If by multiple solar systems, you mean solar systems orbiting not just the Sun but other stars too, yes I think it is very likely that there are other solar systems out there. Recently, astronomers have found evidence for planets around dozens of nearby stars, so this is not so much speculation anymore as it is a matter of fact. The existence of other planetary systems seems to be an observational fact of our 21st Century world.
What we still don't know is, just how unique is our particular type of solar system with its nine planets orbiting in nearly circular paths. The planetary systems we can detect right now, look nothing like ours and have massive planets orbiting in highly elliptical paths that would probably eject all of the smaller bodies in their respective systems. But, the methods now being used are only able to detect these 'pathological' systems given their sensitivity. It is hoped that as more sensitive systems are used in the next few decades, the list of known planetary systems will start to contain examples that look more and more like our own.
Copyright 1997 Dr. Sten Odenwald
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