A solar system is a group of objects that travels through space. It is made up of a star and the planets and other objects that orbit, or travel around, the star. When we talk about a solar system, we are usually talking about our own solar system. Our solar system is made up of the sun, the planets -- including Earth -- and their moons, and many smaller objects that also travel around the sun. Image to left: The sun and its solar system are in the Milky Way galaxy, a large, spiral-shaped group of stars. Credit: World Book illustration by Anne Norcia These smaller objects include asteroids meteoroids, and comets. In the 1990s, astronomers, or scientists who study outer space, discovered several other solar systems. The Sun The sun is the largest object in our solar system. It provides most of the light, heat, and other energy that makes life on Earth possible. The sun'e outer layers are hot and stormy. Materials there erupt, or shoot out, from the sun's surface. These eruptions are made up of hot gases and electrically charged particles, or tiny bits of material. The flow of gases and particles is called the solar wind. The solar wind surrounds everything in the solar system. The Planets The planets orbit the sun in oval-shaped paths. The four inner planets -- Mercury, Venus, Earth, and Mars -- are made up mostly of iron and rocks. They are somewhat similar in size and in makeup. The outer planets are Jupiter, Saturn, Uranus, and Neptune. All the outer planets are giant worlds surrounded by layers of gases. Their insides are liquid, and they may have a chunk of rock at the center.

Image to right: Triton, one of Neptune's moons, is the coldest object in the solar system. Credit: NASA Moons orbit every planet except Mercury and Venus. The inner planets have few moons. Earth has one moon, and Mars has two tiny moons. The giant outer planets, however, have many moons. Jupiter has 16 moons, including the largest moon in the solar system, Ganymede. This moon is even bigger than Mercury. Saturn has 18 moons. Its largest moon, Titan, is also bigger than Mercury. Titan has an atmosphere -- a layer of gases -- that is thicker than Earth's atmosphere. Uranus has 17 moons, and Neptune has 8. The giant planets probably have more moons, yet to be discovered.* Rings of dust, rock, and ice chunks orbit all the giant planets. Saturn's rings are the most familiar, but thin rings also surround Jupiter, Uranus, and Neptune. Comets Comets are objects made up mainly of ice and rock. When a comet gets close to the sun, some of the ice turns into gas. The gas and bits of dust shoot out of the comet. Sunlight pushes against the gas and dust, and the solar wind carries them outward, forming a long tail. Comets come from two regions at the edges of the solar system. Some comets start in the Oort cloud, a far-off cluster of comets. Others come from the Kuiper belt. Asteroids Asteroids are very small planets. Some travel in oval-shaped orbits that pass inside the orbit of Earth or even that of Mercury. Others travel on a circular path among the outer planets. Astronomers believe there are more than 50,000 asteroids. Most asteroids orbit the sun between the orbits of Mars and Jupiter in a region called the asteroid belt.

Meteoroids are chunks of metal or rock smaller than asteroids. When meteoroids fall into Earth's atmosphere, they break up. As that happens, they form bright streaks of light called meteors. Some meteoroids reach the ground. These meteoroids are known as meteorites. Most meteoroids are broken chunks of asteroids. During the 1990s, astronomers discovered a number of meteoroids that came from Mars and from the moon. Image to left: The solar system probably began as a cloud of gas and bits of rock and metal. Credit: World Book illustration by Rob Wood. Formation of the Solar System: Many scientists believe that our solar system formed from a giant, rotating cloud of gas and tiny pieces of rocks and metals. This cloud is known as the solar nebula. According to this theory, the solar nebula began to shrink. As the nebula got smaller, it spun faster and flattened into a disk. Image to right: The spinning cloud flattened out into a disk. Credit: World Book illustration by Rob Wood The scientists believe that particles within the flattened disk then smashed into one another and stuck together to form asteroid-size objects. Some of these objects joined to form the planets. Others formed moons, asteroids, and comets. Image to left: Most of the cloud pulled together in the center and formed the sun. Credit: World Book illustration by Rob Wood Most of the material in the nebula, though, was pulled toward the center and formed the sun. According to the theory, the pressure at the center became great enough to cause the nuclear reactions that power the sun. Eventually, solar eruptions occurred, causing a solar wind. In the inner solar system, the wind was so powerful that it swept away most of the lighter elements. In the outer regions of the solar system, however, the solar wind was much weaker. As a result, much more gas remained on the outer planets. That explains why the inner planets are small, rocky worlds and the outer planets are giant balls of gas. How to cite this article: To cite this article, World Book recommends the following format: "Solar system." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005. Image to right: Some of the rock and metal pieces smashed together. They formed planets, moons and asteroids. Credit: World Book illustration by Rob Wood.