<text>Beatty, J. Kelly, et al., eds., The New Solar System, 2d ed. (1982); Dermott, S. F., The Origin of the Solar System (1978); Frazier, Kendrick, Solar System (1985); Hardy, D. A., Atlas of the Solar System (1982); Hartmann, W. K., Moons and Planets, 2d ed. (1983); Jones, B. W., and Keynes, Milton, The Solar System (1984); Moore, Patrick, et al., The Atlas of the Solar System (1983); Morrison, David, and Owen, Tobias, The Planetary System (1988); O'Leary, Brian, and Beatty, J. Kelly, eds., The New Solar System, 2d ed. (1982); Smoluchowski, Roman, et al., eds., The Galaxy and the Solar System (1987); Time-Life Book Editors, The Far Planets (1989) and The Near Planets (1989).</text>
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<text><span class="style1">inally, the movement of the solar system as a whole through space is defined in terms of the </span><span class="style3"><a href="#" class="group">CELESTIAL SPHERE</a></span><span class="style1">, 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 the velocity, relative to nearby stars, of about 20 km/sec (12 mi/sec). This point, called 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.</span></text>
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<text>CELESTIAL SPHERE</text>
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<text>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 radii of the planetary orbits increase in a regular fashion throughout the solar system. In a similar fashion, the radii of the orbits of the regular satellites of Jupiter, Saturn, and Uranus increase in a regular manner. 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 outflowing solar wind, so that this feature should not have been considered a constraint on theories of the origin of the solar system. The many 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. A 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. 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 a fragment of an interstellar cloud. 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 a giant gaseous protoplanet. From the evolution of such protoplanets there could arise, in the outer solar system, the giant planets that are observed today. In the inner solar system, the possibility exists that giant gaseous protoplanets 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. It must be stressed that all theories of the origin of the solar system currently being formulated respond to and are limited by the rapid accumulation of facts about planetary bodies within the solar system. Because of the rapid rate of progress in such studies, it is generally recognized that such theories are preliminary and simplified, so that ideas and theories in this area of research can be expected to continue to evolve rapidly.</text>
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<text><span class="style1">he sun is also encircled by rings, or disks, of interplanetary dust. One, lying in the zone between the orbital paths of Jupiter and Mars, has long been known and is the cause of </span><span class="style3"><a href="#" class="group">ZODIACAL LIGHT</a></span><span class="style1">. 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, is a third ring only two solar diameters away from the Sun. The dust in this ring is theorized to spiral slowly inward from the outer solar system, due to differential aborption and reradiation of solar energy, until it is vaporized by the Sun and the resulting gases are driven back by the pressure of solar radiation.</span></text>
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<text>ZODIACAL LIGHT</text>
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<text><span class="style1">omets 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 </span><span class="style3"><a href="#" class="group">JAN H. OORT</a></span><span class="style1"> 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 about 1,000 times farther out than the orbits of Neptune and Pluto. The Dutch-American astronomer Gerard Kuiper further suggested a nearer ring of cometary material in the plane of the solar system. Any disturbance of these clouds can send some material plunging into the solar system to be observed as a comet.</span></text>
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<text>JAN H. OORT</text>
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<text>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 radii 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 are probably of relatively lower density than ordinary rocks. Nearly 2,000 of the asteroids have accurately determined orbits and have been given names. It is generally believed that the smaller asteroidal bodies have been created in collisions involving larger ones, so that there probably exist many small bodies that have not been detected by photographic surveys. Many asteroids have orbits that cross the orbit of Mars; some cross the orbit of the Earth or go even further 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 collide with the Earth or one of the other terrestrial planets, and some of the major craters that exist on these planets have probably been caused by such collisions. 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 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. These are sometimes called the L4 and L5 Lagrangian points. Although there has not been clear confirmation of the presence of these dust swarms, they may exist in a manner similar to that of the Trojan asteroids with respect to Jupiter. There have been suggestions that future human colonies in space might be established at one of these Lagrangian points. 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.</text>
