrial planets, mantles of rock formed around metal-rich cores and were covered by thin surface crusts. Lighter elements escaped from the interiors and formed atmospheres and, on Earth, oceans.
In addition to the heat generated by accretion and differentiation, the planets and satellites had a third source of internal heat: the decay of certain radioactive elements within the bodies (see Radioactivity). Since their formation, many of the physical characteristics of the planets have been determined by the manner in which the bodies generated and lost their internal heat. For example, the release of internal heat accounts for the volcanic and tectonic activity that shapes the crusts of the terrestrial planets. (See also Continent; Geology; Plate Tectonics; Volcano.)
These bodies have solid surfaces that have preserved a record of their geologic histories. In smaller bodies such as Earth's moon, Mercury, Mars, and the satellites of the outer planets, the internal heat escapes to the surface relatively quickly. As a result, the surface initially undergoes rapid, violent changes. Then, when most of the body's internal heat has dissipated, the surface features stabilize and remain largely undisturbed as the body ages. Larger bodies, like the Earth and Venus, lose their heat more slowly. In fact, they are still subject to the forces of volcanism and tectonism. The landforms of the terrestrial bodies that lack atmospheres have been shaped primarily by these volcanic and tectonic activities, combined with cratering caused by impacts that occurred during the solar system's formation. The same is true for those terrestrial bodies that have atmospheres, but their landforms have been modified by the action of wind and, in some cases, water.
The evolution of the Jovian planets cannot be reconstructed by analyzing their surface features--they have no solid surfaces. These planets are so large that much of their internal heat is still being released.
Mercury
Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and sets within two hours of the sun. Consequently, little was known about the planet until the Mariner 10 spacecraft made several flybys in 1974 and 1975.
Mercury's surface has several different types of terrain. Planetary scientists can estimate the age of a surface by the number of impact craters on it; in general, the older the surface, the more craters it has. Some regions on Mercury are heavily cratered, suggesting that they are very old surfaces that were probably formed about 4 billion years ago. Between these regions are areas of gently rolling plains that may have been smoothed by volcanic lava flows or by accumulated deposits of fine material ejected from impacts. These plains are also old enough to have accumulated a large number of impact craters. Elsewhere on the planet are smooth, flat plains with few craters. These plains are probably younger and volcanic in origin. Sometime between the formation of the intercrater plains and the formation of the smooth plains, the whole planet may have shrunk as it cooled, causing the crust to buckle and form the long, steep cliffs called scarps.
The largest impact basin on Mercury, Caloris, is about 800 miles (1,300 kilometers) across and is surrounded by mountains that rise to heights of about 1.2 miles (2 kilometers). It was probably created from the impact of a large planetesimal when Mercury was forming. On the opposite side of the planet from Caloris is an area of hilly, lineated terrain that probably resulted from seismic waves caused by the same impact.
Like other airless, solid bodies in the solar system, the entire surface of Mercury is covered with a layer of rubble called regolith. Regolith is composed of material, ranging from dust to boulders, that was scattered when impact craters were formed. This debris was in turn broken up and redistributed by subsequent impacts.
Mercury is very dense and has a magnetic field that is about 1 percent as strong as Earth's. This suggests the existence of a core composed of iron and nickel and constituting about 40 percent of the planet's volume. The surface gravity is about one third as strong as Earth's. A thin atmosphere of hydrogen, helium, potassium, and sulfur surrounds the planet. Radar images taken of Mercury in 1991 show what are considered to be large ice patches at the planet's north pole.
Mercury rotates on its axis three times for every two revolutions around the sun and has a more elliptical orbit than do most other planets. These two characteristics combine to create effects unusual by Earth standards. At aphelion, the point in Mercury's orbit when the planet is farthest from the sun, an observer on Mercury would see a sun that appeared more than twice as large as it does from Earth. At perihelion, when Mercury is nearest the sun, the sun would appear almost four times as large as it does from Earth. Furthermore, the sun would not appear to move steadily across the sky. Instead, its apparent speed would change, depending on the viewer's location on the planet and on the planet's distance from the sun--sometimes the sun would even appear to reverse its course.
