The sun and the other heavenly bodies that move around it are called the solar system.
            There is a great distance between the orbits of Mars and Jupiter. You might expect to find another planet between these two planets. However, instead of one planet, astronomers have found more than 1500 small bodies that vary in diameter from 1 mile to over 400 miles. They are called planetoids, which means little planets. They are also called asteroids. Each one revolves around the sun in its own orbit. Some astronomers think that planetoids may be pieces of a big planet which once revolved around the sun between Mars and Jupiter.
            Besides the planetoids, the solar system contains many comets. Some comets can be seen only through telescopes. From the earth, a comet looks like a hazy, round spot of light. The main part, or head, of a comet may be from 10,000 to 150,000 miles in diameter. A comet usually has a tail. The longest tail ever observed on a comet was 200 million miles long. Astronomers believe that comets are made of gases and little bits of solids. A comet travels in a long, narrow orbit around the sun. One end of the orbit may be very close to the sun, while the other end may reach far beyond Pluto. As a comet travels in its orbit, its tail is always pointed away from the sun. This is because light and other radiations that are traveling away from the sun exert a force on the molecules of gas in the comet and cause the molecules to move in the same direction.
            Some comets have such long orbits that they take hundreds of years to travel around the sun. Others, with shorter orbits, appear every few years. One of the most famous, Halley's comet, appears every 75 years. A comet can be seen only when it comes near both the earth and the sun. Comets shine partly because they reflect sunlight and partly because the sun makes their gases glow.
            Meteors are seen much more often than comets. We usually call them shooting stars or falling stars, but they are not stars at all. Meteors are really small bodies of metal or stone, moving at great speed. They are usually no bigger than the head of a pin, although occasionally they may weigh many tons. A meteor cannot be seen until it falls into the earth's atmosphere. Then friction between it and the air makes it hot enough to give off light. Several billion meteors probably strike the earth's atmosphere every 24 hours. Most of them are burned up, and only the dust that is left falls to earth. Scientists estimate that this dust adds thousands of tons to the mass of the earth every day.
            Some of the larger meteors fall to the earth before they are completely burned up. Then they are called meteorites. Some meteorites are stones, but others are largely metals such as iron and nickel. Very large meteorites may go deep into the earth. Meteor Crater in Arizona is 650 feet deep and over 4000 feet wide. Scientists are quite sure that it was formed a few thousand years ago by a huge meteorite that hit the earth and then exploded.

How Was the Solar System Formed?

            Many hypotheses have been suggested. Many scientists accept this hypothesis:
            Billions of years ago there was a huge cloud of gas whirling in space. It whirled like an enormous wheel. The center of the whirling cloud slowly became smaller and hotter. This became the sun. Around the sun spun huge rings of gases. Slowly the gas in each ring came together into a very hot ball of gas. Gradually these gases cooled. Many of them became liquids. In time, many of the liquids became solids. The balls of gas became the planets and moons.