THE SOLAR SYSTEM
The achievements of astronomy and astrophysics are evident in the rapidly growing knowledge of the extraterrestrial environment, from the solar system to the most remote galaxies. The solar system, as it is known today, comprises the Sun and nine planetsÑMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and PlutoÑalong with numerous asteroids, comets, and smaller objects.
Planets, Asteroids, and Comets
Except for Mercury and Venus, each planet has from 1 to more than 20 natural satellites, including Pluto, whose moon was not discovered until 1978. The four gas-giant planets Jupiter, Saturn, Uranus, and Neptune all have ring systems made up of vast swarms of small icy and rocky fragments circling those planets in the plane of their equators. By far the most spectacular of these ring systems is that of Saturn. The others are less developed.
Between the orbits of Mars and Jupiter lies a belt containing thousands of minor planets, or asteroids. The orbits of most of the asteroids restrict them to the region between Mars and Jupiter, but exceptions of various kinds existÑincluding orbits that cross the Earth's orbit or lie still closer to the Sun.
Comets can attain distances 150,000 times greater than that from the Earth to the Sun. In 1950 the astronomer Jan Oort speculated that the solar system is surrounded at a vast distance by a cloud of comets, and the astronomer Gerard Kuiper later suggested a nearer cloud, as well. (Possible members of this latter cloud began to be sighted in the early 1990s.) The orbits of only a few comets are disturbed sufficiently to bring them near the Earth. Halley's comet, known since 240 ©, swings around the Sun once every 76 years and was visited by probes in 1985-86.
Flyby space probes involving most of the planets, and surface landings on the Moon, Venus, and Mars, have transformed planetary astronomy. No longer must observations be made at great distances. On-site measurement of numerous physical properties is now possible. In studying the planets, the astronomer must also enlist the aid of the chemist, the geologist, and the meteorologist. The Venera and Magellan missions to Venus and the Viking landers on Mars indicate that life as it is known on Earth does not exist on either planet, nor is life possible on the Moon. In spite of such great increases in knowledge, however, the probes and landings have raised as many questions as they have answered. The origin of the solar system, for example, remains unknown.
The Sun is a star with a surface temperature of 5,800 K and an interior temperature of about 15,000,000 K. Because the Sun is the nearest star and is easily observed, its chemical composition and surface activity have been intensely investigated. Among the surface features of the Sun are sunspots, prominences, and flares. It is now known that the maximum number of sunspots occurs approximately every 11 years, that their temperature is approximately 4,300 K, and that they are related to solar magnetic activity in a cycle taking about 22 years to complete. Studies of historical records have also shown long-scale variations in sunspot numbers.
Predictions based on theory indicate that energy-generating processes deep within the Sun and other stars should produce a certain number of chargeless, weightless particles called neutrinos. Efforts to detect solar neutrinos have thus far indicated a far lower rate of neutrino production than current theory seems to require, and revisions of theory may in time prove necessary. On the other hand, physicists and cosmologists are equally interested in the concept that at least some forms of neutrino have mass and undergo transformations inside the Sun.