Temperatures on Mercury vary widely. Planetary scientists often discuss temperatures in terms of the Kelvin scale, an absolute temperature scale in which 0 K is the lowest possible temperature, corresponding to -459.67o F (-273.15o C) (see Heat, "The Measurement of Temperature"). Surface temperatures on Mercury range from about 675 K at "noon" to 100 K just before "dawn" (temperatures of about 295 K are comfortable for humans).
It takes almost 59 Earth days for Mercury to complete one rotation about its axis, but the time between one sunrise and the next is 176 Earth days. The reason for this is that after one rotation, Mercury has completed two thirds of its orbit around the sun, so that the sun is in a different place in Mercury's sky. It takes three rotations, or two Mercurial years, for the sun to reappear in the same place in the planet's sky.

Venus
After the moon, Venus is the most brilliant natural object in the nighttime sky. It is the closest planet to the Earth and is also the most similar to Earth in size, mass, and density. These similarities suggest that the two planets may have had similar histories. Thus, scientists are intrigued by the question of why Venus and Earth are now so different.
Venus rotates once every 243 days in retrograde motion--that is, in a direction opposite to the direction of rotation of most of the other planets--or clockwise when viewed from above the Earth's North Pole. The same side of Venus is always facing Earth when the two planets pass in orbit. Although Venus is close to Earth, the planet is difficult to observe because its surface is completely obscured by thick layers of dense clouds. During the 1970s and 1980s NASA's Pioneer Venus orbiter and the Soviet Venera 15 and 16 orbiters were able to obtain information about the Venusian clouds and surface conditions.
Venus' atmosphere is composed mainly of carbon dioxide, with droplets of sulfuric acid in the upper clouds. The upper atmosphere moves rapidly, completely circling the planet in four days, while the winds at the surface are gentle. The surface temperature is approximately 750 K, even hotter than Mercury's "noon" temperatures.
The large amount of carbon dioxide in Venus' atmosphere accounts for the extremely high temperatures near the planet's surface. Sunlight penetrates the atmosphere, is absorbed by the planet's surface, and is reradiated in the form of heat. The large amount of carbon dioxide in the atmosphere, however, absorbs and traps this reradiated heat, preventing it from being released back into space. As a result of this phenomenon, called the "greenhouse effect," the surface of Venus is hot enough to melt lead, and the rocks may even glow faintly red from their own heat.
Because the clouds allow only about 15 percent of the sun's light to reach the planet's surface, the days on Venus are dim and overcast. Because the dense atmosphere refracts, or bends, light, some light may extend around to the night side of the planet, so the nights may not be completely dark.
In many places the surface has been severely fractured and folded by stresses caused by convection of the Venusian mantle, the part of the planet's interior that lies just above the core. Thus Venus may experience a form of plate tectonics, though the very high surface temperature probably makes the tectonic style quite different from that on Earth. The highest mountain on Venus, Maxwell Montes, is about 7 1/2 miles (12 kilometers) high. It may be a volcano. Radar images indicate that the highlands on Venus have rougher surfaces than do the other terrains.
The Magellan spacecraft arrived at Venus in mid-1990, taking up an orbit that carried it around the planet every three hours while it mapped Venus' cloud-shrouded surface in the best detail ever achieved. The surface of Venus is pocked with large meteor craters. Magellan also found evidence that active volcanoes may exist on Venus and that the surface of the planet is probably only 400 million years old. Further support for the possibility of active volcanoes on Venus came from the Galileo spacecraft. Galileo flew by Venus in early 1990 and found evidence of impulsive electromagnetic events characteristic of lightning. Mission scientists suggested that these events have a volcanic origin.

The Earth-Moon System
The moon's mass is equal to only about 1.2 percent of the mass of Earth, but that percentage is the largest of any satellite-planet combination in the solar system except for Pluto and its satellite, Charon. Although in many ways Earth is a typical terrestrial planet, it occupies a special place in the solar system because it is the only planet known to support life as we know it. Earth's oceans may also be unique because water can exist as a liquid only within a narrow range of temperatures and pressures, and these special conditions are not known to exist on any other planet.
Earth's large size and its high content of radioactive uranium, thorium, and potassium have kept the planet's interior hot. Its geologic history has been active and turbulent, and its surface continues to change (see Earth).
