| SUN | PLANETS | SATELLITES | ASTEROIDS | COMETS | DUST RINGS | ORIGIN | SOLAR APEX |
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THE SOLAR SYSTEM
HISTORY OF SOLAR SYSTEM STUDIES
Since primitive times humanity has been aware that certain of the stars in the sky are not fixed but wander slowly across the heavens. The Greeks gave these moving stars the name planets, or "wanderers." They were the first to predict with accuracy the positions of the planets in the sky, and they devised elaborate theoretical models in which the planets moved around combinations of circles that in turn circled the Earth. The Greek mathematician Claudius Ptolemy systematized an elaborate geocentric scheme of this kind in the 2d century AD, which passed with minor changes through the Middle Ages and on to the Polish astronomer Nicolaus Copernicus. In his work of 1543, Copernicus proposed the idea that planetary motions are centered on the Sun rather than on the Earth, but he retained the description of planetary motions as being a series of superimposed circular motions, mathematically equivalent to the Ptolemaic theory. In the same year Copernicus died. During the 17th century a German mathematician by the name of Johannes Kepler abandoned his forebears' concept of circular motion in favor of an elliptical scheme, in which the motions of the planets describe a simple series of ellipses in which the Sun is at one of the foci. Basing his work on the observations of Tycho Brahe, his former employer and a renowned astronomer, Kepler found (1609, 1619) three important empirical relationships, concerning the motion of the planetary bodies, now known as Kepler's laws. Kepler's labors laid the groundwork for Sir Isaac Newton's law of gravitation (1687), from which it became possible for astronomers to predict with great accuracy the movements and positions of the planets.
Only the planets Mercury, Venus, Mars, Jupiter, and Saturn were known to the ancients. The English astronomer William Herschel accidentally discovered Uranus in 1781 as the result of telescopic observations. Discrepancies between the observed positions of Uranus and those predicted led John Couch Adams and Urbain Jean Joseph Leverrier to propose (1846) that another large planet was exerting a gravitational force on Uranus. In the same year the planet Neptune was found close to its predicted position. In the 20th century smaller apparent discrepancies in the position of Uranus led to predictions of the existence of yet another planet. In 1930, Clyde Tombaugh discovered Pluto close to one of the areas of prediction. Pluto's mass, however, is so small that the discovery was accidental, resulting from intense scrutiny of that part of the sky to which predictions had called attention. It was theorized that a further planet may exist, although recent corrections in the calculated mass of Uranus leave this in doubt.
Galileo Galilei was in 1609 the first to use the telescope for astronomical purposes, and it has since become an essential tool in planetary studies. In the 19th century planetary astronomy flourished, thanks to the construction of large telescopes and their systematic use for planetary observations. Two new tools, the spectroscope and the photographic plate, were also developed in the 19th century and gave rise to the new science of astrophysics. For the first time it became possible to determine not only the orbits and masses of objects in the solar system, but also their temperatures, compositions, and structures. During the early years of the 20th century great advancements took place in the understanding of the physics and chemistry of the planets in the solar system, and during the middle years of the century important further advances were derived from radio astronomy and radar astronomy.
Although most astronomers gradually turned their attention away from the solar system to the study of stars and galaxies, the launch (1957) of the first artificial satellite initiated an age that transformed solar-system studies. Beginning in the 1960s spacecraft accomplished flyby, orbiting, or landing missions to many of the planets. At the present time the reconnaissance of the planets has been accomplished except for Pluto. The U.S. Mariner and Soviet Venera spacecraft have studied the atmosphere and surface of Venus. Mariners and U.S. Viking spacecraft have extensively photographed Mars from orbit, and Viking landers have carried out important initial measurements of surface properties. The investigation of the Moon progressed through the stages of flybys, orbiters, and landers both manned (U.S. Apollo) and unmanned (U.S. Surveyor Ranger, and Lunar Orbiter, and Soviet Luna). Lunar soil samples have also been returned for study from several different landing sites. U.S. Pioneer and Voyager probes have returned data and images from the outer planets and their satellites, except for Pluto, and in 1993 the Voyagers may have glimpsed the heliopause--the outer edge of the solar system, where the solar wind ebbs--about 82 to 130 times farther away from the Sun than the Earth is.
