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67 Strughold proposed in 1956 what he called an “ecosphere of the Sun,” made up of a biotemperature belt, a liquid water belt, and an oxygen belt. These belts all lie at about the same range from the Sun, from the orbit of Venus to beyond Mars. This concept, he suggested, could be applied to other stars.* Scientists further developed the concept of a “zone of habitability” in our solar system, often defining it as a band of orbits within which liquid water could exist on planetary surfaces. Researchers studying the unique role of water in the chemistry of life have concluded that no simple mole- cule can mimic all of water’s biological functions; water is an anomalous liquid with characteristics that seem critical to its biological role. Yet, science editor Philip Ball warned against the idea that, because life on Earth requires water, life anywhere else must have the same requirement. Life, which is adaptive, may simply have found ways to exploit what water has to offer.** Experts disagree about the breadth of habitable zones around other suns. Astronomer Michael Hart argued that only G stars would have con- tinuously habitable zones; other scientists have extended the range to other classes of stars. Ward, Brownlee, and astronomer Guillermo Gonzalez proposed a galac- tic habitable zone—the most hospitable areas of the Milky Way, lying between its dangerous center and its heavy-element-poor outer reaches. In a statement with a certain anthropocentric flavor, they wrote that the galactic habitable zone appears to be an annulus in the disk at roughly the Sun’s distance from the Milky Way’s center. Galactic travelers, if there are any, would tend to roam around the annulus.” Other researchers have defined the galactic zone of habitability as a ring of stars that emerged about 8 billion years ago at 25,000 light-years from the core—slightly closer than the Sun’s distance today. This zone has slowly spread toward and away from the galactic center and now embraces about ten percent of the stars born in the Milky Way. Roughly three- quarters of the stars in the zone are older than the Sun—typically 1 billion years older.”’ A aeelea Ldn ate wee ne bn thn ane. weet fate A tee A suitable star may not be the only environmental factor. Astrophysicist Priscilla Frisch commented that it does not make sense to look for habit- able planets unless you look at the way their stars interact with local envi- ronments. A star passing in and out of dense interstellar cloud fragments would not have a stable environment, nor would its planets.“ Bottom Line, planets. A big win for the believers. It could be even bigger if astronomers detect Earth-like planets. The long-assumed plurality of worlds has been proven, although not the plurality of inhabited worlds. Habitable Zones Habitable Zones