The moon may originally have formed from material scattered by a collision of Earth with a planetesimal about the size of Mars. The resulting material may have settled into orbit around the Earth and accreted to form the moon. There are other models for the moon's formation, but many planetary scientists believe that this theory seems most plausible.
The heavily cratered terrains of the moon, called the terrae, formed about 4 1/2 billion years ago. The lunar terrae crust has been preserved more or less intact, except for repeated impacts by other bodies. The darker areas of the moon, known as maria, are thought to have been formed by lava flows. Over time the weight of the lava caused fractures, called grabens, on the surrounding terrae and folding, or thrust faults, within the maria. Since then the interior of the Moon has been quiescent.

Mars
Since ancient times Mars has been an object of great interest to astronomers. Unlike Venus, Mars generally has no obscuring layer of clouds. In addition, it passes relatively close to the Earth in its orbit. Thus it is a nearly ideal subject for telescopic observation. Over the centuries observers have noted various unusual phenomena on the planet's surface, including a seasonal growing and shrinking of the polar caps and a wave of darkening that appears to sweep from pole to equator during each hemisphere's spring. The explanation of most of these observations had to await the exploratory space missions by the United States and Soviet Union during the 1960s and 1970s.
Mars is about half the size of Earth. Its atmosphere is composed mostly of carbon dioxide and is very thin, exerting about 1/100 the surface pressure that the Earth's atmosphere exerts. The temperature at the planet's surface varies widely during the course of a Martian day, from about 190 K just before dawn to about 240 K in the afternoon. At the center of the planet is probably a small iron or iron sulfide core. If Mars has a magnetic field, it is so weak that no instrument has been able to detect it.
Mars, like the Earth, is tilted on its rotational axis. Consequently it is subject to seasonal variations in climate as first one hemisphere and then the other receives more sunlight during the planet's orbit around the sun (see Seasons). Liquid water cannot exist on Mars's surface because of the low temperature and pressure; water exists only as ice deposited at the poles and perhaps trapped below the surface and as vapor in the atmosphere. However, there is evidence that, in the past, temperatures may have been warmer and the atmospheric pressure higher. Images from the Viking orbiters show surface features that resemble dry riverbeds and gullies. These might have been made by rainfall and runoff, but they might also have been made by subsurface water that escaped to the surface.
Although it is quiescent now, Mars experienced a period of volcanic activity that peaked a few billion years ago. The planet has the largest volcano in the solar system, Olympus Mons. At a height of 17 miles (27 kilometers), the volcano is three times higher than Earth's Mount Everest and covers an area the size of the state of Arizona. It sits on the Tharsis Plateau, a broad, elevated plain dotted with large volcanoes and fractures. The largest fracture system is Valles Marineris, a huge valley about 2,500 miles (4,000 kilometers) long and varying from 2 1/2 to 6 miles (4 to 10 kilometers) in depth. The Tharsis Plateau may have been formed by a rising plume of hot mantle material, but no plate-tectonic activity accompanied this process--the Martian surface consists of a single plate. Other regions on Mars include smooth plains, densely cratered areas, mesas, and rolling hills formed by various combinations of fracturing, volcanism, and atmospheric-related erosion and deposition.
In general, once each Martian year at the beginning of the Southern Hemisphere's spring season, Mars is engulfed by global dust storms. Local temperature differences generate strong winds that lift the dust from the surface to form thick clouds. The clouds block the sunlight, gradually causing the surface temperatures to even out and the winds to subside. Some of the atmospheric dust is deposited in a snowfall of dust and ice in the winter hemisphere. The snow forms a winter cap of carbon dioxide ice, water ice, and dust. During the spring most of the cap evaporates, but some remains as a permanent deposit. As a result, a geologic record of these storms and their variations over the planet's lifetime must be preserved in the permanent layers of dust and ice at the Martian poles.
The phenomenon known as the wave of darkening accompanies the seasonal waning of the polar caps. Near the edge of either polar cap, a general darkening of the surface markings appears in early spring as the cap begins to recede. The darkening then moves away from the cap and sweeps across the equator, finally dissipating in the opposite hemisphere. Attempts to study these waves from spacecraft have failed. No surface changes have been detected that could be associated with this phenomenon, and it is generally agreed that it is some kind of atmospheric effect.
