The solar system is the group of celestial bodies, including the Earth, orbiting around and gravitationally bound by the star known as the Sun, one of at least a hundred billion stars in our galaxy. The Sun's retinue includes 9 planets, about 50 satellites, more than 1,000 observed comets, and thousands of lesser bodies known as minor planets ( asteroids) and meteoroids. 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. Even if only because other stars are too distant for such a size range of objects to be detectable as yet, the Sun is thus far the only star known to be accompanied by such an extensive planetary system that includes objects habitable by life forms. Some relatively nearby stars, however, are accompanied by one or more planets much larger than or in the size range of Jupiter, while others are known to be encircled by swarms of particles of undetermined size. Thus the possibility of a universe filled with many solar systems is strong, although as yet it is not clearly proved.

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 A.D., 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 that 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 Neptune 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. Orbiting craft, including those of the U.S. Viking program, have extensively photographed Mars from orbit, and 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. Ranger, Surveyor, 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.