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Why do you (or others) say they shouldn't be there? I Don't say this Ian....well..I'm reporting what I heard but I'm not stating a fact!!..hence the questionLet me answer my question. Planetary systems are likely to be disrupted by stars passing nearby in their orbits around the centre of the galaxy so the solar system must not have had too many close encounters with stars during its life. The solar system also includes the Oort cloud which is outside of the Kuiper belt from which most of the comets. originate.Most people do not realise how widely separated most stars are and the probability of a close encounter is extremely slow. In the past it was considered that spiral arms in galaxies were areas of increased density and the sun experienced higher risks of close encounters every few hundred million years as it passed through them but that may not be entirely true nowadays they are understood to be more like shock waves where the supernovae resulting from short lived high mass stars spawn the condensation more stars from cold gas in front of them as they orbit. So the stellar density is not too great here it is just that they contain bright young stars and are more visible.