..... Click the link for more information. , dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun sun, intensely hot, self-luminous body of gases at the center of the solar system. Its gravitational attraction maintains the planets, comets, and other bodies of the solar system in their orbits.
..... Click the link for more information. is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass. The principal members of the sun's retinue are the eight major planets planet [Gr.,=wanderer], a large nonluminous ball of rock or gas that orbits a star. The term, once limited to any of the eight solid, nonluminous bodies (major planets) that revolve around the sun, has been extended to include similar bodies discovered revolving
..... Click the link for more information. ; other parts of the solar system are discussed in separate articles: see comet comet [Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun. Comets visible from the earth can be seen for periods ranging from a few days to several months.
..... Click the link for more information. , asteroid asteroid, planetoid, or minor planet, small body orbiting the sun. More than 10,000 asteroids have orbits sufficiently well known to have been cataloged and named; thousands more exist.
..... Click the link for more information. , and meteor meteor, appearance of a small particle flying through space that interacts with the earth's upper atmosphere. While still outside the atmosphere, the particle is known as a meteoroid. Countless meteoroids of varying sizes are moving about the solar system at any time.
..... Click the link for more information. .
The Planets
In order of increasing average distance from the sun, the planets are Mercury Mercury, in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible
..... Click the link for more information. , Venus Venus, in astronomy, 2d planet from the sun; it is often called the evening star or morning star and is brighter than any object in the sky except the sun and the moon.
..... Click the link for more information. , earth earth, in geology and astronomy, 3rd planet of the solar system and the 5th largest, the only planet definitely known to support life. Gravitational forces have molded the earth, like all celestial bodies, into a spherical shape.
..... Click the link for more information. , Mars Mars, in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth.
Physical Characteristics
Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice
..... Click the link for more information. , Jupiter Jupiter , in astronomy, 5th planet from the sun and largest planet of the solar system.
Astronomical and Physical Characteristics
Jupiter's orbit lies beyond the asteroid belt at a mean distance of 483.6 million mi (778.
..... Click the link for more information. , Saturn Saturn, in astronomy, 6th planet from the sun.
Astronomical and Physical Characteristics of Saturn
Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.
..... Click the link for more information. , Uranus Uranus , in astronomy, 7th planet from the sun, at a mean distance of 1.78 billion mi (2.87 billion km), with an orbit lying between those of Saturn and Neptune; its period of revolution is slightly more than 84 years.
..... Click the link for more information. , and Neptune Neptune, in astronomy, 8th planet from the sun at a mean distance of about 2.8 billion mi (4.5 billion km) with an orbit lying between those of Uranus and the dwarf planet Pluto; its period of revolution is about 165 years.
..... Click the link for more information. . The planets orbiting nearer the sun than the earth are termed inferior planets; those whose orbits are larger are called superior planets. The unit for measuring distance in the solar system is the astronomical unit astronomical unit (AU), mean distance between the earth and sun; one AU is c.92,960,000 mi (149,604,970 km). The astronomical unit is the principal unit of measurement within the solar system, e.g., Mercury is just over 1-3 AU and Pluto is about 39 AU from the sun.
..... Click the link for more information. (AU), the average distance between the earth and the sun. The mean distances of the planets from the sun range from 0.39 AU for Mercury to 30.04 AU for Nepture.
Pluto Pluto, in astronomy, a dwarf planet and the first Kuiper belt, or transneptunian, object (see comet) to be discovered (1930) by astronomers. Pluto has an elliptical orbit usually lying beyond that of Neptune.
..... Click the link for more information. , regarded for many years after its discovery as a planet, was reclassified in 2006 as a dwarf planet, which is a planetlike celestial body that does not clear or dominate the region of its orbit. In addition, Pluto is unlike the terrestrial planets—Mercury, Venus, Earth, and Mars—which are rocky, and it is unlike the gas giants—Jupiter, Saturn, Uranus, and Neptune. Its orbit, which is tilted from the plane in which the eight planets travel about the Sun, its size, and its composition more closely resemble those of the objects residing in the Kuiper belt (which were first discovered in 1992; see under comet) than those of a major planet, and Pluto is now recognized as a Kuiper belt, or transneptunian, object.
See the table entitled Major Planets of the Solar System.
Planetary Motion
The motion of the planets was first described accurately by Johannes Kepler at the beginning of the 17th cent.; he showed that the planets move in nearly circular elliptical orbits. Isaac Newton later showed that the laws of planetary motion discovered by Kepler apply also to all other bodies in the solar system and are based on the force of gravitation gravitation, the attractive force existing between any two particles of matter.