For centuries astronomers have considered the possibility that life might exist on Mars. In 1877 the Italian astronomer Giovanni Schiaparelli described a system of interconnecting channels on the planet. The American astronomer Percival Lowell interpreted Schiaparelli's word canali to mean canals and speculated that they were structures that had been built by an advanced but dying Martian civilization. Most astronomers could see no canals, however, and many doubted their reality. The controversy was finally resolved only when pictures returned from the United States Mariner probes in 1969 showed many craters but nothing resembling channels or canals.
Some scientists thought it possible that some type of organism could have existed on Mars because of the presence of water and the possibility that temperatures on the planet were warmer in the past. The United States Viking probes, consisting of two orbiting spacecraft and two landers, were intended in part to search for evidence of past or present forms of life on Mars. The two landers touched down on the planet in 1976 and performed numerous experiments, including a detailed chemical analysis of the Martian atmosphere and soil. No trace of any organic material was found. The next United States probe, the Mars Observer, was launched in September 1992 and was expected to land on Mars in August 1993. Equipped to study the surface composition, volcanic activity, and atmosphere of Mars, the 980-million-dollar spacecraft lost contact with Earth and was presumed lost in September 1993.
A meteorite from Mars that fell to Earth 13,000 years ago and was found in Antarctica contained organic molecules, minerals, and carbonate globules associated with bacterial life. The findings suggested that microbial life may have existed on Mars more than 3 billion years ago.
Scientists announced in 1996 that a meteorite from Mars that fell to Earth 13,000 years ago contained organic molecules, minerals, and carbonate globules that are all associated with bacterial life, providing the first "evidence of primitive life on early Mars." The meteorite was recovered in 1984 by American scientists in Antarctica. The findings suggested that microbial life existed on Mars more than 3 billion years ago, when the planet was wetter and warmer. Skeptical scientists warned against rushing to conclusions, and officials from NASA said that more rigorous scientific examination of the evidence would have to be completed before the extraordinary findings could be validated.
Mars has two small satellites, Phobos and Deimos, that may be captured asteroids. Both are so small that they do not have enough internal gravity to draw them into spherical shapes; instead, they are shaped more or less like potatoes. Phobos is about 17 miles (27 kilometers) long; Deimos is about 9 1/2 miles (15 kilometers) long. Both have rotational periods equal to their orbital periods, so that they always point the same face toward Mars. Deimos has smooth craters that are almost buried in regolith generated by repeated impacts with other bodies. Phobos is also covered with regolith, but it is far more rugged and very heavily cratered.
Phobos is very close to Mars, and its orbit is gradually decaying, so that it is drawing closer to the planet with each orbit. Astronomers estimate that Phobos may fall to the Martian surface sometime in the next 100 million years. Deimos is in a more distant orbit and is gradually moving away from the planet.
Both satellites are very dark and are probably made of a carbonaceous chondrite material. This is a primitive substance that includes many of the first materials to precipitate out of the solar nebula during the creation of the solar system. It is found on many satellites, asteroids, and meteorites.

The Jovian System
Jupiter is larger than all the other planets combined. It gives off nearly twice as much energy as it receives from the sun--heat that was acquired during the planet's accretion as well as heat that is generated as the planet gradually contracts. Jupiter also has the strongest magnetic field of all the planets. The field extends out to ten times the planet's radius and is the source of intense bursts of radio noise.
Jupiter is composed mostly of hydrogen and helium. It has no solid surface, only layers of gaseous clouds. At the planet's center is probably a rocky core with more than ten times the mass of the planet Earth. Temperatures in the core may exceed 25,000 K. Surrounding the core is a liquid hydrogen-helium mixture that has been squeezed into metallic form under the intense pressure of the planet's upper layers.
In October 1989 the Galileo spacecraft was sent into orbit for a six-year journey to Jupiter. A probe was set to be released into the Jovian atmosphere in 1995 to photograph portions of Jupiter over a two-year period.
When viewed through a telescope, Jupiter's topmost clouds appear as dark belts and bright zones that encircle the planet and range from tawny yellow to brown and gray. The colors are most likely caused by ammonia-sulfur compounds. The most obvious feature on the planet is Jupiter's famous Great Red Spot. It is a huge cyclonic storm, as big as two Earth-sized planets placed side by side, and it has been observed from the Earth for more than 300 years.