The Sun is the only star whose surface can be studied in detail from the Earth. This surface presents a scene of churning, turbulent activity, largely dominated by strong magnetic fields. Magnetic lines of force emerging from the solar surface appear as sunspots. Arches of the magnetic lines of force extending across the surface give rise to bright, shining solar prominences. Wave motions generated below the surface of the Sun flicker across the surface and mount into the atmosphere. Brilliant flares appear in the vicinity of sunspots, generating bursts of ultraviolet and X-ray emissions from the Sun and accelerating ions and electrons to create the high-energy particles known as cosmic rays.
The upper levels of the Sun's atmosphere are of very low density, but the solar activity heats the gases there to very high temperatures. Here the electrons are stripped from atoms to form ions, and the two types of particles together form a plasma. The gravitational field of the Sun is unable to retain this superhot plasma, and it streams outward into space as the solar wind. Measurements of the properties of the solar wind are routinely carried out by U.S. spacecraft at many different locations within the solar system.
Most of the mass (99.86 percent) of the solar system is concentrated in the Sun, which thus exerts the gravitational force that holds the scattered members of the system together. There is a remarkable degree of orderliness in the motions of the members of the solar system under the influence of the Sun's gravity. With the exception of the comets, some of the asteroids, and Pluto, the motions of the bodies in the solar system are confined to approximately the same plane, called the plane of the ecliptic. There is a striking similarity in the way in which these bodies revolve and rotate. The planets all revolve around the Sun in the same direction, and the Sun rotates in this direction as well. With only two exceptions, Venus and Uranus, the planets also rotate in this common direction. Many of the planets, particularly in the outer solar system, are accompanied by swarms of satellites, and again, with a few exceptions, these also tend to revolve in a plane close to the plane of the ecliptic and with the same sense of motion. All of these tendencies can be summarized by saying that the angular momentum vectors of the bodies in the solar system are for the most part aligned.
The nine planets of the solar system may be divided into two groups: the inner, or terrestrial, planets, and the outer, or Jovian, planets. This division is based not only on distance from the Sun, but also on the physical properties of the planets.
The Inner Planets
The inner planets are all comparable in size, density, and other characteristics to the Earth and so are generally referred to as the terrestrial, or Earth-like, planets. Included are Mercury, Venus, Earth, and Mars.
The Earth is the largest of the terrestrial planets. By far the most massive constituents of the Earth are the iron core and the rocky mantle and crust. The water in the oceans and the gases in the air form only a thin veneer of volatile materials surrounding the rock of the planet proper. The Sun provides the heat and light that make the Earth habitable for life as we know it. The oceans and atmosphere of the Earth absorb and redistribute the heat in a complex fashion. Various types of geological evidence show that the Earth has passed through ice ages in the past. Various processes were probably involved in their cause, including changes in the Earth's motions, but the exact mechanisms are not yet certain. The early years of the Earth were apparently rather violent, as no geological record is preserved of the first half-billion years of its existence.
The Earth-Moon system is often referred to as a "double planet" system, because the Moon is more nearly comparable in size to the Earth than the other satellites are to their primaries (except for Pluto and its moon). The Earth's Moon is 81 times less in mass than the Earth but only 4 times less in mass than the planet Mercury. It is one of a group of the six largest satellites in the solar system that have approximately comparable mass, and the only such large one in the inner solar system. Compared to the mass of its primary, the Earth, the Moon is abnormally massive. The return of samples from several lunar sites during the Apollo program, and the establishment of stations to measure seismic activity and other physical quantities at these sites, has provided more knowledge about the Moon than currently exists for any other body in the solar system except the Earth. If the Moon has a central iron core, it is unexpectedly small, compared to that of the Earth, and of surprisingly little mass. The bulk of the Moon is mantle and crust that has had an extensive history of melting and chemical differentiation. The Moon contains no atmosphere, and its surface is heavily cratered. Its topmost soil is a very fine-grained substance with little chips of rock sprinkled throughout. This is called the lunar regolith. The Moon is heavily depleted in the more volatile elements and compounds as compared to the Earth.
