Peter Craig, age 12, of Rochester, New York, for his question:

Why do the planets orbit the sun?

It's one thing to know that the planets really do orbit the sun. But this information naturally makes a person wonder why. Some of the clues are in how they do it. Together they move in an orderly system that resembles a multiple carousel. The remarkable similari¬ ties in the patterns are governed by opposing cosmic forces. Late in the 1700 the French astronomer Laplace figured how and why the orbital motions of the planets help to hold the solar system in permanent shape.

Each planet could orbit the sun in any direction and at any angle to the sun. It is remarkable that they all orbit in the same direction and more or less on a level plane with the sun's equator. Their orbits are neatly spaced and their orbital speeds diminish with distance from the sun. Their orbits are nearly circular, with the exception of Pluto's rather elliptical path. The general motion of the entire solar system is from west to east. This is the direction of the sun's rotation, the asteroids, the revolving planets and most of their moons.

Scientists say that this orderly structure was established during the creation of the sun and its planets. The planets must orbit the sun, or its mighty gravity would pull them down into its fiery furnace. Their safety depends upon a precise balance of gravita¬tional attractions between them and the sun. This, of course, is related to mass; the matter packed into a certain volume. The safety factor for each planet involves its dis¬tance from the sun, plus its orbital velocity.

Actually, the planets are falling toward the sun. They never get there because their orbital speeds counteract the pull of its mighty gravity. They keep on falling, but the balance of gravitational forces makes them fall in curved paths at set distances from the sun. Their orbital paths are governed by a law explained by young Isaac Newton. When a body is set in motion, it continues along the same path at the same speed    unless influenced by some outside force.

The planets are not influenced by outside forces strong enough to divert them very far from their paths. Hence, we can assume that they were set in motion while the original substance of the solar system was being molded into shape. Other cosmic forces started this nebulous material spinning    from west to east. As the planets took form, they were boosted around by this motion. It gave them the force to counteract the gravitational pull toward the center of the system. The planets must move around their orbits because otherwise they would fall into the sun.

There is no chance that a planet can come off its course or that two planets could possibly collide. This is because of the cosmic forces that govern them. Their paths are permanent    as long as no outside force disrupts them. But this is not likely because the nearest stars strong enough to pull our solar system out of shape are light years away, across the oceans of space.