The Law of Universal Gravitation
Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles,
..... Click the link for more information. . The sun's gravitational pull is the dominant force in the solar system; the forces exerted by the other celestial bodies on one another produce small shifts and variations, called perturbations perturbation , in astronomy and physics, small force or other influence that modifies the otherwise simple motion of some object. The term is also used for the effect produced by the perturbation, e.g., a change in the object's energy or path of motion.
..... Click the link for more information. , in their orbits. The planets orbit the sun in approximately the same plane (that of the ecliptic ecliptic , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic). Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the
..... Click the link for more information. ) and move in the same direction—counterclockwise as viewed from above the earth's North Pole. A planet's year, or sidereal period, is the time required for it to complete one full circuit around the sun. Mercury's year is 88 earth days, while Neptune's year is 165 earth years. All the planets rotate about their own axes as they revolve around the sun; their periods of rotation vary from just under 10 earth hours for Jupiter to 243 earth days for Venus. The rotation of Venus is from east to west (see retrograde motion retrograde motion, in astronomy, real or apparent movement of a planet, dwarf planet, moon, asteroid, or comet from east to west relative to the fixed stars.
..... Click the link for more information. ). The equatorial planes of the planets are tilted to various degrees with respect to their orbital planes, giving rise to yearly seasons. The smallest tilt, that of Jupiter, is 3°, whereas that of Uranus is 98°, causing its axis of rotation to lie nearly in the plane of the planet's orbit. The tilt of the earth's equatorial plane is 23 1-2°.
Physical Properties
The planets are grouped according to their physical properties. The inner planets (Mercury, Venus, Earth, and Mars), called the terrestrial, or earthlike, planets, are dense and small in size, with solid, rocky crusts and molten metallic interiors. Except for Mercury, they possess gaseous atmospheres from which lighter elements have escaped because of the low gravitational force. The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) all have great volume and mass but relatively low density. Jupiter is heavier than all the other planets combined; it is 318 times as heavy as the earth and 1,300 times as large, making its density only about one fourth that of the earth. Saturn has a mass 95 times that of the earth and a density less than that of water. The atmospheres of the Jovian planets are very thick, merging imperceptibly with the bodies of the planets, and are rich in hydrogen, hydrogen compounds, and helium. Most of the major planets have one or more moons. See satellite, natural.
Origin of the Solar System
Besides explaining the birth of the sun, planets, dwarf planets, moons, asteroids, and comets, a theory of the origin of the solar system must explain the chemical and physical differences of the planets; their orbital regularities, i.e., why they lie almost on the same plane and revolve in the same direction in nearly circular orbits; and also account for the relative angular momentum of the sun and planets arising from their rotational and orbital motions.
The Nebular Hypothesis
The nebular hypothesis, developed by Immanuel Kant and given scientific form by P. S. Laplace at the end of the 18th cent., assumed that the solar system in its first state was a nebula, a hot, slowly rotating mass of rarefied matter, which gradually cooled and contracted, the rotation becoming more rapid, in turn giving the nebula a flattened, disklike shape. In time, rings of gaseous matter became separated from the outer part of the disk, until the diminished nebula at the center was surrounded by a series of rings. Out of the material of each ring a great ball was formed, which by shrinking eventually became a planet. The mass at the center of the system condensed to form the sun. The objections to this hypothesis were based on observations of angular momentum that conflicted with the theory.
The Planetesimal and Tidal Theories
Encounter or collision theories, in which a star passes close by or actually collides with the sun, try to explain the distribution of angular momentum. According to the planetesimal theory developed by T. C. Chamberlin and F. R. Moulton in the early part of the 20th cent., a star passed close to the sun. Huge tides were raised on the surface; some of this erupted matter was torn free and, by a cross-pull from the star, was thrust into elliptical orbits around the sun. The smaller masses quickly cooled to become solid bodies, called planetesimals. As their orbits crossed, the larger bodies grew by absorbing the planetesimals, thus becoming planets.
The tidal theory, proposed by James Jeans and Harold Jeffreys in 1918, is a variation of the planetesimal concept: it suggests that a huge tidal wave, raised on the sun by a passing star, was drawn into a long filament and became detached from the principal mass. As the stream of gaseous material condensed, it separated into masses of various sizes, which, by further condensation, took the form of the planets. Serious objections against the encounter theories remain; the angular momentum problem is not fully explained.
Contemporary Theories
Contemporary theories return to a form of the nebular hypothesis to explain the transfer of momentum from the central mass to the outer material. The nebula is seen as a dense nucleus, or protosun, surrounded by a thin shell of gaseous matter extending to the edges of the solar system. According to the theory of the protoplanets proposed by Gerard P. Kuiper, the nebula ceased to rotate uniformly and, under the influence of turbulence and tidal action, broke into whirlpools of gas, called protoplanets, within the rotating mass. In time the protoplanets condensed to form the planets. Although Kuiper's theory allows for the distribution of angular momentum, it does not explain adequately the chemical and physical differences of the planets.
