Are Most Life-Friendly Stars Older Than the Sun? - Guillermo Gonzalez

Review by Robert Irion

Our solar system has lived in a hospitable part of the Milky Way for nearly 5 billion years. But most of the galaxy's other inhabited systems--if they exist--would have had even longer to nurture life, according to a study on page 59. The analysis intrigues astronomers who dare ponder the conditions for complex life elsewhere, but others warn that we know too little about those conditions for the research to mean much.

The study explores the physical requirements for a "galactic habitable zone" (GHZ), a term coined in 2001 by astronomer Guillermo Gonzalez, now at Iowa State University in Ames, and colleagues. They identified the life-friendly zone as a narrow annulus of stars in the middle of our galaxy's disk, the plane within which our sun revolves. Planetary systems closer to the crowded galactic center would face too much danger from exploding supernovas and passing stars that stir up comets, the team reasoned. And stars in the sparse outskirts wouldn't contain enough heavy elements to spawn planets like Earth.

The logic made sense, says astronomer Charles Lineweaver of the University of New South Wales in Sydney, Australia. In the current study, he and two computational astrophysicists retraced the arguments taking a more rigorous approach. In particular, they applied a detailed model of how the key elements of terrestrial planets have built up in the galaxy since its birth, produced by the flash fusions of supernovas. They also considered what Lineweaver calls "Earth destroyers": giant planets that migrate, formed by stars rich in heavy elements. Finally, the researchers included the time needed for complex organisms to arise. With no easy way to decide this slippery factor, they adopted Earth's time scale of 4 billion years as typical. "We don't assume that complex life exists, or that it is common or rare," Lineweaver notes.

When they crunched the numbers, the researchers found that a habitable ring of stars emerged about 8 billion years ago and 25,000 light-years from the galaxy's core--roughly the sun's distance today. This zone has slowly spread toward and away from the galactic center since then, a spread not evident in the study by Gonzalez and his colleagues. All told, the authors conclude that the GHZ has embraced fewer than 10% of the stars ever born in the Milky Way.

Moreover, about three-quarters of the stars in the zone today are older than Earth--indeed, 1 billion years older, on average. "If you're interested in whether extraterrestrial intelligence has evolved, this should be a sobering result," Lineweaver says. "A billion years is a long, long time."

Gonzalez lauds the work. "Our paper was not as quantitative in terms of the chemical evolution of the galaxy," he says. Other astronomers, however, think the galaxy's influences on extraterrestrial biology are too myriad for a basic astrophysical analysis to grasp. "We hardly understand the origin of life, let alone the evolution of complex life," says astronomer Mario Livio of the Space Telescope Science Institute in Baltimore, Maryland. "Until we do, it is extraordinarily difficult to talk about habitable zones."

Astronomer Virginia Trimble of the University of California, Irvine, who considered the galaxy's habitability in 1997, agrees. "Quantitative isn't necessarily any better unless you can be sure you assigned the right numbers," she says. For instance, if complex life typically takes twice as long to arise as it did on Earth, then older stars nearer the galactic center would be the best abodes--despite the supernovas and close neighbors. "I think the authors may have attached too much importance to the dangers of the environment there," she notes.

Lineweaver encourages the debate. "When life is mentioned, astronomers have winced and haven't talked about it. It's been a taboo," he says. "I'd like to convince the astrobiology community that there is credence to this approach."

File Date: 1.2.04

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