Jupiter spins rapidly on its axis, completing one rotation in less than ten hours. Because of the centrifugal force caused by this rapid rotation, Jupiter's diameter is greater at the equator than it is from pole to pole, giving the planet the shape of a slightly flattened sphere.
Jupiter and its 16 known satellites probably formed as a miniature solar system--a large rotating gaseous ball surrounded by a planetary nebula that eventually evolved into the planet and its satellites. Jupiter has a narrow system of rings, discovered by the Voyager 1 spacecraft in 1979, that are composed of tiny rocks and dust particles.
Jupiter's four largest satellites were the first objects in the solar system to be discovered through the use of a telescope. They were first observed by Galileo in 1610 and so are known as the Galilean satellites. In order of increasing distance from the planet, they are Io, Europa, Ganymede, and Callisto. Io is composed of rock and is bright yellowish orange due to the abundance of sulfur on its surface. In its elliptical orbit Io is continually squeezed in and out like an accordion by the strong gravitational pull of Jupiter and the weaker pull of the other Galilean satellites. This effect, called tidal flexing, generates internal friction and heat in the satellite. As a result Io is volcanically hyperactive--at least ten erupting volcanoes were recorded by Voyagers 1 and 2.
Europa is composed mostly of rock, with a smooth outer shell of recently formed water ice covered with patterns of dark stripes and ridges. Like Io, Europa is internally heated by tidal flexing and is continually renewing its surface. The heat causes water ice to melt and rise to the surface of the outer shell, forming new ice lava flows over regions of the planet.
In contrast, Ganymede and Callisto are cold, geologically inactive satellites. They are slightly larger than Mercury. Both have rock cores that make up about half of their volume; the outer half is ice. Ganymede is covered with light and dark patches. The dark areas are old, heavily cratered surfaces. The light areas are relatively young and contain craters that have bright, radiating streaks of exposed water ice. Ganymede was internally active a few billion years ago, and many broad surface fractures were formed that filled with ice or with water that later turned to ice. The ice was refractured into the complex patterns observed today. Callisto's surface is entirely dark and very densely cratered. Little geologic activity has occurred on its surface for the past few billion years.

In 1992 the Ulysses spacecraft flew by Jupiter to study its magnetic fields, plasma, dust, and X rays. It found that the planet's magnetic field has a clocklike pulsing that extends to the middle of the planet. It also observed that Io is less volcanically active than was previously thought.

The Saturnian System
Like Jupiter, Saturn is a large, gaseous planet composed mostly of hydrogen and helium. It also radiates more than twice as much heat as it receives from the sun. This excess thermal energy is partly from primordial heat and partly from the friction created by the heavier element, helium, gradually sinking through the hydrogen toward the planet's center. Saturn has a magnetic field 1,000 times stronger than Earth's but not as strong as Jupiter's. Saturn's density is so low that it could float in an ocean of water. It probably has a core similar to that of Jupiter. It is covered with cloud bands, some forming cyclonic patterns like Jupiter's, but the colors appear more subdued than do Jupiter's because of an atmospheric haze that covers the clouds. Saturn is surrounded by a spectacular ring system. Galileo observed these rings in 1610, but he did not identify them as rings--he believed Saturn to be a triple planet. In 1655, using a more powerful telescope, the Dutch astronomer Christiaan Huygens was able to see a flat, apparently solid ring around Saturn. Later astronomers were able to identify separate rings.
The cameras of Voyagers 1 and 2 revealed that there are really tens of thousands of rings extending from about 4,300 miles (7,000 kilometers) to 46,000 miles (74,000 kilometers) beyond Saturn's atmosphere. They are made of ice and ice-covered particles that range from the size of a speck of dust to the size of a house. The rings occur in groups, which are referred to as the A ring, the B ring, and so on inward. The gap between the A and B rings is called the Cassini Division. The Voyager cameras observed occasional dark radial spokes in the B ring. These began as thin lines and were then stretched into wedge shapes as the inner rings, which move faster, passed the outer rings. The spokes would disappear after a few hours. Astronomers believe the spokes are probably made of fine particles that have been raised slightly above the rings by electrostatic forces.