Using a chemical approach, H. C. Urey has given evidence that the terrestrial planets were formed at low temperatures, less than 2,200°F; (1,200°C;). He proposed that the temperatures were high enough to drive off most of the lighter substances, e.g., hydrogen and helium, but low enough to allow for the condensation of heavier substances, e.g., iron and silica, into solid particles, or planetesimals. Eventually, the planetesimals pulled together into protoplanets, the temperature increased, and the metals formed a molten core. At the distances of the Jovian planets the methane, water, and ammonia were frozen, preventing the earthy materials from condensing into small solids and resulting in the different composition of these planets and their great size and low density.
The discovery of extrasolar planetary systems planetary system, a star and all the celestial bodies bound to it by gravity, especially planets and their natural satellites. Until the last decade of the 20th cent.
..... Click the link for more information. , beginning with 51 Pegasi in 1995, have given planetary scientists pause. Because it was the only one known, all models of planetary systems were based on the characteristics of the solar system—several small planets close to the star, several large planets at greater distances, and nearly circular planetary orbits. However, all of the extrasolar planets are large, many much larger than Jupiter, the largest of the solar planets; many orbit their star at distances less than that of Mercury, the solar planet closest to the sun; and many have highly elliptical orbits. All of this has caused planetary scientists to revisit the contemporary theories of planetary formation.
Bibliography
See N. Booth, Exploring the Solar System (1996); P. R. Weissman et al., ed., Encyclopedia of the Solar System (1998); J. K. Beatty et al., ed., The New Solar System (4th ed. 1999); B. W. Jones, Discovering the Solar System (1999).
solar system
The Sun, its eight major planets, the dwarf planets and small bodies, and interplanetary dust and gas under the Sun's gravitational control. Another component of the solar system is the solar wind. The Sun contains more than 99% of the mass of the solar system; most of the rest is distributed among the planets, with Jupiter containing about 70%. According to the prevailing theory, the solar system originated from the solar nebula. See also asteroid; Centaur object; Ceres; comet; Earth; Eris; Jupiter; Kuiper belt; Mars; Mercury; meteorite; Neptune; Oort cloud; Pluto; Saturn; Uranus; Venus.
solar system
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solar system [′sō·lər ‚sis·təm]
Solar System
the system of celestial bodies moving in the region where the sun’s gravitational influence is dominant. The solar system includes the sun, planets, planetary satellites, asteroids, comets, meteoroids, and cosmic dust. The observed dimensions of the solar system are determined by the orbit of Pluto, which is approximately 40 astronomical units (AU) from the sun (see Figure 1). On the other hand, the sphere within which stable motion of celestial bodies around the sun is possible extends almost to the closest stars (230,000 AU). Information on the distant outer region of the solar system is obtained through observation of long-period comets that approach the sun and through study of the cosmic dust that fills the entire solar system.
The overall structure of the solar system was uncovered by N. Copernicus in the mid-16th century. He provided evidence for the motion of the earth and the other planets around the sun. His heliocentric system made it possible for the first time to determine the relative distances of the planets from the sun and, consequently, from the earth. The laws governing the motion of the planets were discovered by J. Kepler in the early 17th century, and the law of universal gravitation was formulated by I. Newton in the late 17th century. These laws formed the basis of celestial mechanics, which investigates the motion of the bodies of the solar system. The study of the physical characteristics of bodies in the solar system became possible only after the invention of the telescope by Galileo. In 1609 he first turned the small telescope he had constructed toward the moon, Venus, Jupiter, and Saturn
and made a number of discoveries that were remarkable for his time. While observing sunspots, he detected the sun’s rotation about its axis.
In their physical characteristics the planets are divided into the inner planets (Mercury, Venus, the earth, and Mars) and the outer, giant planets (Jupiter, Saturn, Uranus, and Neptune). Since the physical characteristics of Pluto are qualitatively different from those of the giant planets, it cannot be classified as a giant planet. Figure 2 shows the relative sizes of the sun and planets.
An extensive program of observations conducted in 1963 by the American astronomer C. Tombaugh in search of planets lying beyond the orbit of Pluto failed to yield positive results. Table 1 gives the osculating elements of the planetary orbits (seeORBIT OF A CELESTIAL BODY) according to Oesterwinter and Cohen of the USA (1972). The orbits of the planets are slightly inclined with respect to each other and to the invariable plane of the solar system.