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<text><span class="style1">f the more than 50 known satellites in the solar system, only three circle the inner planets. Earth has its abnormally massive Moon, and Mars has two tiny satellites, </span><span class="style3"><a href="#" class="group">DEIMOS</a></span><span class="style1"> and </span><span class="style3"><a href="#" class="group">PHOBOS</a></span><span class="style1">. Very dark and heavily cratered, the Martian satellites may resemble the 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 that are suggestive of miniature solar systems. Jupiter has four giant satellites, each comparable in mass to Earth's Moon, called the Galilean satellites for their discoverer. The internal densities of these satellites are now reasonably well known as the result of measurements made by the flyby Pioneer spacecraft. The innermost two Galilean satellites, </span><span class="style3"><a href="#" class="group">IO</a></span><span class="style1"> and </span><span class="style3"><a href="#" class="group">EUROPA</a></span><span class="style1">, are largely rocky in composition. On the other hand, the outer two giant satellites, </span><span class="style3"><a href="#" class="group">GANYMEDE</a></span><span class="style1"> and </span><span class="style3"><a href="#" class="group">CALLISTO</a></span><span class="style1">, are of a lower density, suggestive of 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 further, very small satellites were discovered by Voyager spacecraft. Orbiting far from these regular satellites are the irregular satellites, in two swarms of much smaller bodies, each only a few kilometers in radius. Eight of these bodies are so far known to exist, and there are indications of additional members. The orbits of these satellites are inclined at substantial angles with respect to the plane of Jupiter's equator, and the orbits themselves are quite elliptical. Four of these small 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, </span><span class="style3"><a href="#" class="group">TITAN</a></span><span class="style1">, is larger than the planet Mercury and is unique among the satellites in the solar system in that it has 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 satellites of Uranus visible to Earth-based telescopes 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. These satellites are relatively small, comparable in size to the lesser regular satellites of Saturn. Several much smaller satellites were discovered by Voyager 2. The unusual system of Neptune contains one major satellite, </span><span class="style3"><a href="#" class="group">TRITON</a></span><span class="style1">--whose mass is not exactly known but may be comparable to that of the Moon--which moves in a circular but inclined retrograde orbit. Neptune also has a smaller, direct-rotating satellite. A single moon of Pluto was discovered on June 22, 1978, and named Charon. It appears to have about 5-10 percent of the mass of Pluto, meaning that it is the solar system's largest moon compared to its planet.</span></text>
<text><span class="style1">he 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 PlanetsThe 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 </span><span class="style3"><a href="#" class="group">EARTH</a></span><span class="style1"> 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 indicate that the Earth has passed through ice ages in the past, but it is not known whether some unknown variability in the Sun, the great complexity of the Earth's atmospheric weather system, or some other factor has been responsible for these (see also </span><span class="style3"><a href="#" class="group">MILANKOVITCH THEORY</a></span><span class="style1">). 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 </span><span class="style3"><a href="#" class="group">MOON</a></span><span class="style1"> 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 </span><span class="style3"><a href="#" class="group">VENUS</a></span><span class="style1">, 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 </span><span class="style3"><a href="#" class="group">MARS</a></span><span class="style1">, 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 to be a possible abode for other forms of life within the solar system, and apparent seasonal differences in its appearance were attributed to the presence of life. Experiments performed by the Viking spacecraft landers, however, found no evidence for the presence of Martian life forms, however, and it has been found that