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 about 150 million km (about 93 million mi). The most distant known planet, Pluto, orbits the sun at an average distance of 39.482 AU. 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.
Through the end of the 1980s, this solar system was the only planetary system known to exist, although a number of relatively nearby stars had been found to be encircled by swarms of orbiting material of indeterminate size (see Vega) or to be accompanied by suspected brown dwarfs. Since 1991, however, astronomers have detected more than 100 planets orbiting other stars, including more than a dozen multiple-planet systems. Many astronomers think it likely that solar systems of some sort are numerous throughout the universe (see Astronomy; Galaxy; Star).
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 changing 600 million metric tons of hydrogen into helium each second, with a net conversion to energy of at least 4 million tons of mass, the sun is so massive (2 × 1030 kg, or 4.4 × 1030 lb) that it can continue to shine at its present brightness for at least 4.5 billion years. This stability has allowed life to develop and survive on 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 cycles of approximately 11 years; sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on earth; and a continuous stream of protons, electrons, and ions leaves the sun and moves out through the solar system, spiraling 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 piloted U.S. Apollo spacecraft (see Space Exploration).
The Major Planets.
Nine major planets are 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 composed mainly 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 earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet--about 462° C (about 864° 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 water ice beneath a layer of 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. Saturn rivals Jupiter, with a much more intricate ring structure; one of Saturn's satellites, Titan, is the only moon in the solar system known to have a dense atmosphere. Uranus and Neptune are deficient in hydrogen compared with the two giants; Uranus, also ringed, has the distinction of rotating at nearly 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.
Some scientists contend that Pluto is not really a planet but a large object in the Kuiper Belt, a zone of icy debris that extends about 30-50 AU from the sun. In March 2004, astronomers announced the discovery of Sedna, the most distant planet-like object known to orbit the sun; this icy world, with a diameter of more than 1000 km (more than 620 mi), reaches perihelion, its closest approach to the sun, at 76 AU, beyond the Kuiper Belt and the orbit of Pluto.
THE MAJOR PLANETS (Data in parentheses indicate estimates.) Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto Average distance from sun (AU)1 0.387 0.723 1.00 1.524 5.023 9.537 19.191 30.069 39.482 Period of revolution about sun (years) 0.241 0.615 1.00 1.881 11.857 29.4 84.02 164.79 247.92 Eccentricity of orbit2 0.206 0.007 0.017 0.093 0.048 0.054 0.047 0.009 0.249 Inclination of orbit (degrees)3 7.0 3.39 0.0 1.8 1.305 2.484 0.77 1.769 17.14 Mass (earth = 1)4 0.055 0.815 1.00 0.107 317.82 95.16 14.371 17.147 0.0022 Radius (earth = 1)4 0.383 0.949 1.00 0.533 11.209 9.449 4.007 3.883 0.18 Mean density (earth =1)4 0.984 0.951 1.00 0.714 0.241 0.127 0.236 0.317 (0.4) Rotation period (about axis) 58.646 days 243 days5 23.934 hr 24.62 hr 9.925 hr5 10.656 hr5 17.24 hr5 16.11 hr 6.387 days Inclination of equator to orbit (degrees) 0 177.3 23.45 25.19 3.12 26.73 97.86 29.58 119.61 Surface gravity (earth = 1)4 0.38 0.91 1.00 0.38 2.1 0.74 0.86 1.1 0.08 Number of known satellites6 0 0 1 2 63 31 27 13 1 1 A distance of 1 AU is equivalent to 150 million km (93 million mi). A circle has an eccentricity of 0.0, a parabola 1.0. The inclination of a planetary orbit is measured with respect to the plane of the earth's orbit. The earth's mass = 5.98 × 1027 g, its mean radius = 6371 km, its mean density = 5.515 g/cm3, and its surface gravity at the equator = 9.766 m/sec2. The rotation of Venus and Uranus is retrograde; for Jupiter and Saturn, rotation periods vary with latitude, but the rotation of the interior can be measured by observation of radio emissions. As of February 2004.
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 maximum diameter of about 970 km (about 603 mi), to bodies that measure less than 1 km (less than 0.6 mi) in diameter. Some asteroids are perturbed into eccentric orbits that can bring them closer to the sun. A few binary systems have been discovered, in which the larger of two asteroids appears to have its own smaller satellite.
When the orbits of asteroids or asteroid fragments intersect that of the earth, they are called meteoroids. When meteoroids 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 recent impact craters.
Some meteors and interplanetary dust may also come from comets, which are aggregates of dust and frozen gases usually measuring 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 76-year periods. Its most recent return was in 1986. Comets that take less than 200 years to orbit the sun are known as short-period comets; most of these are thought to originate in the Kuiper Belt. Other bodies, known as long-period comets, may take millions of years to orbit the sun; most of these comets are thought to originate in the Oort Cloud, a vast region between 50,000 and 150,000 AU from the sun.
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 Uranus and Neptune, was itself determined, in 1989, to 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.
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. 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 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, 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 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 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) 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 quite 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 4.5 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 (see Nova and Supernova). 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 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. T.C.O., TOBIAS C. OWEN, M.S., Ph.D.
See also Exobiology.
For further information on this topic, see the Bibliography, sections 381. Cosmology-384. Planets, 386. Sun.