The next inner planet toward the Sun is Venus, long considered a mystery planet because it is shrouded in clouds that hide the details of its underlying surface. Venus is nearly as large and as massive as the Earth, contains relatively little water, and has nothing resembling the oceans of the Earth. Instead, carbon dioxide in an amount comparable to that in the carbonate rocks of the Earth fills the Venusian atmosphere, producing a pressure at the surface about 100 times higher than that at the surface of the Earth and a temperature far too high to support life of any kind as we know it. Venus has a slow retrograde rotation, so that it rotates in a direction opposite to that of most of the other objects in the solar system.
The next planet outward from the Earth away from the Sun is Mars, which is only about one-tenth of the mass of the Earth. Its tenuous atmosphere is composed principally of carbon dioxide, with a pressure at the surface more than 100 times smaller (0.7 percent) than that at the surface of the Earth. The surface of Mars can be considered to be roughly divided into two hemispheres, one a surface of ancient, heavily cratered terrain and the other a geologically younger terrain having a much lower density of cratering. Mars has long been suspected as a possible abode for some form of life, at least in the geological past. Although experiments performed by the Viking spacecraft landers produced no evidence for life, the controversial claim was made in 1996 that certain meteorites found on Earth and thought to be of Martian origin showed possible traces of bacterialike fossils. It is hoped that further probes sent to the planet may yield more information on this issue.
The planet nearest the Sun is Mercury, with a mass half that of Mars and with only a trace atmosphere consisting of such elements as helium, sodium, and hydrogen. Its surface is heavily cratered. Mercury has an interesting resonance with its orbital motion, presenting one face and then the other during its closest approaches to the Sun.
The Outer Planets
The terrestrial planets just described have in common a rocky composition whose major constituents have high boiling points. It is believed that the entire solar system was formed from the gravitational contraction of a large cloud of gas and dust composed mainly of hydrogen and helium and only a small percentage of heavier elements. The Sun's composition is believed to be essentially the same as that of the original nebula. The inner planets lost most of their lighter, volatile elements early as a result of their proximity to the hot Sun, whereas the more distant outer planets were able to retain their light gases. The result is that the outer planets became far more massive and were able to hold very extensive atmospheres of light gases such as hydrogen, as well as light, icy substances such as water (H(2)O), ammonia (NH(3)), and methane (CH(4)).
The most massive planet in the solar system, with about one-thousandth the mass of the Sun and more than 300 times the mass of the Earth, is Jupiter. Composed primarily of hydrogen and helium, Jupiter may have an interior composed of ice (and other frozen volatiles) and rocks, or both, exceeding several times one Earth mass of rocky material and three Earth masses of the ices. The total amount of material heavier than hydrogen and helium is probably in the range of 10-20 Earth masses. Jupiter rotates rapidly on its axis, so that its figure is significantly flattened toward its equatorial plane, and the gases in its surface show a banded structure along lines of latitude. Jupiter radiates into space about twice as much energy as it absorbs from the Sun, with the additional heat emerging from the interior of the planet. Spacecraft also revealed that Jupiter is ringed.
The next planet outward from Jupiter is the strikingly ringed Saturn, another gas giant also thought to be composed predominantly of hydrogen and helium. Its mass is slightly less than a third that of Jupiter, but it also appears to have something approaching 20 Earth masses of heavier materials in the form, presumably, of icy or rocky materials. Saturn also rotates rapidly, is highly flattened toward its equatorial plane, and exhibits a banded structure along latitude lines.
