Solar System

As the sun rushes through space at a speed of 150 miles (240 kilometers) per second, it takes many smaller bodies along with it. The sun and its smaller companions together are known as the solar system. Together, these bodies are making a revolution around the Milky Way that takes 225 million years. These other members of the solar system range in size from the giant planet Jupiter to microscopic particles called micrometeorites and even smaller particles—atoms and molecules of the interplanetary gas. Earth is one of the larger bodies of the solar system, though it is quite small when compared with the sun or Jupiter.

Astronomers do not know exactly how far out the solar system extends. When it is at its farthest point from the sun, some 4.5 billion miles (7.2 billion kilometers)—a point called the aphelion—Pluto is the most distant known planet. Many comets, however, have orbits that take them even farther out, up to several hundred times the distance of Pluto. Even at that distance the sun's gravitational force can pull the comet back. Some hundred billion comets form a tenuous halo in the outer parts of the solar system. Each is like a giant snowball, 1,000 to 10,000 feet (300 to 3,000 meters) in diameter.

The Solar System in Space

The solar system centers on the sun, one of a huge group of stars swirling around in a huge pinwheel-shaped mass called the Milky Way galaxy . There are more than 100 billion stars in the galaxy. Astronomical distances are so huge that astronomers often use the light-year as the unit of distance. One light-year is equal to the distance light travels in a year, or 5,880,000,000,000 miles (9,463,000,000,000 kilometers).
The galaxy is about 100,000 light-years across. The solar system's nearest neighbor in the galaxy is the triple star system Alpha and Proxima Centauri, which is 4.3 light-years away from our sun. Outside the Milky Way there are billions more galaxies stretching out through space. Evidence suggests that there are also at least two planets orbiting a pulsar outside the solar system and more than 50 planets orbiting extrasolar stars.
Astronomers cannot see to the end of the universe, which is the vast space that contains the galaxies and all other matter and energy that anyone knows to exist. However, galaxies and other objects have been detected that are thought to be between 5 billion and 15 billion light-years away from the sun. Compared with such distances, our solar system occupies a very tiny amount of space.

Parts of the Solar System

Even though the solar system is tiny compared with the distances to galaxies or even to nearby stars, it is enormous when compared with distances on Earth. It is also diverse, with conditions ranging from the hot, gaseous sun to the cold darkness of Pluto.

The Sun.

The sun is the central member of the solar system. Its gravitational force holds the other members in orbit and governs their motions. It far outweighs all other components of the solar system combined. In fact, the sun contains more than 99 percent of the mass of the entire solar system.
The sun is, however, only an average-sized star. If it were as far away from Earth as most stars are, it would look no larger or brighter than its neighbors. But since it is by far the nearest star and the only star whose surface details may be observed, it is also one of the major sources of information that scientists have about how stars behave
The sun provides nearly all the heat and light and other forms of energy necessary for life on Earth. In fact, the sun provides virtually all the energy of the solar system. Its gravitational attraction governs the motions (or kinetic energy) of the planets and other bodies. Radiation from its surface bathes the planets in all the electromagnetic radiation they receive, with some minor exceptions. These exceptions include the faint light from stars, the disintegration of radioactive materials on the planets, emissions of long-wave radiation by the planet Jupiter, and the radio waves and X rays from remote space.

The Planets.

The largest and most massive members of the solar system, after the sun, are the nine known planets. Even so, their combined mass is less than 0.2 percent of the total mass of the solar system.
The planets travel around the sun in regular orbits that are nearly circular in shape. Mercury's orbit lies nearest the sun. Next is Venus, then Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and finally Pluto. Pluto's orbit is the most elliptical of any of the planets. When Pluto is nearest to the sun (at perihelion), it is nearer to the sun than is Neptune.
The motions of the planets are similar in significant ways. All the planets orbit the sun in very nearly the same plane, which is the plane of the sun's equator. Pluto is the most divergent; its orbital plane makes an angle of about 17° to the plane of Earth's orbit around the sun. Mercury is next, making a 7° angle to Earth's orbital plane. The planes of all the other planetary orbits lie within 3.5° of Earth's orbit.The planets may be grouped according to their nearness to the sun or according to their physical properties. For example, Mercury and Venus, whose orbits lie between the sun and Earth, are called inferior planets. The planets whose orbits lie beyond Earth's orbit are the superior planets. Alternatively, the planets may be divided by location into inner planets (Mercury, Venus, Earth, and Mars) and outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto).
The reason for this division is that the four inner planets are similar in composition—mostly silicate rock and iron in varying proportions—while the four major outer planets, Jupiter to Neptune, are huge, not very dense, and have deep gaseous atmospheres. Since Jupiter is the outstanding representative of this group, these four planets are also known as the Jovian planets. These planets are composed mostly of hydrogen and helium in liquid and gaseous form. Pluto is an exception. It is much smaller than the other planets and is composed mostly of nitrogen.
Seven of the planets have smaller bodies—their natural satellites—circling them. Jupiter, Saturn, and Uranus have more than 20 moons each. Earth and Pluto each have one moon. These moons are so large with respect to the planets they orbit that each of the two planet-moon systems is sometimes considered a double planet system. Jupiter's Ganymede and Saturn's Titan are larger than the planet Mercury. The planetary rings around Jupiter, Saturn, Uranus, and Neptune are made up of innumerable tiny satellites.

Asteroids, or Minor Planets.

