Barbara Geller, age 15, of Great Falls, Mont., for her question:
WHAT'S THE DIFFERENCE BETWEEEN A PLANET AND A MOON?
A planet is any of the nine largest objects that travel around our sun. Earth is a planet and the rest of the planets in our solar system are Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
A moon is a natural satellite of a planet. Earth has one moon orbiting around it. There are at least 32 moons circling the planets with at least 12 going around Jupiter alone.
The sun, a star, is a giant, shining ball of hot gases. All light and nearly all heat on the planets come to them from the sun. The planets can be seen only because they reflect the light of the sun.
Most astronomers agree that there are nine planets in our solar system. But they are almost certain that many of the stars in the universe also have planets orbiting around them.
And how do you like this for possibilities: Astronomers tell us that there are more than 100 billion stars in the galaxy, or family of stars. Also, there are more than 100 billion other galaxies estimated in the universe.
Suppose that only one star in every galaxy had a planet like 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. The moons of other planets vary in size and geological make up.
When the United States astronauts returned from the moon, they brought with them many samples of soil and rocks. From this material, scientists are continuing to learn much about the composition of the heavenly body.
The soil on the moon, the scientists say, is made up of tiny pieces of ground 7up rock, bits of glass and scattered chunks of rock that range in size from five to 20 feet.
Our moon orbits the earth at a mean distance of 238,858 miles and it has a diameter of 2,160 miles.
All of the planets move around the sun in the same direction. Each planet rotates as it revolves around the sun. The planet's rotation period, 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.
The planets differ in these main ways: distance from the sun, atmosphere and rotation.