SOLAR SYSTEM

assemblage consisting of the Sunan average star in the Milky Way Galaxyand those bodies orbiting around it: 9 major planets, at least 60 planetary satellites, countless asteroids and comets, and the vast interplanetary medium. Four of the major planets have ring systems, and seven have one or more satellites. The several thousand minor planets, or asteroids, are predominantly in orbits between Mars and Jupiter, while most of the several billion comets travel around the Sun in a spherical shell approximately 50,000 times farther out than the Earth. The interplanetary mediuman exceedingly tenuous plasma (ionized gas) laced with concentrations of dustextends outward from the Sun to great distances. Observations of the motions of the Sun, the Moon, and the visible planets by early investigators gave rise to the science of astronomy. These objects are still studied today in an attempt to understand their origin and evolution, which can aid in determining whether there may be other similar systems among the millions of stars in the galaxy. assemblage of planets, natural satellites, asteroids, comets, meteoroids, and interplanetary dust and gas under the gravitational control of the Sun. The Sun contains more than 99 percent of the mass of the entire solar system; most of the remaining mass is distributed among the nine major planets. According to the prevailing nebular theory of the origin of the solar system, the Sun and the planets formed simultaneously from the gravitational collapse of a large cloud of dust and gas some 4.6 billion years ago. The centrally located Sun accreted most of the infalling gas, growing massive enough to trigger energy-producing fusion reactions in its hot, compressed core. The Sun thus developed into a star. None of its encircling companions gained sufficient mass to generate a significant amount of light and heat, and so became dark, comparatively cool planets. The planets can be divided into two principal groupsterrestrial and Jovian. The terrestrial group is made up of the four innermost planets: Mercury, Venus, Earth, and Mars. The Jovian planets are the outer gaseous giants: Jupiter, Saturn, Uranus, and Neptune. The outermost planet of the solar system, Pluto, is distinct from the members of either group, having a makeup more characteristic of an icy satellite than a planet proper. The terrestrial planets lie between 0.39 and 1.52 astronomical units (AU) from the Sun. (One astronomical unit equals 149,598,000 km, or 92,955,800 miles, and represents the mean distance between the Earth and the Sun.) All four planets are small (less than 13,000 km in diameter), rocky, and have thin or negligible atmospheres. It is thought that the heat from the young Sun prevented the abundant gases in the original solar nebula from condensing in these planets. These bodies thus were not able to retain extensive gassy envelopes, and so they remained bare, rocky cores whose atmospheres (if any) resulted from internal degassing. Between 5.2 and 30.4 AU, the gas in the solar nebula was cool enough for several abundant gases to condense, enabling the embryonic outer planets to grow very massive and retain huge atmospheres of light gases, predominantly hydrogen and helium. Jupiter, Saturn, Uranus, and Neptune all appear to be similar in structure, and none of them has an accessible surface. Of the four gaseous giants, Jupiter is the largest, with a diameter of nearly 143,000 km. Except for Neptune, each of these planets is surrounded by at least one full ring and by a complex satellite system. With a diameter of only about 2,300 km, Pluto is by far the smallest major planet. There may be planets beyond Pluto. The existence of such bodies may be responsible for the still-unexplained deviations in the orbits of Uranus and Neptune. The inner and outer planets are separated by a gap between the orbits of Mars and Jupiter that is filled by tens of thousands of minor planets, known as asteroids. Although some of them, such as Ceres and Pallas, are several hundred kilometres in diameter and spherical in shape, the majority are irregular chunks of rock or metal only a few metres across. The asteroids are probably fragments of a number of slightly larger bodies that failed to accrete into a single planet because of the influence of Jupiter's strong gravitational pull. Some asteroids have very elongated orbits that take them well outside the asteroid belt and close to the other planets. Asteroids or asteroidal fragments can thus collide with the Earth (as well as with various other planetary bodies), and excessive bombardment by large objects of this type in the past has given rise to extensive cratering on the surfaces of the inner planets and their satellites. By now, however, most of the larger asteroidal bodies have disappeared; some of the smaller fragments enter the Earth's atmosphere to become bright meteors. If such an asteroidal fragment survives the fiery entry and reaches the surface, it is called a meteorite. Most meteoroids are much smaller than those originating in the asteroid belt. They are fragile, low-density dust particles that come from comets worn down by repeated passes through the inner solar system. Comets are thought to originate in a huge swarm of cosmic debris known as the Oort cloud, the outer boundary of which is estimated to be 50,000 AU or more from the Sun. When the orbit of one of these comet nuclei is disturbed by the gravity of a nearby star, the nucleus may travel out of the Oort cloud and into the inner solar system in an elongated orbit. As the incipient comet approaches the Sun, its outer layer of icy matter melts and vaporizes, releasing gases and dust that form a spherical, diffuse cloud (the coma) around the nucleus. Eventually, some of the gas molecules and dust particles are ejected from the coma, forming one or two tails millions of kilometres in length. Other components of the solar system include such phenomena as the solar wind, a stream of charged atomic particles that continually flows from the outermost region of the Sun. Protons and electrons in the solar wind have been detected by space probes out as far as the orbit of Neptune. Additional reading General works The basic orbital and physical data about planets, satellites, and selected comets and asteroids are revised each year in The Astronomical Almanac (annual). The Observer's Handbook (annual), published by the Royal Astronomical Society of Canada, provides excellent information for observing these objects with the naked eye or small telescopes. Overviews of the solar system include J. Kelly Beatty and Andrew Chaikin (eds.), The New Solar System, 3rd ed. (1990); and David Morrison and Tobias Owen, The Planetary System (1988). The natural satellites in the solar system are examined in Joseph A. Burns and Mildred Shapley Matthews (eds.), Satellites (1986), a collection of papers on aspects of satellites in general and on particular satellites or groups of satellites. William B. Hubbard, Planetary Interiors (1984), discusses the physics and chemistry of the interiors of all planets except Pluto and the Jovian satellites using much mathematics. Bruce C. Murray, Michael C. Malin, and Ronald Greeley, Earthlike Planets (1981), analyzes the surfaces of Mercury, Venus, the Earth, the Moon, and Mars. International reports of research on asteroids, comets, meteorites, planets, satellites, and other objects of the solar system can be found in The Astrophysical Journal (semimonthly); The Astronomical Journal (monthly); Astronomy and Astrophysics (semimonthly); Icarus (monthly), a journal of solar system studies; JGR: Journal of Geophysical Research (monthly); and Annual Review of Earth and Planetary Sciences. Tobias Chant Owen The origin of the solar system Volumes of original technical articles by different authors on the topic are Richard Greenberg, Andr Brahic, and Mildred Shapley Matthews (ed.), Planetary Rings (1984); David C. Black and Mildred Shapley Matthews (eds.), Protostars & Planets II (1985); John F. Kerridge and Mildred Shapley Matthews (eds.), Meteorites and the Early Solar System (1988); and S.K. Atreya, J.B. Pollack, and Mildred Shapley Matthews (eds.), Origin and Evolution of Planetary and Satellite Atmospheres (1989). The formation of the inner planets has been extensively studied by George W. Wetherill, Formation of the Earth, Annual Review of Earth and Planetary Sciences, 18:205256 (1990), a review of his and others' work in the field. An excellent collection of papers on the general subject of solar system origin appears in H.A. Weaver and L. Danly (eds.), The Formation and Evolution of Planetary Systems (1989). Tobias Chant Owen