To Jim Traicoff, age 12, of Peoria, Ill., for his question:

What theory exp1ains how the planets formed?

At one time, scientists thought that our solar system might have been caused by an accident. This theory was discarded a generation ago, and in the light of new evidence several new theories have been suggested. Nowadays most scientists suspect that the birth of a solar system may be not at all unusual, and the heavens may teem with stars that have orbiting planets.

Mankind is a curious fellow  always seeking the answers to his endless questions. Our remote ancestors did not have the skills or the tools to find all the answers they wanted. So they used their imaginations to invent explanations to satisfy their curiosity. The world, they thought, was flat. Same said it sat on the back of a big turtle; others said that it was held on the shoulders of a giant.

Modern science, of course, uses more Sensible methods to solve its problems. Facts and more facts are gathered and proved. Someone gets an idea, a theory, of how the known facts may or may not be related. Still more tests are necessary to prove whether the theory is right or wrong. At present, many scientists lean toward a certain theory of the origin of our planets. But the definite proof of the theory is nowhere in sight.

The most widely accepted theory assumes that the planets are indeed children of the sun. Some billions of years ago, it is suggested, our sun was much bigger and its gases much thinner. It might have been a hazy ball of glowing gases as big as the present solar system. Then it began to shrink. Its gases became hotter and denser.

If this happened, it is reasonable to suppose that clouds of stray gases were left behind by the shrinking sun. Cosmic forces could have set these gassy clouds swirling in eddies. Left in the cold emptiness Of space, the gases would cool and freeze into solid lumps of matter.

There may have been many of these small bodies which have been called protoplanets, and the sun, though small.er, continued to exert its mighty gravity upon them. They swirled around their shining parent gathering more stray debris  left by the shrinking sun. Each planet may have formed from the joining of several smaller protoplanets. This theory agrees with most Vf the facts We know about the solar system  but so far no one has found a way to prove it to be true.

Not so long ago, scientists thought that the planets may have been formed by gases pulled out of the sun. They suggested that a star passing near our sun might set up tidal forces to make this occur. Such an accident might happen once in same 15 billion years  which would mean that a solar system is a rare event. Recent data, hweyer, suggests that solar systems are by no means rare, and maybe countless suns have families of orbiting planets similar to our own.