Gavin Van Estey, age 11, of Fredericton, New Brunswick, Canada, for his question:

Can there be life on other planets?

Some 400 years ago, scholars had to face the fact that our planet revolves around the sun and our Earth is not the center of the Universe. Now modern scientists are challenging our minds with an even bigger idea It seems likely that our Earth is not the only populated planet in the Universe.

At the moment, we cannot prove that life exists on other worlds. But the evidence that this must be so is overwhelming    and growing. A few decades ago, many experts insisted that life is possible only on the planet Earth. Nowadays, most scientists agree that life can exist and most likely does exist on countless other planets    and perhaps also on some of their moons.

Serious minded scientists base these conclusions on known facts and figures. For example, we know that the living things of earth are built mainly from carbon and oxygen, hydrogen and nitrogen. We now know that these and other life building elements are plentifully strewn through the universe.

So the basic ingredients are here, there and everywhere. Next we need to find possible places where the miraculous creation of life could occur. Such conditions would provide radiation or some other energy for basic elements to form compounds called amino acids. For these assorted chemicals must form complex protein molecules. And one of these is DNA, the blueprint that governs the processes and multiplication of life itself.

In the past, it was assumed that such conditions existed only in our unique solar system    on the unique planet Earth. We now know that other solar systems exist, several quite close to home. Our sun is an average star of 100 billion assorted stars in our cart wheeling Galaxy. Millions, perhaps billions of similar galaxies are strewn throughout the vast reaches of the universe.

There is evidence that many, perhaps most, average stars have planetary systems. In this case, there must be billions of earth type planets in our Galaxy alone. So, the life making ingredients are everywhere and the number of possible places past counting. Surely the miracle of life must have happened again, again and again.

When we think of life, we have to think in terms of the living things of the Earth. But earth's inhabitants range from giant whales and redwoods to microscopic organisms that are neither plants nor animals. Most, but not all, need oxygen. Some can endure below freezing temperatures and some are at home in boiling heat. So let's not expect those numerous lively worlds to be inhabited with earth type forms of life.

Some scientists estimate that wiser and more advanced civilizations must exist in older parts of our Galaxy. Perhaps some have advanced and declined. According to these careful estimates, at this time, there must be at least 50 and perhaps as many as 50 million intelligent alien populations    not counting the billions of possibilities in other galaxies. It's nice to realize that most likely we are not all alone in the vast universe.