NASA - National Aeronautics and Space Administration

An orbiting solar telescope known as the Solar and Heliospheric Observatory (SOHO) studies the sun's interior, its atmosphere, and the solar wind, a stream of electrically charged particles that flow from the sun's surface. The European Space Agency launched the telescope in 1995. Image credit: NASA/ESA/Solar & Heliospheric Observatory

A solar system is a group of heavenly bodies consisting of a star and the planets and other objects orbiting around it. We are most familiar with our own solar system, which includes Earth, eight other major planets, and the sun. Our solar system also includes many smaller objects that revolve around the sun, such as asteroids, meteoroids, and comets; and a thin cloud of gas and dust known as the interplanetary medium. More than 100 moons, also called satellites, orbit the planets.

Besides the sun, Earth, and Earth's moon, many objects in our solar system are visible to the unaided eye. These objects include the planets Mercury, Venus, Mars, Jupiter, and Saturn; the brightest asteroids; and occasional comets and meteors. Many more objects in the solar system can be seen with telescopes.

Since the 1990's, astronomers have discovered many planets orbiting distant stars, though the planets cannot be seen directly. By studying the masses and orbits of these planets, astronomers hope to learn more about solar systems in general. For example, our own solar system contains four small, rocky planets near the sun -- Mercury, Venus, Earth, and Mars -- and four giant, gaseous planets farther out -- Jupiter, Saturn, Uranus, and Neptune. Astronomers were surprised to find that other stars have giant, gaseous planets in close orbits. For example, a planet nearly the size of Jupiter orbits the star 51 Pegasi closer than Mercury orbits our own Sun.

Our solar system

The sun is much larger than Earth. From the sun's center to its surface, it is about 109 times the radius of Earth. Some of the streams of gas rising from the solar surface are larger than Earth. Image credit: World Book illustration by Roberta Polfus

The sun is the largest and most important object in our solar system. It contains 99.8 percent of the solar system's mass (quantity of matter). The sun provides most of the heat, light, and other energy that makes life possible.

The sun's outer layers are hot and stormy. The hot gases and electrically charged particles in those layers continually stream into space and often burst out in solar eruptions. This flow of gases and particles forms the solar wind, which bathes everything in the solar system.

Planets orbit the sun in oval-shaped paths called ellipses, according to a law of planetary motion discovered by German astronomer Johannes Kepler in the early 1600's. The sun is slightly off to the side of the center of each ellipse at a point called a focus. The focus is actually a point inside the sun -- but off its center -- called the barycenter of the solar system.

The inner four planets consist chiefly of iron and rock. They are known as the terrestrial (earthlike) planets because they are somewhat similar in size and composition. The outer planets, except for Pluto, are giant worlds with thick, gaseous outer layers. Almost all their mass consists of hydrogen and helium, giving them compositions more like that of the sun than that of Earth. Beneath their outer layers, the giant planets have no solid surfaces. The pressure of their thick atmospheres turns their insides liquid, though they may have rocky cores.

Pluto is so far from Earth that even powerful telescopes reveal little detail of its surface. The Hubble Space Telescope gathered the light for the pictures of Pluto shown here. Image credit: NASA

A distant object called Pluto has been referred to as the ninth planet since its discovery in the 1930's. But Pluto has so many unusual features that some astronomers think it may not be a planet at all. For example, it travels around the sun in an elongated oval path much different from the nearly circular orbits of the other planets. Unlike the other outer planets, Pluto is small and solid. But Pluto contains only 1/500 the mass of Earth.

During the 1990's, astronomers discovered dozens of small rocky objects orbiting the sun beyond Neptune and Pluto. Astronomers had long suspected that the outer solar system had such a band of rocky material, called the Kuiper (KY pur) belt. The belt is named for the Dutch-born American astronomer Gerard P. Kuiper, who first predicted its existence. Pluto may merely be the largest of the objects in the Kuiper belt.

Ganymede, a moon of Jupiter, has craters and cracks on its surface. Asteroids and comets that hit Ganymede made the craters. The cracks are due to expansion and contraction of the surface. Image credit: NASA

Moons orbit all the planets except Mercury and Venus. The inner planets have few moons. Earth has one, and Mars has two tiny satellites. The giant outer planets, however, resemble small solar systems, with many moons orbiting each planet. Jupiter has 25 moons that are at least 6 miles (10 kilometers) in diameter, and many smaller satellites. Jupiter's four largest moons are known as the Galilean satellites because the Italian astronomer Galileo discovered them in 1610 with one of the first telescopes. The largest Galilean satellite -- and the largest satellite in the solar system -- is Ganymede, which is even bigger than Mercury and Pluto. Saturn has 25 moons that are at least 6 miles (10 kilometers) in diameter, and several smaller satellites. The largest of Saturn's moons, Titan, has an atmosphere thicker than Earth's and a diameter larger than that of Mercury or Pluto. Uranus has 21 moons, and Neptune has 11. The giant planets probably have more small moons not yet discovered. Pluto has one moon.