Beyond Saturn are Uranus and Neptune, two planets of similar size. Uranus has a mass about 15 times and Neptune a mass about 17 times that of the Earth. Hydrogen and helium predominate in the atmospheres of both planets. The planetary interiors lie hidden beneath thick atmospheres, but data from Voyager 2 suggest that Uranus has a superheated water ocean, up to 10,000 km (6,000 mi) deep, surrounding an Earth-size core of molten rock materials. Neptune has an active atmosphere and, apparently, some form of internal energy source. The rotation period of Uranus is a little longer than 17 hours and that of Neptune a little longer than 16 hours. Uranus is unique among the planets in being tilted on its rotation axis by about 98 degrees with respect to the plane of the ecliptic, so that its rotation is retrograde. Uranus and Neptune both have ring systems.
Pluto is a planet whose characteristics were largely unknown until 1978. Pluto's diameter is 2,284 km (1,416 mi). The density of the planet is about the same as that of water, so Pluto may be composed of an ice-rock mixture. Pluto has a rather elliptical orbit that at times takes the planet closer to the Sun than Neptune ever reaches. Since 1979, for example, Pluto has lain within Neptune's orbit, and it will continue to do so until 1999. This would ordinarily be a rather unstable state of affairs, but perturbations of the Pluto orbit caused by Neptune occur in such a way that a collision between the two planets cannot happen. Astronomers have also observed perturbations in the orbits of Uranus and Neptune. Pluto is too small to cause them, and the Pioneer probes have detected no other gravity sources to which they could be attributed. A few scientists have hypothesized a tenth planet as the cause. Others think that more recent estimates of Neptune's mass account for the perturbations.
Of the more than 50 known satellites in the solar system, only three circle the inner planets. Earth has its Moon, and Mars has Deimos and Phobos. Very dark and heavily cratered, the Martian satellites resemble chondritic meteorites (fragile, low- density, stony-type meteorites that contain large amounts of carbon, water, and other volatile substances).
Most of the outer planets have large swarms of satellites attending them. In many cases the satellites are arranged in regular orbits suggestive of miniature solar systems. Jupiter has four giant satellites, each comparable in mass to Earth's Moon, called the Galilean satellites. The internal densities of these satellites are now reasonably well known. The inner two Galilean satellites, Io and Europa, are largely rocky in composition. On the other hand, the outer two giant satellites, Ganymede and Callisto, are of a lower density, suggesting a much higher ice content. Closer to Jupiter than these Galilean satellites is a much smaller one, Amalthea. These five satellites lie in the plane of Jupiter's equator and have very nearly circular orbits. Because of this ordered arrangement, they are called the regular satellites. Three additional regular satellites, all very small, were discovered by Voyager spacecraft.
Orbiting far from these satellites are two swarms of so-called irregular satellites, each of them only a few kilometers in radius. Eight of these bodies are so far known to exist, and there are indications of additional members. The satellites are called irregular because their orbits are inclined at substantial angles with respect to the plane of Jupiter's equator, and the orbits themselves are quite elliptical. Four of these satellites rotate in a direct (west to east) sense, but the others rotate in a retrograde (east to west) sense.
Saturn also has a system of regular satellites. One of these, Titan, is larger than the planet Mercury and is unique among the satellites in the solar system in having a substantial atmosphere. Four other satellites of Saturn have diameters of more than 1,000 km (600 mi), but the rest are much smaller. One of them, Phoebe, has a retrograde orbit. Studies of Voyager data have brought the total number above 20.
The five major satellites of Uranus are closely clustered in the plane of the Uranian equator, so that the plane of their orbits is also rotated 98 degrees to the plane of the ecliptic. The planet also has several smaller satellites.
The unusual system of Neptune contains one major satellite, Triton, whose mass is comparable to that of the Moon. The satellite moves in a circular but inclined retrograde orbit and has a very thin atmosphere. Neptune also has seven smaller, direct-rotating satellites.
A single moon of Pluto was discovered on June 22, 1978, and named Charon. Charon's diameter is 1,160 km (721 mi), which is about half the diameter of Pluto, making it the solar system's largest moon compared to its planet. Like the Earth and Moon, Pluto and Charon can be considered a double-planet system.
