Extraterrestrial life: Why's it so quiet out there?


Outer space aliens almost surely exist -- but technological civilizations don't survive very long.

February 28, 1999|By John R. Alden | John R. Alden,Special to the Sun

Are we alone in the Universe? Some very solid science indicates that the answer is "No." From what we know about stars, planets and chemistry, there is every reason to believe that living things -- carbon-based organisms that grow, reproduce and evolve -- are relatively common in our galaxy. But if life is as widespread as our current understanding would indicate, and natural selection is as powerful as biologists believe, why haven't we been contacted by any other intelligent species? Why is it so quiet out there?

Among people who think about extraterrestrial life, this question is called Fermi's Paradox. "Where are they?" the great physicist asked some 50 years ago. Where are the advanced beings from other solar systems that science leads us to expect? The most likely explanation is that they didn't survive long enough for us to hear from them.

Within the last six months, at least a dozen serious and thought-provoking books have come out dealing with non-earthly life. These include surveys of astrophysics and planetary geology, accounts of recent discoveries of planets orbiting other stars, and speculative musings about the search for intelligent alien life. All, however, orbit around a formula called the Drake equation.

In 1961, a radio-astronomer named Frank Drake devised a scheme for estimating the number of technological civilizations (defined pragmatically as those that communicate with radio) that currently exist in our galaxy.

Astrophysicist Armand Delsemme, in "Our Cosmic Origins" (Cambridge, 322 pages, $24.95), explains it succinctly. "If ten new technological civilizations emerge per year in the Galaxy, and if their average longevity is 1,000 years, then there are, on average, a constant 10,000 civilizations capable of exchanging radiowave signals with us."

Ten thousand planets nattering and chattering away, and we haven't heard a word of it. A cynic might say, "How typical." A better question is, "Why not?" Perhaps our electronic ears aren't acute enough, or we haven't pointed them in the right direction. But the best explanation may be that most technological civilizations just don't survive very long.

The stars in the sky may be uncountable, but a reasonable estimate for the rate that stars form in our galaxy is about 10 per year. Perhaps only two or three of those 10 are sun-like (not fast-burning blue giants, double stars, or one of several forms of stellar dwarfs).

For those sun-like stars, according to planetary geologist Bruce Jakosky in his superb "The Search for Life on Other Planets" (Cambridge, 326 pages, $59.95), the odds are high that at least one habitable planet -- a planet of earthlike size and composition whose orbit is neither too close nor too far from its sun -- will condense out of the new star's protoplanetary disk.

The odds that carbon-based life will appear on such a planet also seem very good. Our earth is about 4.5 billion years old, and for its first 700 million years it was regularly blasted by comets and asteroids 30 to 100 times more powerful than the meteor that wiped out the dinosaurs. Nevertheless, according to "Comets and the Origin and Evolution of Life" (Thomas, Chyba and McKay, eds. Springer-Verlag, 296 pages, $29), life apparently originated on earth "before the end of the heavy bombardment" -- perhaps as early as 3.9 billion and certainly by 3.5 billion years ago.

Recent analyses of a meteorite from Mars (and yes, scientists can tell -- the oxygen isotope ratios in Martian rocks are systematically different from the ratios in rocks from Earth) hint that life may have appeared on Mars, too.

In a careful and cautious assessment of those studies, Jakosky emphasizes that the case for life on Mars remains unproved. Yet if carbon-based life arose on both Mars and Earth, it is hard to believe that it didn't appear on many (or even most) other Earth-like planets orbiting sun-like stars.

There is no way to know how often multicellular life will evolve from simple organisms like those that appeared on Earth 4 billion years ago. Here, that evolutionary step took an unimaginably long time, more than 3 billion years. It was another 600 million or 700 million years before self-aware creatures using tools and fire appeared. Then, in a blink of the archeological eye, these creatures developed speech, domesticated plants and animals, smelted iron and created complex political organizations. Within the last century, we invented radio. In a bit more than 4 billion years, our solar system produced an intelligent technological species.

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