THERE ARE ABOUT 100 billion galaxies in the observable universe, each with hundreds of billions of stars. What are the chances that there's any interesting life out there?
In 1961, astronomer Frank Drake proposed a simple answer: We can assume that some stars have planets, some planets host single-celled life forms, some of those life forms survive to develop intelligence, and some intelligent beings leave an electromagnetic trace before they expire.
Carl Sagan once estimated that in the Milky Way alone there must be over a million detectable civilizations. Today, Drake sticks to his original estimate of 10,000.
Peter Ward is sick of these loose overestimates. ''You can't turn on the TV without seeing aliens,'' the co-author of ''Rare Earth: Why Complex Life is Uncommon in the Universe'' (Copernicus Books, 2000) complained to a Cambridge auditorium packed withastronomers, UFO enthusiasts, and other onlookers last week. Ward was facing off against Harvard paleontologist Charles Marshall at a debate hosted by the Harvard-Smithsonian Center for Astrophysics. In 1996, Ward and Marshall worked together on a paper arguing that a major drop in sea level, in addition to the infamous asteroid, had wiped out the dinosaurs. But when it comes to the distribution of intelligent life in the universe, they couldn't agree less.
"Maybe I shouldn't count myself as intelligent life," quipped Ward, a professor of geology at the University of Washington. ''The first stop on my book tour was a science fiction convention. A little girl told me I was the devil for taking the aliens away.'' The Australia-born Marshall, for his part, retains the taste for discovery that propelled him from childhood dino-mania to a career in evolutionary biology. ''Life is capable of more trajectories than physics or astronomy might predict,'' he said. ''I don't know if life is teeming out there. But it could be.''
Ward and Marshall agree that the universe is full of microbes. Recent studies have shown that interstellar clouds can generate amino acids, the building blocks of proteins. Meteors falling to earth usually contain a variety of organic compounds. And cells can survive under extremes of temperature, pressure, and pH, and may be able to travel from planet to planet on comets.
But to flourish, even simple life needs liquid water, and this limits it to planets in the habitable zone: far enough from a star not to be boiling, but close enough not to be freezing. And to get complex life - anything more intricate than a flatworm - it seems that you need, first of all, a decent atmosphere.
Here on Earth, it took 3 billion years of steady temperatures to build up enough oxygen to support animals. The fact that our planet lies in a habitable zone does not itself guarantee such steadiness. Ward thinks that plate tectonics also do us a great service: When one plate slides under another, an updraft of magma brings carbon dioxide to the surface, eventually warming up the atmosphere through the greenhouse effect. But once the atmosphere gets warmer, excess carbon dioxide is removed by the calcium in the magma, and it gets cooler again. ''For billions of years, we've been bouncing around in a very fine temperature range because of the thermostat of plate tectonics,'' he said. ''How common is that in the universe? We don't know.''
To survive long enough to evolve any complexity, Ward went on, life must also avoid being destroyed by space debris. Earth is shielded by Jupiter's gravitational field, which slows down incoming comets. But in most solar systems, a Jupiter-sized planet has such an erratic orbit that it will eventually fling any nearby planet away from the star. We may be uniquely lucky to live in such a safe neighborhood.
Marshall is unimpressed by scenarios that emphasize life's fragility. "The question is, how hard is it to sterilize a planet?" he asked. In the total devastation following the Mount Saint Helens eruption, biologists were staggered to find plants protected by animals that fell on them. Even at Hiroshima, a few people survived at close range to the explosion. ''We should expect such surprises,'' said Marshall. ''Life will find a way.''
Life also makes its own way. Take the Cambrian explosion of 445 million years ago, when a host of scuttling sea creatures burst into a world that hadn't seen much more than worms. What accounts for such rapid evolutionary change? ''If you have a bunch of plant life, and someone is able to develop a few genes for jaws, then everyone had better watch out,'' said Marshall. It's no surprise that eyes and legs, crucial for hiding from predators, appeared at the same time. ''With an increase in selective pressures, complexity is bound to arise. You don't need special conditions.''
But Peter Ward was not convinced. ''If complexity is inevitable, then what was going on for the three billion years between the first cell and the Cambrian explosion?'' Just a slow and steady buildup of oxygen due to the presence of simple life-forms and liquid water. ''Without the thermostat of plate tectonics, the right conditions just don't last very long,'' he said.
Of course, there may be more to life than what we can guess now. ''We have some idea of what conditions were necessary for us to evolve,'' Marshall said, ''but we don't know if they're the only possible ones.'' While Ward prefers to limit the discussion to life as we know it - carbon-based organisms with DNA - Marshall thinks we should expect the unexpected.
Some audience members found this approach a bit too vague. ''One data point is better than none,'' one said. ''Can you quantify the problem?''
Many scientists are working on it. Astrobiology, defined broadly as the study of life in the universe, is now serious science. NASA and the National Science Foundation invest tens of millions of dollars every year in it. Astronomers search for new planets, geologists prospect for evidence of water on already known planets, and biochemists piece together the origins of life on earth. Still, it's mostly theory for now. And hitchhiking to the nearest star still takes 300,000 years.
''The fact that neither of us has any numbers, shows that we're going on next to nothing,'' said Marshall. ''But my sense of faith is that the universe is so unimaginably rich that it will turn out that life is common, and that scientific reasoning, while powerful, can lock us into a narrow view of what is possible.''