Introduction

The solar system took shape 4,600,000,000 years ago, when it condensed within a large cloud of gas and dust. Gravitational attraction holds the planets in their elliptical orbits around the Sun. Besides the Earth, five major planets (Mercury, Venus, Mars, Jupiter, and Saturn) have been known from ancient times. Since then, only three others have been discovered: Uranus by accident in 1781, and Neptune and Pluto in 1846 and 1930, respectively, after deliberate searches.

The average Earth–Sun distance was originally defined as the astronomical unit (a.u.) and provides a convenient measure for distances within the solar system. The astronomical unit is now defined dynamically (using Kepler's third law), and it has the value 1.49597870 ´ 1013 centimetres. The semimajor axis of the Earth's orbit is 1 + (3.1 ´ 10-8) a.u. Mercury, at 0.39 a.u., is the closest planet to the Sun, while Pluto, at 39.5 a.u., is the farthest. The planes of the planetary orbits (other than that of Pluto) are all within a few degrees of the ecliptic, the plane that contains the Earth's orbit around the Sun. As viewed from far above the Earth's North Pole, all planets move in the same (counterclockwise) direction in their orbits.

All of the planets, apart from those closest to the Sun (Mercury and Venus), have satellites very diverse in appearance, size, and structure, as revealed through closeup observations from long-range space probes. Four planets—Jupiter, Saturn, Neptune, and Uranus—have rings consisting of small rocks and particles that are confined to disklike systems as they orbit their parent planets.

Most of the mass of the solar system is concentrated in the Sun, with its 1.99 ´ 1033 grams. Together, all of the planets amount to about 2.7 ´ 1030 grams, with Jupiter alone accounting for 71 percent of this. The solar system also contains a very large number of much smaller objects. In order of decreasing size, these are the asteroids (also called minor planets), comets, meteoroids, and dust particles.

The four terrestrial planets, Mercury, Venus, Earth, and Mars, along with the Moon, have average densities in the range 3.9–5.5 grams per cubic centimetre (g/cm3), setting them apart from the outer planets, whose densities are all close to 1 g/cm3, the density of water. The compositions of these two groups of planets must therefore be significantly different. This is probably attributable to the conditions that prevailed during the early development of the solar system. Planetary temperatures now range from about 500° C on Mercury's surface through the typical 20° C on Earth to -135° C on Jupiter and down to -230° C on Pluto.

The surfaces of the terrestrial planets and many satellites show extensive cratering produced by high-speed impacts. On Earth, with its large quantities of water and an active atmosphere, many of these cosmic footprints have eroded, but remnants of very large craters can be seen in satellite and aerial photographs of the terrestrial surface. On Mercury, Mars, and the Moon, the absence of water and any significant atmosphere has left the craters unchanged for billions of years, apart from disturbances produced by infrequent later impacts. Cratering on the largest scale seems to have ceased about 3,000,000,000 years ago, but there is clear evidence for a continued cosmic drizzle of small particles, with the larger objects churning (gardening) the lunar surface and the smallest producing microscopic impact pits in crystals in the lunar rocks.

Theories Of Origin

The origin of the Earth, Moon, and solar system as a whole is a problem that has not yet been settled in detail. The Sun probably formed by condensation of the central region of a large cloud of gas and dust, with the planets and other solar-system bodies forming soon after, their composition strongly influenced by the temperature and density gradients in the evolving solar nebula. Less-volatile materials could condense into solids relatively close to the Sun to form the terrestrial planets. The abundant, volatile lighter elements could condense only at much greater distances.

The origin of the planetary satellites is not settled. There is still the question as to the origin of the Moon, and professional opinion has been oscillating between theories that see its origin and condensation simultaneous with the formation of the Earth, to an explanation in terms of a large impact on the Earth resulting in the expulsion of material that subsequently formed the Moon. For the outer planets with their multiple satellites, many very small and quite unlike one another, the picture is even less clear. Some of the objects have icy surfaces, while others are heavily cratered, and at least one, Jupiter's Io, is volcanic. Some of the satellites may have formed along with their parent planets, and others may have formed elsewhere and been captured.

