Solar System

I

Introduction

Solar System, the system principally consisting of the Sun; the eight planets and their satellites; the dwarf planets and their satellites; the asteroids, meteoroids, Kuiper Belt objects, and comets; and interplanetary dust and gas. The dimensions of this system are specified in terms of the mean distance from the Earth to the Sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant planet, Neptune, has an orbit at a mean distance of 30 AU from the Sun. The boundary between the solar system and interstellar space—called the heliopause—is estimated to occur at between 106 and 160 AU (see Heliosphere). The comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more. The solar system was the only planetary system known to exist until 1999. In the 1980s a number of relatively nearby stars were found to be encircled by swarms of orbiting material of indeterminate size (see Vega) or to be accompanied by objects suspected to be brown dwarfs. In 1999, four years after the first confirmed detection of an extrasolar planet, two teams of astronomers detected the first extrasolar multiple-planet system, comprising at least three gas-giant planets around the Sun-like star Upsilon Andromidae, some 44 light years from Earth. It is believed that planetary systems of some sort are numerous throughout the universe. See Astronomy.

II

The Sun and the Solar Wind

The Sun is a typical star, of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun's hot, dense interior (see Nuclear Energy). Although this nuclear fusion is converting 600 million tonnes of hydrogen each second, the Sun is so massive (2 × 10²⁷ tonnes) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on the Earth.

For all the Sun's steadiness, it is an extremely active star. On its surface dark sunspots bounded by intense magnetic fields come and go in 11-year cycles; sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on the Earth; and a continuous stream of protons, electrons, and ions leaves the Sun and moves out through the solar system, spiralling with the Sun's rotation. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back from the Moon's surface by Apollo spacecraft. See Space Exploration.

III

The Planets

There are eight planets. They are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice.

Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Venus has a carbon dioxide atmosphere 90 times thicker than that of the Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet—about 477° C (890° F). The Earth is the only planet with abundant liquid water and life. Strong evidence exists that Mars once had water on its surface (and still has below the surface, in the form of ice), but now its carbon dioxide (CO₂) atmosphere is so thin that the planet is predominantly dry and cold, with polar caps of solid carbon dioxide, or dry ice. Jupiter is the largest of the planets. Its hydrogen and helium atmosphere contains pastel-coloured clouds, and its immense magnetosphere, rings, and satellites make it a planetary system unto itself. Saturn rivals Jupiter, with a much more intricate ring structure and more satellites, including one with a dense atmosphere—Titan. Uranus and Neptune are deficient in hydrogen compared with the two giants; Uranus, also ringed, has the distinction of rotating at 98° to the plane of its orbit.

IV

Other Constituents

The dwarf planets are round or nearly round bodies in cleared orbits around the Sun that are not satellites of another body and are too small to be considered planets but too large to be considered asteroids or objects of the Kuiper Belt, a ring of ice and rock objects orbiting the Sun beyond Neptune to a distance of about 50 AU. The designation was introduced in August 2006 by the International Astronomical Union (IAU), and the first three bodies to be named dwarf planets were Pluto, Ceres, and Eris. The distant Pluto, which is similar to the larger, icy satellites of Jupiter or Saturn, and which has an eccentric orbit so elliptical that it is often closer than Neptune to the Sun, had previously been considered the ninth and outermost planet of the solar system, but this changed with the discovery in 2005 of Eris, an object beyond the Kuiper Belt in an area of icy bodies known as the “scattered disc”. Not only does Eris have an orbit 8.59 billion km (5.33 billion mi) further out, on average, than that of Pluto, but its diameter is also greater than that of Pluto. At the same time that the IAU voted to downgrade Pluto (and not declare Eris to be the tenth planet), it also re-designated Ceres, in orbit between Mars and Jupiter, as a dwarf planet. Until then Ceres has been considered the largest of the asteroids since its discovery in 1801.

The asteroids are small rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from Pallas, which has a diameter of 480 km (300 mi), to microscopic grains. Some asteroids are perturbed into eccentric orbits that can bring them closer to the Sun. Smaller bodies orbiting the Sun are called meteoroids. Some collide with the Earth and appear in the night sky as streaks of light, known as meteors. Recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much about primitive conditions in our solar system. The surfaces of Mercury, Mars, Venus, and several satellites of the planets (including the Earth's moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On the Earth that record has been eroded away, except for a few recent impact craters.

Some interplanetary dust may also come from comets, which are basically aggregates of dust and frozen gases about 5 to 10 km (3 to 6 mi) in diameter. Comets orbit the Sun from within the Kuiper Belt and, principally, the Oort Cloud. Many comets orbit the Sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system. As comets approach the Sun, they release their dust and gases to form a spectacular coma and tail. Under the influence of Jupiter's strong gravitational field, comets can sometimes adopt much smaller orbits. The most famous of these is Halley's comet, which returns to the inner solar system at 75-year periods. Its most recent return was in 1986. In July 1994 fragments of Comet Shoemaker-Levy 9 bombarded Jupiter's dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the tremendous kinetic energy of the fragments was converted into heat through massive explosions, some resulting in fireballs larger than the Earth.

The surfaces of the icy satellites of the outer planets are scarred by impacts from comet nuclei. Indeed, the asteroidal object Chiron, with an orbit between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly, some of the asteroids that cross Earth's orbit may be the rocky remains of burned-out comets.

The Sun has been found to be encircled by three rings of interplanetary dust. One of them, between Jupiter and Mars, has long been known as the cause of zodiacal light. The other two rings, one lying only two solar widths away from the Sun, the other occurring in the region of the asteroids, were discovered in 1983.