Solar System

• Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history.
• Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet—about 477° C (about 890° F).
• Earth is the only planet with abundant liquid water and known life.
• Strong evidence exists that Mars once had liquid water on its surface, but now its carbon dioxide atmosphere is so thin that the planet is dry and cold, with polar caps of frozen water and solid carbon dioxide, or dry ice.
• Jupiter is the largest of the planets. Its hydrogen and helium atmosphere contains pastel-colored clouds, and its immense magnetosphere, rings, and satellites make it a planetary system unto itself. One of Jupiter's largest moons, Io, has volcanoes that produce the hottest surface temperatures in the solar system. At least four of Jupiter's moons have atmospheres, and at least three show evidence that they contain liquid or partially-frozen water.
• Saturn rivals Jupiter, with a much more intricate ring structure and more satellites. One of Saturn's moons, Titan, has an atmosphere thicker than that of any other satellite in the solar system.
• Uranus and Neptune are deficient in hydrogen compared with Jupiter and Saturn; Uranus, also ringed, has the distinction of rotating at 98° to the plane of its orbit.
• Pluto seems similar to the larger, icy satellites of Jupiter or Saturn. Pluto is so distant from the Sun and so cold that methane freezes on its surface.

IV. Other Orbiting Bodies

The asteroids are small rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from Ceres, which has a diameter of 1,003 km (623 mi), to microscopic grains. Some asteroids are perturbed, or pulled by forces other than their attraction to the Sun, into eccentric orbits that can bring them closer to the Sun. If the orbits of such bodies intersect that of Earth, they are called meteoroids. When they appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much information about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including Earth's Moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On Earth that record has eroded away, except for a few recently found impact craters.

Some meteors and interplanetary dust may also come from comets, which are basically aggregates of dust and frozen gases about 5 to 10 km (about 3 to 6 mi) in diameter.

Comets orbit the Sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system. As comets approach the Sun, they release their dust and gases to form a spectacular coma and tail. Under the influence of Jupiter's strong gravitational field, comets can sometimes adopt much smaller orbits. The most famous of these is Halley's Comet, which returns to the inner solar system at 75-year periods. Its most recent return was in 1986. In July 1994 fragments of Comet Shoemaker-Levy 9 bombarded Jupiter's dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the tremendous kinetic energy of the fragments was converted into heat through massive explosions, some resulting in fireballs larger than Earth.

Comets circle the Sun in two main groups. The Kuiper belt is a ring of debris that orbits the Sun beyond the planet Neptune. Many of the comets with periods of less than 500 years are members of the Kuiper belt. The Oort cloud is a theoretical, spherical cloud of comets about halfway between the Sun and the heliopause, the imaginary boundary that marks the end of the Sun's influence. Astronomers believe that comets with very long periods reside in the Oort cloud. A chunk of dust and ice may stay in the Oort cloud for thousands of years. Nearby stars sometimes pass close enough to the solar system to push an object in the Oort cloud into an orbit that takes it close to the Sun.

Many of the objects that do not fall into the asteroid belts, the Kuiper belt, or the Oort cloud may be comets that will never make it back to the Sun. The surfaces of the icy satellites of the outer planets are scarred by impacts from such bodies. The asteroid-like object Chiron, with an orbit between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly, some of the asteroids that cross the path of Earth's orbit may be the rocky remains of burned-out comets. Chiron and similar objects called the Centaurs probably escaped from the Kuiper belt and were drawn into their irregular orbits by the gravitational pull of the giant outer planets, Jupiter, Saturn, Neptune and Uranus.

The Sun was also found to be encircled by three rings of interplanetary dust. One of them, between Jupiter and Mars, has long been known as the cause of zodiacal light, a faint glow that appears in the east before dawn and in the west after dusk. The other two rings, one lying only two solar widths away from the Sun, the other occurring in the region of the asteroids, were discovered in 1983.

V. Movements of the Planets and Their Satellites

If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. All of the planets except Venus and Uranus rotate on their axes in this same direction. The entire system is remarkably flat—only Mercury and Pluto have obviously inclined orbits. Pluto's orbit is so elliptical that it is sometimes closer than Neptune to the Sun.

The satellite systems mimic the behavior of their parent planets and move in a counterclockwise direction, but many exceptions are found. Jupiter, Saturn, and Neptune each have at least one satellite that moves around the planet in a retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits are highly elliptical. Jupiter, moreover, has trapped two clusters of asteroids (the so-called Trojan asteroids) leading and following the planet by 60° in its orbit around the Sun. (Some satellites of Saturn have done the same with smaller bodies.) The comets exhibit a roughly spherical distribution of orbits around the Sun.

Within this maze of motions, some remarkable patterns exist: Mercury rotates on its axis three times for every two revolutions about the Sun; no asteroids exist with periods (intervals of time needed to complete one revolution) 1/2, 1/3, ..., 1/n (where n is an integer) the period of Jupiter; the three inner Galilean satellites of Jupiter have periods in the ratio 4:2:1. These and other examples demonstrate the subtle balance of forces that is established in a gravitational system composed of many bodies.

VI. Theories of Origin

Despite their differences, the members of the solar system probably form a common family. They seem to have originated at the same time; few indications exist of bodies joining the solar system, captured later from other stars or interstellar space.

Early attempts to explain the origin of this system include the nebular hypothesis of the German philosopher Immanuel Kant and the French astronomer and mathematician Pierre Simon de Laplace, according to which a cloud of gas broke into rings that condensed to form planets. Doubts about the stability of such rings led some scientists to consider various catastrophic hypotheses, such as a close encounter of the Sun with another star. Such encounters are extremely rare, and the hot, tidally disrupted gases would dissipate rather than condense to form planets.

Current theories connect the formation of the solar system with the formation of the Sun itself, about 4.7 billion years ago. The fragmentation and gravitational collapse of an interstellar cloud of gas and dust, triggered perhaps by nearby supernova explosions, may have led to the formation of a primordial solar nebula. The Sun would then form in the densest, central region. It is so hot close to the Sun that even silicates, which are relatively dense, have difficulty forming there. This phenomenon may account for the presence near the Sun of a planet such as Mercury, having a relatively small silicate crust and a larger than usual, dense iron core. (It is easier for iron dust and vapor to coalesce near the central region of a solar nebula than it is for lighter silicates to do so.) At larger distances from the center of the solar nebula, gases condense into solids such as are found today from Jupiter outward. Evidence of a possible preformation supernova explosion appears as traces of anomalous isotopes in tiny inclusions in some meteorites. This association of planet formation with star formation suggests that billions of other stars in our galaxy may also have planets. The high frequency of binary and multiple stars, as well as the large satellite systems around Jupiter and Saturn, attest to the tendency of collapsing gas clouds to fragment into multibody systems.