The solar system is comprised of the Sun and the celestial bodies which it holds in orbit around it by its gravitational force. The Sun itself makes up 99.8 percent of the solar system's mass, while Jupiter, the largest planet, contains only 0.1 percent. The other planets, their satellites and rings, asteroids, comets, dust and debris constitute the remainder.
Originally, the Earth was considered to be the center around which the Sun, Moon, and the five visible planets revolved. Comets, however, were not believed to be a part of the solar system, but rather a phenomenon of the Earth's atmosphere. This geocentric view of the solar system was also called the Ptolemaic view after the ancient Greek author, Ptolemy, who first published the idea in the second century A.D. Philosophers and scientists in Rome and later Europe adhered to this theory for the next thousand years, in large part because of its agreement with Christian doctrine and the unquestioning confidence scholars placed in classical Greek teachings.
The European Renaissance, however, set in motion a series of discoveries that radically changed humanity's view of the solar system. In the sixteenth century Polish astronomer Nicholas Copernicus found that if he placed the Sun at the center of the system rather than the Earth, the complex calculations required to predict the positions of planets would no longer be required. Copernicus published his discovery in 1507; however, most scientists ignored his findings until the early seventeenth century when Galileo accepted his heliocentric theory of the solar system. Although Galileo had incurred the wrath of the Church for his views, they were quickly adopted by the European scientific community.
In 1772 German astronomer Johann Elert Bode popularized a mathematical scheme--first devised by Johann Titius of Wittenberg--for determining the relative distance from the Sun to the planets. The scheme involved a ratio between the relative distances to the planets, but without knowing the actual distance from one planet to another, none of the distances could be determined. When William Herschel discovered Uranus in 1781, it was found to fit within Bode's law nearly exactly, but the "law" broke down and lost its importance after Neptune was discovered far from its expected distance in 1846. It is not surprising that the Titius-Bode law finally did break down, for it is nothing more than numerical coincidence, with no physical basis.
As scientists and philosophers began to investigate the organization of the solar system, they also began to speculate on its formation. Philosopher Immanuel Kant (1724-1804) proposed a theory which became known as the "nebular hypothesis." He suggested, in 1755, that as the Sun condensed from a great nebula of gas, rings were left behind that coalesced into the planets. French astronomer Pierre Laplace arrived at the same conclusion in 1796, yet no evidence exists that Laplace knew of Kant's earlier theory. The nebular hypothesis was widely accepted during the nineteenth century, but it was eventually supplanted by the hypothesis of geologist Thomas Chrowder Chamberlin (1843-1928) and astronomer Forest Ray Moulton (1872-1952) who proposed that the planets had formed by the accretion of particles (planetesimals).
In the 1940s, the nebular hypothesis was resurrected by Carl von Weizäcker (1912-), and it became more popular than ever. Today it is believed the solar system formed about 4.5 billion years ago from a rotating, hot cloud of gas and dust.
Until the development of sophisticated telescopes, space probes, and satellites, information concerning the nature and composition of the Sun and its planets was difficult to obtain. For example, it was known that the Sun was a sphere of mostly hydrogen gas (spectroscopic study of its light by William Huggins in the 1860s had shown that), but it was not until 1938 that Hans Bethe defined the nuclear process that powers the Sun and other stars.
Today, the planets and matter that revolve around the Sun can be classified into several different categories. Nearest the Sun are the solid terrestrial planets, Mercury, Venus, Earth, and Mars. Beyond Mars is the asteroid belt, a vast collection of space debris. The largest object in the belt, a minor planet named Ceres, is about 584 miles (940 km) in diameter, while the smallest bits are just particles of dust. Next follows the gaseous giant planets: Jupiter, Saturn, Uranus and Neptune, along with their systems of moons. Although Pluto is also a member of this outer group, it is more like a terrestrial planet. It has been suggested Pluto is a moon that escaped the gravitational pull of Neptune, but the discovery in 1978 that Pluto had a moon of its own has cast doubt on this theory.
Another component of the solar system is the comets. Danish astronomer Tycho Brahe, having observed a comet from 1577 to 1578, first dared to refute the theory that comets were "atmospheric exhalations" and placed comets among the other members of the solar system. During the early twentieth century, Dutch astronomer Jan Oort suggested that they originate in a great cloud, now known as the Oort cloud, far beyond the orbit of the gas giants. Comets are composed chiefly of ice, dust and gas. A passing star or planet could gravitationally perturb this cloud, hurling comets toward the inner solar system.
With the exception of Pluto, all the planets have been visited by satellite probes (although a mission to Pluto, the Pluto-Kuiper Express, is planned for launch in 2003 or 2004). As much as they have increased our knowledge about the solar system, there are still more questions than answers. For example, is there a large "Planet X" in orbit around the Sun beyond Pluto? A detailed survey of the sky by the Infrared Astronomical Satellite (IRAS) in 1983 failed to detect any such bodies, but speculation continues.
In the late 1980s, an entirely new class of solar system objects was discovered. Called Kuiper belt objects, they are small, probably asteroid-sized objects residing in the outer solar system. A few of them orbit in the vicinity of Saturn, but the great majority are located beyond the orbits of Neptune and Pluto. Because they are so far away, they are very faint and difficult to detect; as of 1998, only about 70 had been discovered. Astronomers suspect there may be a vast population of Kuiper Belt objects, and surveys are underway to detect them. These objects may yield important clues about the formation of the solar system, and represent the frontier of solar system research t the end of the twentieth century.
Questions have also arisen concerning other stars and the possibility that they are the center of their own solar systems. In 1943 astronomers under the direction of Peter Van de Kamp (1901-) observed irregularities in the behavior of a star which suggested the presence of an unseen companion eight times the mass of Jupiter. Another planet was found influencing the star Lalande 21185 in 1960. In 1963 evidence was found of a body only 1.5 times the mass of Jupiter circling Barnard's Star. In 1992, planets were discovered orbiting a pulsar, and the late 1990s saw a spate of discoveries of planets around more solar-like stars. It now seems that solar systems like our own may be a common component of the Universe.
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