At least twenty satellites orbit Saturn. The largest of these is Titan, intermediate in size between the planets Mercury and Mars. Titan is half rock and half ice, with an atmosphere of nitrogen and methane that exerts about 1 1/2 times the surface pressure of the Earth's atmosphere. Its surface is very cold and is obscured by haze. It may be covered by oceans of liquid methane. The six other major satellites are Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus. Most of these have icy, cratered surfaces. Enceladus has a smooth, bright surface of apparently pure ice. Iapetus has a large patch of material as dark as asphalt that nearly covers the leading hemisphere (the side of the satellite facing in the direction of orbital motion).
The remaining satellites of Saturn are all small, icy, and irregular in shape. Some are called shepherd satellites because their orbits are located at the edges of rings, apparently helping to keep the ring material in place. The F ring of Saturn has two shepherd satellites whose gravitational forces may be responsible for the ring's braided, or twisted, appearance. Two of the smallest satellites have almost the same orbit but do not move at exactly the same speed. When the faster one overtakes the slower one, they do not collide; instead they attract each other gravitationally and exchange places.

Uranus
Uranus is another large gaseous planet. It is denser than Jupiter and Saturn and is composed of hydrogen, helium, substantial amounts of water, and probably some methane, ammonia, rock, and metal. Trace amounts of methane in its upper atmosphere give it a blue-green color. The temperature in the upper atmosphere is only about 60 K, but the temperature increases with atmospheric depth. Underneath the thick clouds there may be an immense ocean of water that, though it is heated to several thousand degrees Kelvin, does not boil away because of the intense pressure from the atmosphere above it. The core of the planet is most likely rock and metal.
Uranus' rotational axis is tilted an unusually great 98 degrees from a hypothetical line perpendicular to the ecliptic plane. (The ecliptic plane is the theoretical plane created by extending Earth's orbit around the sun to form a vast, flat surface.) Thus the planet lies on its side with its north pole pointing slightly below the plane. During the course of its 84-year orbit around the sun, Uranus points first one pole toward the sun, then its equator, and then the other pole. It is thought that a catastrophic collision between Uranus and another body, perhaps a large comet, may have knocked the planet on its side. Uranus rotates in retrograde, or clockwise, motion about once every 17 hours. The planet has a strong magnetic field in which the magnetic north pole is tilted an exceptionally great 60 degrees from the rotational north pole.
Uranus has 15 known satellites, which are composed mostly of ice and are heavily cratered. The five major satellites are Miranda, Ariel, Umbriel, Titania, and Oberon. Oberon's surface is very old and heavily cratered, indicating that the body has been geologically inactive during most of its existence. Titania is covered by only small craters and shows evidence of early geologic activity. Ariel and Umbriel are the brightest and darkest satellites, respectively. Ariel has a young surface that contains some small craters, many faults, and some apparent ice flows. Umbriel is uniformly very dark and heavily cratered. The darkness of the surface suggests that it is relatively young, but the large number of craters indicates that the surface is old.
Miranda, the smallest and innermost of the major satellites, is partly young terrain covered with ridges and scarps and oddly shaped, sharp-cornered regions. Another part resembles the grooved terrain of Jupiter's Ganymede. A third area has a very old surface with many craters. Such extensive and diverse geologic activity is highly unusual for such a small, cold moon. Miranda may have been broken apart by an impact with a comet or another satellite, probably more than once, reaccreting each time to form the strange jumble of terrains now observed.
Uranus has a system of narrow, sharp-edged rings made of some unusually dark material very unlike Saturn's bright, icy rings. They are not uniformly thick, and in some places certain rings are so thin that they disappear. It is possible that Uranus' rings are relatively young compared to Saturn's and are still being formed. Voyager 2 recorded two small shepherd satellites in orbit close to the rings. There are probably more such satellites, but they are too small and dark to be seen. Some astronomers have suggested that the dark material of the rings and the small satellites, and possibly the dark coating on Umbriel, may be carbonaceous chondritic material.

Neptune
Little was known about the planet Neptune, which was discovered in 1846, until the Voyager 2 encounter in 1989. Its mass is comparable to that of Uranus, and it has a similar composition. Its thick atmosphere of hydrogen, helium, and some methane gives it a bluish color.
