Mary Mustapha, age 10, of Sergeant Bluff, Iowa, for her question:
WHAT'S'THE DIFFERENCE BETWEEN A PLANET AND A MOON?
When the United States astronauts returned from the moon, they brought with them many samples of soil and rocks. From this material, scientists are learning much about the composition of the heavenly body. The soil, they say, is made up of tiny pieces of ground up rock, bits of glass and scattered chunks of rock from five to 20 feet deep.
A planet is any of the nine largest objects that travel around the sun. The earth is a planet and so are Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
A moon is a natural satellite of a planet. The earth has one moon traveling around it. There are more than 30 natural satellites circling the planets with at least 13 going around Jupiter alone.
The sun and the stars are giant, shining balls of hot gases. The planets and the moons are dark, solid bodies. All light and nearly all heat on the planets come to them from the sun. The planes can be seen only because they reflect the light of the sun.
Most astronomers say there are probably no more than nine planets in our solar system. But they are almost certain that most of the stars in the universe have planets circling around them.
And how do you like this for possibilities: Astronomers tell us there are more than 100 billion stars in the galaxy, or family of stars, that includes the sun. Also, there are over 100 billion other galaxies that can be seen in the universe. Suppose that only one star in every galaxy had a planet like the earth, and intelligent life existed on one of every million of these planets. This would mean there would be at least 100,000 planets with intelligent life.
Our moon is a silent, lonely place with no life of any kind. Scientists believe all other moons are similar to the earth's.
All of the planets move around the sun in the same direction. Each planet rotates as it revolves around the sun. The planets' rotation periods, or the time required to spin around once, range from less than 10 hours for Jupiter to 243 days for Venus. The earth rotates once every 24 hours, or once a day.
Each planet spins around its rotational axis. The temperature, atmosphere, surface features, lengths of days and nights and other conditions on the planets vary widely. They depend on these three considerations: the planet's distance from the sun, the planet's atmosphere and the planet's rotation.