The planet parade: for millennia, planets were just mysterious, wandering points of light in the night sky. Now they are destinations

Natural History, May, 2004 by Neil deGrasse Tyson

Yet Bruno was neither the first nor the last person to posit some version of those ideas. Others who espoused the concept of multiple inhabited worlds range from the fifth century B.C. Greek philosopher Democritus to the nineteenth-century French novelist Honore de Balzac. Bruno was just unlucky to be born at a time when you could get executed for such thoughts.

During the twentieth century, astronomers figured that life could exist on other planets, as it does on Earth, only if those planets orbited their host star within the "habitable zone"--a swath of space neither too close nor too far. No doubt life as we know it requires liquid water, but it was just an assumption that life also requires starlight as its ultimate source of energy.

Then carne the discovery that some objects in the outer solar system, such as Jupiter's moons Io and Europa, are heated by energy sources other than the Sun. In both cases, the stress of Jupiter's tides on the solid moons pumps energy to their interiors, melting ice and giving rise to environments that might sustain life independent of solar energy.

Even right here on Earth, new categories of organisms, collectively called extremophiles, thrive in conditions inimical to human beings. The concept of a habitable zone incorporated an initial bias that room temperature is just right for life. But some organisms thrive at several hundred degrees Fahrenheit, and find room temperature downright hostile. To them, we are the extremophiles.

So, armed with the knowledge that life can appear in places vastly more diverse than previously imagined, astrobiologists are broadening the earlier, and more restricted, concept of a habitable zone. And, just as Bruno and others had suspected, the roster of confirmed exosolar planets continues to grow by leaps and bounds. Their number has now risen pasta hundred--all discovered in the past decade.

Once again we resurrect the idea that life might be everywhere, just as our ancestors had imagined. But today we do so without risk of being immolated, and with the newfound knowledge that life is hardy, and that the habitable zone may be as large as the universe itself.

Astrophysicist NEIL DEGRASSE TYSON is the Frederick P. Rose Director of the Hayden Planetarium in New York City.