Approximately 90 percent of the natural satellites of the planets are grouped around the outer planets. Indeed, Jupiter and Saturn, together with their satellites, make up, as it were, miniature solar systems. Some satellites are extremely large; for example, Jupiter’s satellite Ganymede is bigger than the planet Mercury. In addition to ten satellites, Saturn has a system of rings consisting of a large number of small bodies whose motion obeys Kepler’s laws; these bodies are also essentially satellites of Saturn. The radius of the outermost ring is 2.3 radii of Saturn—that is, the rings lie within Roche’s limit.
By 1976, the exact orbits of more than 2,000 asteroids had been calculated; the orbits lie for the most part between the orbits of Mars and Jupiter. In some cases, the orbits of the asteroids differ considerably in shape and position from the orbits of the planets. The inclinations of the asteroids’ orbits to the plane of ecliptic can be as great as 52°, and the eccentricities of their orbits can be as great as 0.83. Because of their large eccentricities, some asteroids approach the sun closer than does Mercury and move as far away from the sun as Saturn’s orbit. The total number of asteroids observable by present-day telescopes is estimated at 40,000.
When viewed from the north celestial pole, the motion of the planets and planetary satellites and the rotation of these bodies about their axes are counterclockwise—that is, in the direction customarily referred to as direct. Exceptions are the rotation of Venus and Uranus and the retrograde motion of some satellites around their primaries. The distances between the orbits of the planets are described by the empirical Titius-Bode law.
Comets differ markedly in appearance, dimensions, and orbital characteristics from other bodies in the solar system. The periods of revolution of comets can be as great as several million years; at aphelion such comets approach the boundaries of the solar system and experience gravitational perturbations from nearby stars. The orbits of comets vary in inclination from 0° to 180°. The total number of comets is estimated to be hundreds of billions.
Meteoroids (seeMETEOR) and cosmic dust fill all space in the solar system. The motion of cosmic dust is influenced not only by
| Table 1. Elements of the planetary orbits (1973 data) | ||||
|---|---|---|---|---|
| Planet | Mean distance from sun (AU) | Eccentricity of orbit | Inclination of orbital plane to ecliptic (degrees) | Period of revolution about sun (years) |
| Mercury ............... | 0387 | 0.206 | 7.00 | 0.24 |
| Venus ............... | 0.723 | 0.007 | 3.39 | 0.62 |
| Earth ............... | 1.000 | 0.016 | 1.00 | |
| Mars ............... | 1.524 | 0.093 | 1.85 | 1.88 |
| Jupiter ............... | 5.203 | 0.043 | 1.31 | 11.86 |
| Saturn ............... | 9.539 | 0.056 | 2.49 | 29.46 |
| Uranus ............... | 19.19 | 0.046 | 0.77 | 84.02 |
| Neptune ............... | 30.06 | 0.008 | 1.77 | 164.79 |
| Pluto ............... | 39.75 | 0.253 | 17.15 | 250.6 |
the attraction of the sun and planets but also by solar radiation; the motion of electrically charged particles is influenced also by the magnetic fields of the sun and planets. Inside the earth’s orbit the density of the cosmic dust increases, and the dust forms a cloud surrounding the sun that is visible from the earth as the zodiacal light.
The question of the stability of the solar system is closely connected with the presence of secular terms in the semimajor axes, eccentricities, and inclinations of the planetary orbits (seePERTURBATIONS OF CELESTIAL BODIES). Classical methods of celestial mechanics, however, do not take into account small dissipa-tive factors, such as the continuous loss of mass by the sun, that may play an important role in the evolution of the solar system over long periods of time. The solar system participates in the rotation of the Galaxy and moves in an approximately circular orbit at a speed of about 250 km/sec. The solar system’s period of revolution about the center of the Galaxy has been determined to be about 200 million years. The problem of the origin of the solar system is one of the most important questions in modern science (seeCOSMOGONY). The solution of this problem is particularly difficult because the solar system is the only such system we have knowledge of. Suppositions have been advanced that the nearest stars have dark companions of planetary dimensions. The existence of such companions is extremely likely, but conclusive confirmation has not yet been provided. The age of the solar system is estimated at 5 billion years.
The space age has furnished astronomy with new means of studying the solar system. Soviet and American space probes are intensively investigating the inner planets of the solar system. Soviet space probes have made soft landings on the moon, Venus, and Mars. The first manned landing on the surface of the moon was made in 1969 by American astronauts. Between 1972 and 1974 the American space probes Pioneer 10 and Pioneer 11 were launched, crossed the asteroid belt, and made close passes of Jupiter. Plans are being made for flights to periodic comets and for the soft landing of a spacecraft on an asteroid approaching close to the earth. Mankind is beginning to gain practical mastery of the inner region of the solar system.
G. A. CHEBOTAREV
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