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About planets... SOLAR SYSTEM consists of a star called the sun and all the objects that travel around it. The solar system includes (1) The earth and eight other planets, along with the satellites (moons) that travel around most of them (2) Planet-like objects called asteroids (3) Chunks of iron and stone called meteoroids (4) Bodies of dust and frozen gases called comets and (5) Drifting particles called interplanetary dust and electrically charged gas called plasma that together make up the interplanetary medium. The solar system is shaped like a disk. It is only a tiny part of a galaxy (family of stars) called the Milky Way. The Milky Way consists of hundreds of billions of stars, including the sun. Some of the other stars may also have planets orbiting them. The Milky Way, which also has a disk like shape, is about 100,000 light-years across and about 10,000 light-years thick at its center. A light-year is the distance light travels in one year at a speed of 186,282 miles (299,792 kilometers) per second. The solar system is less than one light-day (the distance light travels in one day) across. Distances within the solar system are measured in astronomical units (AU). One astronomical unit equals the average distance between the earth and the sun--about 93 million miles (150 million kilometers). The distance between the sun and Pluto, which is usually the outermost planet, averages about 39 AU. Parts of the Solar System The Sun is the center of the solar system. Its mass is about 740 times as great as that of all the planets combined. The huge mass of the sun creates the gravitation that keeps the other objects traveling around the sun in an orderly manner. The sun continuously gives off energy in several forms--visible light; invisible infrared, ultraviolet, X-rays, and gamma rays; radio waves; and plasma. The flow of plasma is called the solar wind. The surface of the sun changes continuously. Bright spots called plages and dark spots called sunspots frequently form and disappear. Gases often shoot up violently from the surface. Planets are the largest objects that orbit the sun and are not satellites of another object. Going outward from the sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Pluto, and Neptune. Pluto is usually the farthest planet from the sun. Every 248 years, however, Neptune becomes the most distant planet. Pluto moves inside the orbit of Neptune and remains there for about 20 years. Pluto entered Neptune's orbit on Jan. 23, 1979, and it will stay there until March 15, 1999. Unlike the sun, the planets do not produce their own energy. Instead, the planets reflect the heat and visible light produced by the sun. The two largest planets, Jupiter and Saturn, send out radio radiation. Jupiter's radio |
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