The Solar System

Solar System, the system consisting of the sun; the nine planets and their satellites; the asteroids, comets, and meteoroids; and interplanetary dust and gas. The dimensions of this system are specified in terms of the mean distance from the earth to the sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant known planet, Pluto, has an orbit at 39.44 AU from the sun. The boundary between the solar system and interstellar space—called the heliopause—is estimated to occur near 100 AU. The comets, however, achieve the greatest distance from the sun; they have highly eccentric orbits ranging out to 50,000 AU or more. The solar system is the only planetary system known to exist, although in the 1980s a number of relatively nearby stars were found to be encircled by swarms of orbiting material of indeterminate size (See Vega) or to be accompanied by objects suspected to be brown dwarfs. Many astronomers think it likely that planetary systems of some sort are numerous throughout the universe. See Astronomy.

The Sun and the Solar Wind

The sun is a typical star, of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the sun's hot, dense interior (See Nuclear Energy). Although this nuclear fusion is converting 600 million tonnes of hydrogen each second, the sun is so massive (2 × 1027 tonnes) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on the earth.

For all the sun's steadiness, it is an extremely active star. On its surface dark sunspots bounded by intense magnetic fields come and go in 11-year cycles; sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on the earth; and a continuous stream of protons, electrons, and ions leaves the sun and moves out through the solar system, spiralling with the sun's rotation. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back from the moon's surface by Apollo spacecraft. See Space Exploration.

The Major Planets

Nine major planets are currently known. They are commonly divided into two groups: the inner planets (Mercury, Venus, earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto). The inner planets are small and are composed primarily of rock and iron. The outer planets (except Pluto) are much larger and consist mainly of hydrogen, helium, and ice.

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 the 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 (890° F). The earth is the only planet with abundant liquid water and life. Strong evidence exists that Mars once had water on its surface, but now its carbon dioxide (CO2) atmosphere is so thin that the planet is dry and cold, with polar caps of solid carbon dioxide, or dry ice. Jupiter is the largest of the planets. Its hydrogen and helium atmosphere contains pastel-coloured clouds, and its immense magnetosphere, rings, and satellites make it a planetary system unto itself. Saturn rivals Jupiter, with a much more intricate ring structure and more satellites, including one with a dense atmosphere—Titan. Uranus and Neptune are deficient in hydrogen compared with the two giants; 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.

Other Constituents

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 1,000 km (620 mi), to microscopic grains. Some asteroids are perturbed into eccentric orbits that can bring them closer to the sun. Smaller bodies orbiting the sun are called meteoroids. Some collide with the earth and appear in the night sky as streaks of light, known as meteors. Recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including the earth's moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On the earth that record has been eroded away, except for a few recent impact craters.

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

The surfaces of the icy satellites of the outer planets are scarred by impacts from comet nuclei. Indeed, the asteroidal object Chiron, with an orbit between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly, some of the asteroids that cross earth's orbit may be the rocky remains of burned-out comets.

The sun has been 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. 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.

Movements of the Planets and Their Satellites

If one could look down on the solar system from far above the North Pole of the earth, the planets would appear to move around the sun in an anticlockwise 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 at present closer than Neptune to the sun, and will be so until 1999.

The satellite systems mimic the behaviour of their parent planets, but many more exceptions are found. Jupiter, Saturn, and Neptune each have one or more satellites that move around the planets in retrograde orbits (clockwise instead of anticlockwise) and several satellite orbits are highly elliptical. Jupiter, moreover, has trapped two clusters of asteroids (the so-called Trojan asteroids), which lead and follow the planet by 60° in its orbit around the sun. (Some satellites of Saturn have similarly trapped smaller bodies.) The comets exhibit a roughly spherical distribution of orbits around the sun.

Within this maze of motions, some remarkable resonances exist: Mercury rotates on its axis three times for every two revolutions about the sun; no asteroids exist with periods 1/2, 1/3, …, 1/n (where n is an integer) of 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.

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 later captures 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 envelope and a larger than usual dense iron core. (It is easier for iron dust and vapour than for lighter silicates to coalesce near the central region of a solar nebula.) At larger distances from the centre of the solar nebula, gases condensed into such solids as are found today from Jupiter outwards. 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.