Rings of dust, rock, and ice chunks encircle all the giant planets. Saturn's rings are the most familiar, but thin rings also surround Jupiter, Uranus, and Neptune.

Comets are snowballs composed mainly of ice and rock. When a comet approaches the sun, some of the ice in its nucleus (center) turns into gas. The gas shoots out of the sunlit side of the comet. The solar wind then carries the gas outward, forming it into a long tail.

Astronomers divide comets into two main types, long-period comets, which take 200 years or more to orbit the sun, and short-period comets, which complete their orbits in fewer than 200 years. The two types come from two regions at the edges of the solar system. Long-period comets originate in the Oort (oort or ohrt) cloud, a cluster of comets far beyond the orbit of Pluto. The Oort cloud was named after the Dutch astronomer Jan H. Oort, who first suggested its existence. Short-period comets come from the Kuiper belt, the band of rocky objects orbiting the sun just beyond Pluto. Many of the objects in the Oort cloud and the Kuiper belt may be rocky chunks known as planetesimals left over from the formation of the solar system.

The asteroid Ida is about 35 miles (55 kilometers) long. It is one of thousands of asteroids in the asteroid belt, a region between the orbits of Mars and Jupiter. Image credit: NASA

Asteroids are minor planets. Some have elliptical orbits that pass inside the orbit of Earth or even that of Mercury. Others travel on a circular path among the outer planets. Most asteroids circle the sun in a region called the asteroid belt, between the orbits of Mars and Jupiter. The belt contains more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate that there are more than 750,000 asteroids in the belt with diameters larger than 3/5 mile (1 kilometer). There are millions of smaller asteroids. Astronomers have even found several large asteroids with smaller asteroids orbiting them.

Meteoroids are chunks of metal or rock smaller than asteroids. When meteoroids plunge into Earth's atmosphere, they form bright streaks of light called meteors as they disintegrate. Some meteoroids reach the ground, and then they become known as meteorites. Most meteoroids are broken chunks of asteroids that resulted from collisions in the asteroid belt. During the 1990's, astronomers discovered a number of meteoroids that came from Mars and from the moon. Many tiny meteoroids are dust from the tails of comets.

Heliosphere is a vast, teardrop-shaped region of space containing electrically charged particles given off by the sun. Scientists do not know the exact distance to the heliopause, the limit of the heliosphere. Many astronomers think that the heliopause is about 9 billion miles (15 billion kilometers) from the sun at the blunt end of the "teardrop."

Formation of our solar system

Many scientists believe that our solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula. According to this theory, the solar nebula began to collapse because of its own gravity. Some astronomers speculate that a nearby supernova (exploding star) triggered the collapse. As the nebula contracted, it spun faster and flattened into a disk.

The nebular theory indicates that particles within the flattened disk then collided and stuck together to form asteroid-sized objects called planetesimals. Some of these planetesimals combined to become the nine large planets. Other planetesimals formed moons, asteroids, and comets. The planets and asteroids all revolve around the sun in the same direction, and in more or less the same plane, because they originally formed from this flattened disk.

Most of the material in the solar nebula, however, was pulled toward the center and formed the sun. According to the theory, the pressure at the center became great enough to trigger the nuclear reactions that power the sun. Eventually, solar eruptions occurred, producing a solar wind. In the inner solar system, the wind was so powerful that it swept away most of the lighter elements -- hydrogen and helium. In the outer regions of the solar system, however, the solar wind was much weaker. As a result, much more hydrogen and helium remained on the outer planets. This process explains why the inner planets are small, rocky worlds and the outer planets, except for Pluto, are giant balls composed almost entirely of hydrogen and helium.

Other solar systems

Several other stars have disk-shaped clouds around them that seem to be solar systems in formation. In 1983, an infrared telescope in space photographed such a disk around Vega, the brightest star in the constellation Lyra. This discovery represented the first direct evidence of such material around any star except the sun. In 1984, astronomers photographed a similar disk around Beta Pictoris, a star in the southern constellation Pictor.

By the early 2000's, astronomers had discovered that more than 50 stars like our sun have planets orbiting them. In almost all cases, they found only one planet per star. All the planets found are probably gaseous with no solid surface.

Contributor: Jay M. Pasachoff, Ph.D., Field Memorial Professor of Astronomy and Director, Hopkins Observatory of Williams College.

How to cite this article: To cite this article, World Book recommends the following format: Pasachoff, Jay M. "Solar system." World Book Online Reference Center. 2004. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar518960.

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