Other Worlds: The Search for Life in the Universe.

THERE is very little, if any, unexplored territory left on our planet. We know, therefore, that there are no lost civilizations in the heart of Africa, no Lost Worlds in the Amazon jungle where dinosaurs still flourish, no Yetis in the high Himalayas. Those who wish to imagine strange new creatures or undiscovered civilizations nowadays must let their thoughts wander among the stars.

Among the stars there are plenty of potential dwelling places for fabulous creatures. The Milky Way alone contains a hundred billion stars, and scientists believe that a good fraction of these have planets. It is a pretty safe bet, then, that many millions of habitable planets exist in our galaxy. Michael D. Lemonick, an award-winning senior science writer for Time, has written an enjoyable journalistic account of the search for these planets and for the life which many hope and believe they harbor.

In spite of the antiquity of speculation on the subject, it was only a few years ago that scientists found the first real evidence of planets orbiting other stars. The heart of Lemonick's book, and the most interesting part of it, is the story of two of the world's leading ''planet hunters,'' Geoffrey Marcy, a professor at San Francisco State University, and his irrepressible post-doctoral assistant, Paul Butler. Lemonick is good at bringing to life the world of scientific research and the personalities of those involved in it. One gets caught up in the story of Marcy and Butler's eight-year struggle to perfect the techniques that would allow them to detect the almost undetectable, and the thrilling payoff when at last a new planet does swim into their ken.

Of course, the discovery of planets outside our solar system is an epoch-making event for pure astronomy. But the really big excitement came when a team of scientists announced in the summer of 1996 that they had actually found evidence of Martian life. What they found was only bacteria, and extinct ones at that, but, so we were told, it was extraterrestrial life all the same. The announcement was made in an unprecedented NASA press conference, and it led to an equally unprecedented statement by the President of the United States. However, many scientists were skeptical at the time, and it seems that they were right to be so. The evidence from Mars was flimsy to start with, and the problems with it are becoming more obvious. But it makes an interesting story, especially as told by Lemonick. He has a gift for giving his accounts immediacy and drama, and can explain scientific ideas without making them boring. In the last section of his book he tells about the SETI program. SETI stands for ''Search for Extraterrestrial Intelligence.'' This quixotic enterprise is conducted by a faithful few who cadge time on radio telescopes to listen for messages from outer space.

What are the chances that intelligent life does exist elsewhere in our galaxy? True believers, like the late Carl Sagan, are confident that the galaxy teems with life. How could it not, they say, with so many billions of stars? This argument, if one can call it that, fails to take account of some basic facts.

The first fact is that it is very hard for life to get started from inanimate matter, and no one really understands how it happened on earth 3.8 billion years ago. One cannot invoke evolution by natural selection. Natural selection presupposes that there already exist organisms with a genetic code and the ability to reproduce. The first, one-celled organism must have been fashioned in some other way.

The second fact is that the simplest living thing that anyone can imagine is remarkably complicated. It has been estimated that it would have a genetic blueprint with at least two hundred thousand ''bits'' of information. This is equal to the amount of information that could be encoded in a sequence of about forty thousand letters of the alphabet. How easily could such a genetic sequence arise by pure chance in a ''primordial soup'' of chemicals?

To get some idea, let us start by asking how easily a specific sequence of only eight letters -- say, the name ''Lemonick,'' can arise by chance. How many pages of random letters would the proverbial ''monkey with a typewriter'' have to type before the sequence LEMONICK would be likely to crop up? The answer is about one hundred million pages. What about a sequence that is only a little longer -- say, MDLEMONICK? That would require about one hundred trillion pages. Note the geometric escalation in the odds. Suppose that one were a little more ambitious, and wanted a specific eighty-character line of text to appear? Then, even if every atom in the observable universe were a monkey, and all of those monkeys were speed-typists, and all of them had been typing for the whole age of the universe up till now, there would still be a negligible chance that they would have produced that one line of text. Recall now that to make the first living thing on a planet is, in effect, to hit upon a sequence not of just eighty letters but of forty thousand.

While the number of stars in the galaxy is indeed ''astronomical,'' it is nonetheless paltry compared to what appears to be needed for life to arise by chance. This is true as well of the number of stars in the entire observable universe. How, then, did life ever get started on earth? It seems that there are only two plausible possibilities: a) it was a miracle, or b) the universe is infinitely large and contains an infinite number of planets. Whether the universe is infinite or finite in size is not known, and in all likelihood will never be known for certain. But however large the universe as a whole may be, there is simply no scientific argument that would lead one to expect that extraterrestrial life exists in the part of the universe which we can observe with telescopes, let alone contact or visit.

FOR many people, there seems to be something more involved in this search for extraterrestrials than simple scientific curiosity. Very likely that something is atheism. As long as man appears to be unique and ''alone in the universe,'' he seems to be an anomaly, an exception, even, perhaps, a miracle. How much more natural the human race would seem if intelligent life were shown to be the quite ordinary outcome of planetary chemistry. And so, among Martian rocks and radio waves from outer space, the secularist searches for evidence which would allow him to announce, with strange triumph, that man is nothing particularly special.

For the religious believer, in contrast, extraterrestrial life is not an issue of paramount importance. God could just as well have made one or many races of rational beings. The existence of alien life would certainly be of great interest, but it would make no difference to religious duty or doctrine.

This brings me to a highly regrettable aspect of Lemonick's book. Although he is a scrupulously accurate reporter about scientific matters, he does not always try very hard to get his facts straight about religion. If he had, he would know that it has never been a ''Christian dogma'' that there is only one inhabited world. He would know that Giordano Bruno was burned at the stake for his radical theological doctrines rather than for his speculations about other worlds. And he would know that Pope John Paul II did not ''rescind the Church's longstanding objections to Darwin's theory of evolution.'' (The Catholic Church never condemned or even objected to Darwin's theory, and Pope John Paul II said nothing, theologically, that went beyond what Pope Pius XII said in 1950 in Humani Generis.)

Fortunately, though, Lemonick is more interested in the science of the story than in its supposed philosophical and religious implications. And so, for all those healthy souls for whom science is just science, and alien creatures are just something fun to think about, like dinosaurs and Yetis and lost civilizations, I recommend this book as a pleasant read.