the Martian surface apparently contains oxidizing agents highly incompatible with any form of organic life. The planet closest to the Sun is </span><span class="style3"><a href="#" class="group">MERCURY</a></span><span class="style1">, a planet whose mass is half as great as that of Mars. Mercury has only a trace atmosphere, consisting of such elements as helium, sodium, and hydrogen. Its surface is heavily cratered. The planet possesses an interesting resonance with its orbital motion, presenting first one face and then the other during its closest approaches to the Sun. The Outer PlanetsThe terrestrial planets just described have in common a rocky composition whose major constituents have high boiling points and are therefore described as refractory. It is believed that the entire solar system, including the Sun, 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 atoms such as oxygen, silicon, and iron. The Sun's composition, which is about three-quarters hydrogen and nearly one-quarter helium, with less than two percent heavy elements, 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, cold, outer planets were able to retain their light gases. The result is that the outer planets became far more massive than the terrestrial planets and were able to hold very extensive atmospheres of light gases such as hydrogen, as well as light, icy substances such as water, ammonia, and methane. 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 </span><span class="style3"><a href="#" class="group">JUPITER</a></span><span class="style1">. 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 unknown but 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. Infrared measurements from high-flying aircraft on the Earth and from flyby spacecraft have determined that Jupiter radiates into space about twice as much energy as it absorbs from the Sun; the additional heat emerges from the interior of the planet. Spacecraft also revealed that Jupiter is ringed. The next planet outward from Jupiter is the strikingly ringed </span><span class="style3"><a href="#" class="group">SATURN</a></span><span class="style1">, 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 </span><span class="style3"><a href="#" class="group">URANUS</a></span><span class="style1"> and </span><span class="style3"><a href="#" class="group">NEPTUNE</a></span><span class="style1">, 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. Although Neptune receives comparatively little energy from the Sun, it has an active atmosphere and apparently has some form of internal energy source. The rotation period of Uranus is a little more than 17 hour; 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. </span><span class="style3"><a href="#" class="group">PLUTO</a></span><span class="style1"> is a planet whose characteristics were largely unknown until the discovery of its moon in 1978. This discovery suggested a value for Pluto's diameter of about 3,000 km (1,900 mi), and for its moon of about 1,200 km (750 mi). The density of the planet is about the same as that of water, so that it 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. From 1979 until 1999, for example, Pluto will be within Neptune's orbit. 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 occur. Astronomers have also observed perturbations in the orbits of Uranus and Neptune. Pluto is too small to cause these irregularities, and the Pioneer spacecraft have detected no other sources of gravity. Some scientists hypothesize that a tenth planet, called "Planet X," is responsible for these perturbations.</span></text>
<text>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.</text>
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<text><span class="style1">ince 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 (see </span><span class="style3"><a href="#" class="group">ASTRONOMY, HISTORY OF</a></span><span class="style1">). In his work of 1543, Copernicus proposed that planetary motions 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 the 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 </span><span class="style3"><a href="#" class="group">KEPLER'S LAWS</a></span><span class="style1">. Kepler's labors laid the groundwork for Sir Isaac Newton's law of </span><span class="style3"><a href="#" class="group">GRAVITATION</a></span><span class="style1"> (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 residual discrepancies in the apparent positions of Uranus and Neptune led to predictions of the presence of still another planet, and in 1930, Clyde Tombaugh discovered the planet Pluto close to one of the areas of prediction. Pluto's mass, however, is so small that the discovery is now considered to have been an accident resulting from intense scrutiny of that part of the sky to which the predictions had called attention. Yet another planet may remain to be discovered. Galileo 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 (see </span><span class="style3"><a href="#" class="group">ASTRONOMY AND ASTROPHYSICS</a></span><span class="style1">). 