The major planets in the solar system are greatly outnumbered by the swarms of smaller bodies called minor planets, or asteroids, and by the even more numerous and smaller bodies known as meteoroids. Most of the asteroids exist within the relatively large gap lying between the orbits of Mars and Jupiter, whereas meteoroids are randomly distributed. A few large asteroids have radiuses of a few hundred kilometers, but most are much smaller. The smaller meteoroids produce meteor trails when they enter the Earth's atmosphere, and the larger ones form meteorite craters. A large number of the asteroids appear similar to the carbonaceous chondritic meteorites, and they are probably of relatively lower density than ordinary rocks. Nearly 2,000 have accurately determined orbits and have been given names. It is generally believed that most smaller asteroidal bodies have been created in collisions involving larger asteroids. Probably very many still smaller bodies exist that have not been detected by photographic surveys because of their size.
Many asteroids have orbits that cross the orbit of Mars. Some cross the orbit of the Earth or go even deeper into the inner solar system. These are called the Apollo asteroids. It has been suggested that many of the meteorites that strike the Earth are chips of the Apollo asteroids caused by collisions. These asteroids can also collide with the Earth or one of the other terrestrial planets. Some of the major craters that exist on these planets have more than likely been caused by such collisions, and chips from such collisions with Mars and the Moon have reached the Earth's surface as meteorites as well.
Other asteroidal bodies, called Trojan asteroids, have been observed both 60 degrees ahead of Jupiter in its orbit and 60 degrees behind. These positions of special orbital stability are called Lagrangian points. It is possible that similar swarms of dust particles are concentrated in the Moon's orbit, both 60 degrees ahead of the motion of the Moon and 60 degrees behind it (sometimes called the L4 and L5 Lagrangian points), but there has been no clear confirmation of this.
Until recently it was believed that minor planets were confined to the inner solar system. In 1977, however, an object was discovered called Chiron, a body some hundreds of kilometers in radius that orbits between Saturn and Uranus. This object has since been classified as a huge comet, as mentioned in the following discussion.
Comets are sometimes spectacular objects from the outer regions of the solar system, as far away as a substantial fraction of the distance to the nearest star. They appear to be typically a few kilometers in radius and are composed largely of icy substances. Their chemistry is, however, clearly complex. As a comet enters the inner solar system, it emits large amounts of volatile materials that are transformed by the energy of sunlight and of the solar wind into a variety of individual atoms, molecules, and ions, mostly of the common materials carbon, nitrogen, oxygen, and hydrogen, and combinations that include these. Many complex molecules have been detected by spectroscopic analysis of comet tails. Comets also emit a large number of tiny dust particles.
The Dutch astronomer Jan H. Oort recognized (about 1950) that most of the apparently fresh comets coming into the inner solar system started from initial distances beyond 50,000 astronomical units (the distance from the Earth to the Sun is defined as one astronomical unit). Furthermore, he recognized that the ease with which planetary perturbations can change the orbits of the comets meant that typical comets were unlikely to endure many orbital passages through the inner solar system. Because several comets are observed each year, this means that there must be a very large reservoir of them in the outer solar system. Oort suggested that a thick shell of cometary material surrounds the Sun very far beyond the orbits of Neptune and Pluto. This shell has not yet actually been observed.
The Dutch-American astronomer Gerard Kuiper further suggested that a much nearer ring of cometary material also exists, with its inner edge about 37 astronomical units from the Sun. Any disturbances of these clouds could send material plunging into the solar system to be observed as a comet. The object called Chiron is now conjectured to have had such an origin. Beginning in the 1990s, several objects thought to be members of the Kuiper belt have been sighted beyond Pluto.
The sun is also encircled by rings, or disks, of interplanetary dust. One such ring, lying in the zone between the orbital paths of Jupiter and Mars, has long been known and is the cause of zodiacal light. Another ring was found in the region of the asteroids, between Mars and Jupiter, by the Infrared Astronomy Satellite (IRAS) launched in 1983. Also detected in 1983, by a team of Japanese and Indonesian astronomers, was a third ring only two solar diameters away from the Sun. The dust in this ring is theorized to spiral slowly inward, because of differential absorption and reradiation of solar energy, until it is vaporized by the Sun. The resulting gases are driven back by the pressure of solar radiation.