Lunar exploration

During the U.S. Apollo missions, a total sample weight of 381 kilograms was collected; 300 grams of lunar material also was returned by three unmanned Soviet Luna space vehicles. Less than 10 percent of the samples has so far been distributed for analysis, but planetary science has been revolutionized by these expeditions. A wide range of laboratory techniques has been employed on the lunar samples. The results of the analysis have enabled investigators to determine the composition and age of the lunar surface. Seismic techniques have made it possible to probe the lunar interior. In addition, a retroreflector left on the Moon's surface by Apollo astronauts returns a high-power laser beam emitted from the Earth, enabling researchers to monitor on a regular basis the Earth–Moon distance to an accuracy of a few centimetres. This experiment provides data that can be used in calculations of the dynamics of the Earth–Moon system.

Study Of Planets

Mercury is too hot to retain an atmosphere, but Venus' brilliant white appearance is the result of its being completely enveloped by thick clouds of carbon dioxide. Below the upper clouds it has a hostile atmosphere containing clouds of sulfuric acid droplets. The cloud cover shields the planet's surface from direct sunlight, but the energy that does filter through warms the surface, which then radiates at infrared wavelengths. The long waves of infrared radiation are trapped by the dense clouds, resulting in a very high surface temperature of almost 480° C. Radar can penetrate the thick Venusian clouds and has been used to map the planet's surface. The Martian atmosphere is very thin, only about 0.006 that of the Earth, and composed mostly of carbon dioxide (95 percent), with very little water vapour.

The outer planets have atmospheres composed largely of light gases. For example, hydrogen and helium, along with some methane and ammonia, have been detected on Jupiter. Each of the planets rotates on its axis, and nearly all of them rotate in the same (counterclockwise) direction, as viewed from above the ecliptic. The two exceptions are Venus, which rotates in the clockwise direction beneath its cloud cover, and Uranus, which has its rotational axis very nearly in the plane of the ecliptic.

Some of the planets have magnetic fields. The Earth's field extends outward until it is disturbed by the solar wind, an outward flow of protons and electrons from the Sun that carries a magnetic field along with it. Through processes not yet fully understood, protons and electrons from the solar wind and cosmic rays populate two doughnut-shaped regions called the Van Allen radiation belts, the inner of which extends from about 1,000 to 5,000 kilometres above the Earth's surface and the outer from roughly 15,000 to 25,000 kilometres. In these belts, trapped particles spiral along paths that take them around the Earth while bouncing back and forth between the Northern and Southern hemispheres. During periods of increased solar activity, these regions of trapped particles are disturbed, and some of the particles move down into the Earth's atmosphere where they collide with atoms and molecules to produce auroras.

Study Of Minor Bodies

Approximately 3,500 asteroids have now been identified. Most have orbits close to the ecliptic and move in the asteroid belt located between 2.3 and 3.3 a.u. from the Sun. Only about 250 of these objects are larger than 100 kilometres, and their total mass is thought to be roughly 1/2,000 that of the Earth. Comets are considered to come from a vast reservoir, the Oort cloud, which orbits the Sun at distances of 30,000 to 100,000 a.u.

More than 600 comets have so far been discovered. Most make only a single pass through the inner solar system, but some are deflected by Jupiter or Saturn into orbits that allow them to return at predictable times. Comet Halley is the best known of these periodic comets, with its next return predicted for AD 2060. About 30 comets have periods of less than 100 years. Comet masses have not been well determined, but most are thought to be less than 1018 grams, or 1,000,000,000 times smaller than the Earth.

Even smaller than comets are the meteoroids, lumps of stony material. Meteoroids vary in size from small rocks to large boulders weighing a ton or more. A few have orbits that bring them into the Earth's atmosphere and down to the ground as meteorites. These are classified into three broad groups: stony or chondrites (about 93 percent), iron (5.7 percent), and stony-iron (1.5 percent). Smaller meteoroids that enter the atmosphere may heat up sufficiently to vaporize and appear as meteors. Many, perhaps most, of the meteors occur in showers and follow orbits that seem to be identical with those of certain comets, thus pointing to a cometary origin. For example, each May the Earth crosses the orbit of Comet Halley, and the Eta Aquarid meteor shower becomes visible. Micrometeorites, the smallest meteoroidal particles, can be detected from Earth-orbiting satellites or be sufficiently slowed by atmospheric friction to be collected by specially equipped aircraft flying in the stratosphere and returned for laboratory inspection.