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 </span><span class="style3"><a href="#" class="group">RADIO ASTRONOMY</a></span><span class="style1"> and </span><span class="style3"><a href="#" class="group">RADAR ASTRONOMY</a></span><span class="style1">. 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 ushered in an age that transformed solar-system studies. During the 1960s, 1970s, and 1980s spacecraft accomplished flyby, orbiting, or landing missions to many of the planets. At the present time the reconnaissance of the planets in the solar system has been accomplished for Mercury through Neptune. The U.S. </span><span class="style3"><a href="#" class="group">MARINER</a></span><span class="style1"> spacecraft have provided a good model of the atmosphere of Venus, and the Soviet </span><span class="style3"><a href="#" class="group">VENERA</a></span><span class="style1"> spacecraft have returned pictures from the surface of that planet. Mariner and </span><span class="style3"><a href="#" class="group">VIKING</a></span><span class="style1"> (U.S.) spacecraft have extensively photographed Mars from orbit, and the Viking landers have carried out important initial measurements of surface properties. U.S. </span><span class="style3"><a href="#" class="group">PIONEER</a></span><span class="style1"> and </span><span class="style3"><a href="#" class="group">VOYAGER</a></span><span class="style1"> probes have returned data and images from the outer planets Jupiter, Saturn, Uranus, and Neptune. The investigation of the Moon has progressed through the stages of flybys, orbiters, and landers both of the manned variety (U.S. Apollo) and the unmanned variety (U.S. </span><span class="style3"><a href="#" class="group">RANGER</a></span><span class="style1">, </span><span class="style3"><a href="#" class="group">SURVEYOR</a></span><span class="style1">, and </span><span class="style3"><a href="#" class="group">LUNAR ORBITER</a></span><span class="style1">, and Soviet </span><span class="style3"><a href="#" class="group">LUNA</a></span><span class="style1">). The success achieved in bringing to the Earth samples from several different lunar landing sites has made possible a continuing series of laboratory investigations and further intensive study of Earth's satellite (see </span><span class="style3"><a href="#" class="group">SPACE EXPLORATION</a></span><span class="style1">).</span></text>
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<text>ASTRONOMY AND ASTROPHYSICSASTRONOMY, HISTORY OFGRAVITATIONKEPLER'S LAWSLUNALUNAR ORBITERMARINERPIONEERRADAR ASTRONOMYRADIO ASTRONOMYRANGERSPACE EXPLORATIONSURVEYORVENERAVIKINGVOYAGER</text>
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<text><span class="style1">he solar system is the group of celestial bodies, including the Earth, orbiting around and gravitationally bound by the star known as the </span><span class="style3"><a href="#" class="group">SUN</a></span><span class="style1">, one of at least a hundred billion stars in our galaxy. The Sun's retinue includes nine </span><span class="style3"><a href="#" class="group">PLANETS</a></span><span class="style1">, at least 54 </span><span class="style3"><a href="#" class="group">SATELLITES</a></span><span class="style1">, more than 1,000 observed </span><span class="style3"><a href="#" class="group">COMETS</a></span><span class="style1">, and thousands of lesser bodies known as minor planets (</span><span class="style3"><a href="#" class="group">ASTEROIDS</a></span><span class="style1">) and meteoroids (see </span><span class="style3"><a href="#" class="group">METEOR AND METEORITE</a></span><span class="style1">). All of these bodies are immersed in a tenuous sea of fragile and rocky interplanetary dust particles, perhaps ejected from comets at the time of their passage through the inner solar system or resulting from minor planet collisions. The Sun is the only star known to be accompanied by such an extensive planetary system. A few nearby stars are now known to be encircled by swarms of particles of undetermined size, however (see </span><span class="style3"><a href="#" class="group">PLANETARY SYSTEMS</a></span><span class="style1">), and evidence indicates that a number of stars are accompanied by giant planetlike objects (see </span><span class="style3"><a href="#" class="group">BROWN DWARF</a></span><span class="style1">). Thus the possibility of a universe filled with many solar systems remains strong, though as yet unproved.</span></text>
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<text>ASTEROIDSBROWN DWARFCOMETSMETEOR AND METEORITEPLANETARY SYSTEMSPLANETSSATELLITESSUN</text>
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<text>Solar System HyperTextBook 2.0 Level 1</text>
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<text>Solar SystemHISTORY OF SOLAR SYSTEM STUDIESTHE SUNTHE PLANETSTHE SATELLITESASTEROIDS AND METEOROIDSCOMETSDUST RINGSORIGIN OF THE SOLAR SYSTEMSOLAR APEXBibliography</text>
 (January 1993).iso/Files/Education/S/Solar System HTB 2.0 L1.cpt/Solar System HTB 2.0 L1/BMAP_7917.png){kind=link}
 (January 1993).iso/Files/Education/S/Solar System HTB 2.0 L1.cpt/Solar System HTB 2.0 L1/PATs.png){kind=link}