For more than 300 years there has been serious scientific discussion of the processes and events that led to the formation of the solar system. For most of this time lack of knowledge about the physical conditions in the solar system prevented a rigorous approach to the problem. Explanations were especially sought for the regularity in the directions of rotation and orbit of objects in the solar system, the slow rotation of the Sun, and the Titius-Bode law, which states that the radiuses of the planetary orbits increase in a regular fashion throughout the solar system. In a similar fashion the radiuses of the orbits of the regular satellites of Jupiter, Saturn, and Uranus increase regularly. In modern times the slow rotation of the Sun has been explained as resulting from the deceleration of its angular motion through its magnetic interaction with the solar wind. Thus this feature in itself should not have been considered a constraint on theories of the origin of the solar system.
The numerous theories concerning the origin of the solar system that have been advanced during the last three centuries can be classified as either dualistic or monistic. One common feature of dualistic theories is that another star once passed close to the Sun, and tidal perturbations between the two stars drew out filaments of gas from which the planets condensed. Theories of this type encounter enormous difficulties in trying to account for modern information about the solar system, and they have generally been discarded. By contrast, monistic theories envisage a disk of gas and dust, called the primitive solar nebula, that formed around the Sun. Many of these theories speculate that the Sun and the planets formed together from the primeval solar nebula. This type of theory has dominated thinking about the origin of the solar system since World War II. A photograph taken in 1984 of a nearby star, Beta Pictoris, appears to show a solar system forming in this way from a disk of surrounding material.
The large amount of activity that has taken place in the last 20 years in the renewed exploration of the solar system has also provided a great impetus for renewed studies of the origin of the system. One important component of this research has been the detailed studies of the properties of meteorites that has been made possible by modern laboratory instrumentation. The distribution and abundance of the elements within different meteoritic mineral phases has provided much information on the physical conditions present at the time the solar system began to form. Recent discoveries of anomalies in the isotopic compositions of the elements in certain mineral phases in meteorites promise to give information about the local galactic interstellar environment that led to the formation of the solar system. Investigations of the properties of other planets has led to the new science of comparative planetology, in which the differences observed among the planets not only lead to a better understanding of the planets, but also pose precise new questions concerning the mechanisms by which the planets may have been formed.
Studies of the stars within our galaxy have shown that the age of our galaxy is much greater than the age of the solar system. Therefore, processes observed in the formation of stars within our galaxy today are likely to be found relevant to the formation of our solar system. Stars appear to form in groups or associations, as a result of the gravitational collapse of clouds of gas and dust in the interstellar medium. Modern monistic theories envisage the gas and dust in the primitive solar nebula to be the collapsed remnant of such materials.
There has been much discussion of how the planets might have formed from the primeval solar nebula. In recent years attention has focused on the possibility that two types of gravitational instabilities might have played an important role in this process. One type is a gravitational instability in the gas of the primitive solar nebula, from which there would be formed giant gaseous protoplanets whose evolution could lead, in the outer solar system, to the giant planets observed today. In the inner solar system, giant gaseous protoplanets could have formed rocky cores at their centers, which survived the stripping away of the gaseous envelopes caused by gravitational and thermal forces from the growing Sun.
The other form of gravitational instability involves the condensed materials in the solar nebula. Small dust particles that may have been present in the gas of the solar nebula could be expected to settle toward the midplane of the nebula if the gas were not subject to extensive turbulent churning. Gravitational instabilities acting on a thin dust layer might have formed bodies ranging from tens to hundreds of kilometers in radius. Collisions among these bodies may have played a major role in accumulations of material to form the planets.
Finally, the movement of the solar system as a whole through space is defined in terms of the celestial sphere, the imaginary sphere of the heavens that has Earth at its center. The solar system appears to be moving toward a point on the sphere at a velocity, relative to nearby stars, of about 20 km/sec (12 mi/sec). This point, the solar apex, lies in the constellation Hercules near the star Vega, at a right ascension of about 18 hours and a declination of about 30 degrees north.
