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Despite Goldin's caution about assuming the existence of Earthlike planets, few astronomers doubt they are out there. If other solar systems do contain Earthlike worlds, says NASA exobiologist Michael Meyer, at least some should fall into the "habitable zone"--the region, governed by a planet's distance from its star, where water is liquid rather than solid or gaseous. "The good news," he says, "is that if our solar system is typical, there's a 50% chance that a planet will be in the right zone."
That is crucial, observes David Des Marais, a NASA biogeochemist. Liquid water is an ideal medium in which carbon-based organic chemicals can dissolve and react with one another in myriad ways. Why carbon, necessarily? Because, says Des Marais, "it is such a versatile chemical. It makes so many different and complex compounds. And it's the fourth most abundant element in the universe." Carbon compounds literally litter the cosmos, drifting through interstellar space in giant molecular clouds and making up a significant percentage, by mass, of comets and asteroids. Some scientists are convinced that the basic building blocks of life fell to Earth from space and that the same could easily happen anywhere.
Whether life would inevitably arise from those building blocks is still an open question. With only one example, it is impossible to say whether life on Earth was a fluke or a foregone conclusion. But most biologists cautiously lean toward the latter. Life on this planet emerged surprisingly quickly--as early as a few hundred million years after Earth formed. At the time, the planet was intensely volcanic, with the occasional leftover asteroid screaming in every few million years--yet primitive life forms persisted and flourished.
Until a few years ago, biologists were at a loss to understand how life could have arisen under such conditions. But laboratory experiments have convinced them that self-replicating molecules are relatively easy to assemble. And the discovery of hot-water volcanic vents deep in the ocean, surrounded by rich ecosystems of exotic life, implies that a hot, young, volcanic planet might in fact be an ideal incubator.
That suggests, though it does not prove, that biology will take hold if it possibly can, even under hostile conditions. In fact, biologists have not quite given up on our own solar system yet. They think Mars may have had a brief fling with one-cell life that could have left fossil evidence behind. Some even hold out the hope that microorganisms are still surviving somewhere under the Martian surface. Attention is also turning to Europa, one of Jupiter's moons; its icy white surface could conceal oceans of liquid water, and perhaps some sorts of living organism. Both possibilities are likely targets of future NASA investigation.
Alien life of any sort would make biologists ecstatic, of course, but it is the prospect of intelligent life that fires most people's imagination. "That final step from life to intelligent life is probably the longest shot of all," observes Des Marais. Even so, the small band of astronomers devoted to the search for broadcasts from high-tech extraterrestrials is encouraged: their 35-year quest has always rested on the assumption that planets exist outside Earth's solar system, and the fact that they have been proved right makes the search seem considerably less quixotic.