Like the other gaseous planets, Neptune rotates rapidly, once every 16.1 hours, and has a slightly larger diameter at the equator than at the poles. The atmospheric temperature has been found to be at about 60 K, higher than expected for a body so far from the sun. Its high temperature suggests that Neptune has another, possibly internal, heat source. The planet probably has a rocky core surrounded by water ice and liquid methane, which in turn are surrounded by hydrogen and helium gases.
Neptune has eight known satellites. The largest satellite, Triton, revolves around Neptune in a direction opposite to that of most other satellites in the solar system. Nereid, the second largest, revolves in the normal direction but in a very eccentric orbit.
Pluto and Charon
Pluto was discovered in 1930, and its satellite, Charon, was discovered in 1978. Pluto is a tiny, low-density planet made up of 97 percent nitrogen and small amounts of frozen carbon monoxide and methane. Until this discovery in 1992, Pluto was believed to have consisted of ice and rock. The planet's diameter is estimated at about 1,430 miles (2,300 kilometers). Charon is about 750 miles (1,200 kilometers) across. The combined mass of Pluto and Charon is about 450 times less than that of Earth. Consistent variations in brightness can be observed on Pluto, indicating that its surface is irregular. Astronomers have used these variations to help determine the planet's rotational period: 6 days, 9 hours, and 17 minutes, in Earth time.
Pluto's orbit is much more elliptic than those of the other planets and is tilted 17 degrees from the ecliptic plane. When it is at perihelion, Pluto is nearer the sun than is Neptune. Pluto's eccentric orbit and its physical similarities to icy satellites originally led some astronomers to believe that Pluto did not have the same origin as the other planets. One theory suggested that Pluto and Charon may once have been satellites of Neptune but were pulled away from Neptune's gravitational field. Most scientists, however, no longer believe this model is physically plausible.

Motions of the Planets

Planets revolve around the sun in elliptical orbits, with the sun at one focus of the ellipse (see Geometry, "Curves"). They move in the same direction (counterclockwise when viewed from above the Earth's North Pole) and in nearly the same plane.
The true orbital motions of the planets were first accurately described in the 1600s by the German astronomer Johannes Kepler. He formulated three laws that he observed to govern planetary motion. First, the planets' orbits around the sun are not exactly circular but are slightly elliptic. Second, the velocities of the planets in their orbits are such that an imaginary line drawn from a planet to the sun sweeps across equal areas in equal periods of time. As a result, the planets move faster when their orbits bring them closer to the sun and more slowly when they are farther away. Kepler's third law states that the square of a planet's period of revolution around the sun is proportional to the cube of the planet's average distance from the sun. In addition to their orbital motions, all planets rotate about their axes. Most rotate from west to east; only Venus, Uranus, and Pluto rotate from east to west. The rotational axes of all the planets except Uranus and Pluto are more or less perpendicular to the ecliptic plane. (See also Kepler, Johannes.)
The motions of the planets as observed from Earth, called their apparent motions, are complicated by Earth's own revolution, rotation, and slightly tilted axis. The Earth rotates from west to east, so that both the stars and the planets appear to rise in the east each morning and set in the west each night. Observations made at the same time every night will show that a planet usually appears in the sky slightly to the east of its position the previous night. Periodically, however, a planet will appear to change direction for several nights and move slightly to the west of its previous position. When a body moves opposite to an established direction, its movement is known as retrograde motion. A planet's reversal in direction when viewed from Earth is properly called apparent retrograde motion because it is only an illusion that occurs whenever Earth "overtakes" an outer planet. For example, because Saturn requires 29.46 Earth years to complete one orbit around the sun, the Earth often passes between Saturn and the sun. As this occurs, over the course of several nights Saturn appears to stop moving eastward, reverse its direction against the background of stars, and move in retrograde motion. Eventually, it appears to stop its retrograde motion and again move eastward.
Mercury and Venus also have unique apparent motions. Both appear to move from one side of the sun to the other. When either planet is east of the sun it appears to observers on Earth as an evening star, and when west of the sun, as a morning star.
Satellites revolving around the planets follow the same laws of orbital motion as do the planets, and their orbital planes nearly coincide with the orbital planes of the planets they circle. Most satellites, including Earth's moon, rotate around their axes once for each revolution around the planet. As a result, these satellites always show the same side to the planet.