Galactic Habitable Zone

If the planetary system is not far enough from the galactic center, then close encounters with passing stars become more likely. This will trigger hazards from comets that move into the inner planetary systems. Moreover, outbursts from supernovae and from the supermassive black hole at the center could cause strongly enhanced shortwavelength radiation bursts desfroying the complex molecules needed for life. 

Observations of extrasolar planetary systems have shown also that when the metallicity of the star becomes too high more massive planets orbiting close to the stars are likely. These could have destroyed Earth-sized objects there. 

In the early stages of galaxy formation the heavy elements to form terrestrial planets were only present near the center of the galaxy because there the concentration of stars was largest and some stars already had evolved and became supemovae that enriched the interstellar medium with heavier elements there. This was not a safe environment because the danger of nearby supernovae explosions was high. 

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