Brian Hurd, age 11, of Oak Hill, W.Va., for his question:

HOW DID OUR SOLAR SYSTEM START?

There was nobody around to take notes, so we must use indirect evidence to trace the origin of our magnificent solar system. Most likely the whole thing was created at the same time. The sun and its family of nine planets, their moons and various other small bodies are made of the same basic materials and apparently more or less the same age.

We cannot prove exactly how the solar system began, but most experts agree on a theory. This is an educated guess, backed up by a lot of evidence. It all began in an enormous cosmic cloud that occupied our corner of the Milky Way. Its gaseous particles and dusty atoms contained all the basic materials to form the various elements from which our solar system is made.

For a long time, this hazy cosmic cloud had no special shape.  Then, billions of years ago, the force of gravity began to mold it into a flat disk, like a huge revolving wheel. During the next 80 million years or so, most of the dusty gases were concentrated in the center and smaller helpings gathered in wider and wider rings.

The center contained about 90% of the original cloudy material and about 10% was concentrated in the circles. The center, of course, was the unborn sun, and the rings were unborn planets. As the whole thing spun around like a wheel, materials in the rings congealed to form solid globes surrounded by shells of gases. Some of the ring material formed smaller globes that were captured to become moons of the planets.

It seems likely that the planets and their moons were created in the light of the distant stars. For the sun was not ignited until later. This happened as its gaseous materials condensed, generating enough heat and pressure to start nuclear activity in the sun's core. This may have happened about 5 billion years ago.

The infant earth was bigger and heavier than it is today. Its atmosphere was thicker and much deeper. Gradually its heavier materials sank to the center. And when the sun was ignited, pressures from its nuclear furnace blew much of our gaseous atmosphere out into space. Most likely this is the basic story, though future evidence may cause us to change some of the details.