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ASTRONOMERS have speculated for decades that water-dependent life can only evolve on planets that lie within a “habitable zone” around stars. Now evidence is emerging that our Galaxy also has a habitable zone where planets like Earth are most likely to form. Guillermo Gonzalez of Iowa State University in Ames says that of the 69 extrasolar systems found so far, those lying further from the galactic centre contain fewer heavy elements and may have planets in more circular orbits, similar to our Solar System. Gonzalez says the surfeit of heavy elements towards the galactic centre leads to systems with more…