Dust Debris Around Stars

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. 

If a habitable zone is restricted to the presence of liquid water, then we must also include the interiors of giant planets and the ice covered Galilean Moons of Jupiter, or the icy satellites of Saturn and maybe even other places. Therefore, the search for life must not be restricted to the study of circumstellar habitable zones. But it seems that life requires much more than just liquid water. 

Before the first extrasolar planets were detected, the observational evidence of dust around stars was a first hint to the existence of extrasolar planetary systems. 

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Disks containing dust debris are found around stars (see e.g. Fig. 6.6). These disks are believed to be equivalent to our Kuiper Belt. There seem to be two different types of such disks ... 

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