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It's an age-old inquiry: Are we alone in the universe? In pondering it, scientists have scanned the sky for extraterrestrial signals and sent messages out into space. Taking a different approach, astronomers have now identified the most likely places in our galaxy for other inhabited solar systems to exist. The analysis suggests that up to 10 percent of the stars in the Milky Way could offer conditions necessary to support complex life.
Charles Lineweaver of the University of New South Wales in Australia and his colleagues modeled the evolution of our galaxy and analyzed the requirements for the so-called galactic habitable zone (GHZ). The team traced the distribution of four prerequisites for life: the presence of a host star, sufficient heavy metals to form terrestrial planets, enough time for biological evolution and a location that is safe from deadly supernovae. The findings, published today in the journal Science, indicate that the GHZ is a slowly spreading region located about 25,000 light-years from the galaxy's center. The stars encompassed by it formed between four billion and eight billion years ago; three quarters of them are a billion years older than the sun.
The scientists caution that the work is very preliminary. For instance, they make no assumptions about whether or not complex life is probable or if it is rare or common in the galaxy. Lineweaver and his collaborators propose that their results will be directly tested within a few decades, as more information about planets outside the solar system becomes available.