There are many smaller bodies that circle the sun in orbits that lie, for the most part, between Mars and Jupiter. These are the asteroids, sometimes called minor planets. Ceres is the largest, with a diameter of more than 600 miles (965 kilometers). Few asteroids have diameters larger than 100 miles (160 kilometers). Most are probably no larger than 1 mile (1.6 kilometers) across. It is estimated that millions of asteroids of boulder size exist in the solar system.
The total mass of all asteroids in the solar system adds up to only about three times that of Ceres. Many of the smaller asteroids are thought to be fragments caused by collisions between the larger asteroids. Some of the fragments may collide with Earth as meteorites. Scientists are then able to determine their compositions and ages. Some asteroids are thought to contain samples of the first materials to coalesce out of the great cloud from which the solar system itself is believed to have formed.

Comets.

At irregular intervals, a fuzzy spot of light, perhaps with a tail streaming away from it, appears in the sky. Such appearances of comets are spectacular but infrequent. Most comets that are detected are visible only with a telescope. Occasionally one can be seen with the unaided eye, and several times a century a comet will appear that can be seen even in the daytime.
Comets contain dust particles and ices of various substances that are gases on Earth. As a comet approaches the sun, the ices turn to vapor, forming a hazy, gaseous coma around the remaining swarm of solid particles, the nucleus. As the comet moves even closer to the sun, even more material is vaporized. Radiation and high-energy particles streaming out from the sun push this material away from the comet in a long tail that always points away from the sun.
Astronomers have been unable to determine the mass of a comet because a comet is not sufficiently massive to affect the orbits of objects it approaches. For example, one comet passed near Jupiter's satellites without affecting their orbital motions. The orbit of the comet, however, was shortened to about one quarter its original length.
Such evidence has led astronomers to conclude that comets have less than one billionth the mass of Earth, and probably most of them have even smaller masses. Comets contain icy nuclei a mile or more across. Gases and fine particles stream away from the nucleus as it disintegrates in the heat of the sun. Comets either dissolve completely, ending up as swarms of tiny particles, or ultimately appear as small asteroidal bodies, without tails, still orbiting around the sun.

Matter Between the Planets.

A great deal of matter—debris from comets, rock and metal fragments like miniature asteroids—is orbiting in interplanetary space. These fragments are meteoroids. Often a meteoroid will collide with Earth's atmosphere, where it is usually vaporized by heat from the friction against air molecules. The bright streak of light that occurs while the particle vaporizes is a meteor. Occasionally a large chunk of rock and metal survives the journey to the ground. Such remnants are meteorites
Particles even smaller than the meteoroids exist in the space between the planets. Tiny particles less than one ten-thousandth of an inch in diameter fall to Earth in a continuous rain. Some astronomers estimate that up to 100 tons of these micrometeorites land on Earth each day.
Micrometeorites are particles of interplanetary dust. This dust seems to be particularly dense along the plane of the planets' orbits around the sun, which is also the plane of the zodiacal constellations . On a clear night a faint glow is visible along the line of the zodiac, following the setting sun or preceding the rising sun. This glow can be almost as bright as the Milky Way. Scientists believe that it is caused by sunlight reflected from the interplanetary dust concentrated along the plane of the planets' orbits.
The sun itself contributes much material to the vast spaces between the planets. Along with the stream of radiation that continuously leaves its surface, the sun gives off electrically charged particles—electrons and nuclei of atoms. This flow is the solar wind, which spreads beyond the planets and escapes the solar system. The part of the solar wind that encounters Earth causes the auroras

Space Probes.

Human beings have added many pieces of matter to the solar system. These are the space probes and artificial satellites that have left Earth's atmosphere since the launching of Sputnik I in 1957. Artificial Earth satellites provide information on conditions in Earth's upper atmosphere and just beyond. Studies have been made of Earth's magnetic field, of solar radiation encountering Earth's atmosphere, and of the composition and density of micrometeorites and solar wind particles surrounding Earth's atmosphere.
Probes and manned space vehicles have landed on the moon . Unmanned craft have taken close-up photographs and gathered data at all the planets except Pluto. Unmanned probes have been sent to Venus, Mars, Jupiter, and Saturn. These probes were equipped with instruments to study the conditions (temperature, pressure, density, and chemical composition) of the atmospheres of those planets. The probes sent to Venus and Mars also surveyed the planets'surfaces.

Past and Future of the Solar System

Various theories have been proposed to describe the origin of the solar system. Because it happened so long ago, and because the distances involved are huge by Earth standards, it is extremely difficult to acquire sufficient evidence to test theories of the origin of the solar system. Information from space probes is the most important source of evidence.
The protoplanet theory, developed by Gerard P. Kuiper and Thomas Chrowder Chamberlin, suggests that the solar system was formed as a by-product of the formation of the sun itself. A great cloud of interstellar matter contracted and formed the solar nebula, which then developed a dense center, the protosun. As the outer part of the cloud rotated around the protosun, gravity caused dense clusterings to form within the solar nebula. These clusterings further contracted into slowly spinning protoplanets. As the protosun contracted because of the pull of gravity, it heated up and blew much of the rest of the cloud off into space. The protoplanets also lost their outer envelopes, but enough remained to contract into the present planets. On a much smaller scale, protomoons evolved into moons.The future of the solar system probably depends on the behavior of the sun. If current theories of stellar evolution are correct, the sun will have much the same size and temperature for 4 billion or 5 billion more years. By then, all of its hydrogen will have been burned. Other nuclear reactions involving helium and heavier atoms will begin. Then it will grow much brighter and larger, turning into a red giant and expanding beyond the orbit of Venus, perhaps even engulfing Earth. Much later, when all of its nuclear energy sources are exhausted, the sun will begin to cool down, evolving into a white-dwarf star. As its temperature decreases, it will become a dense nonluminous black dwarf of dead matter. Around it will orbit the remaining planets. They will have turned into frozen chunks, orbiting their shrunken star.