How special is the Solar System?
  Most mechanisms proposed for the formation of planets are modified versions of the mechanism proposed for the solar system. Here we argue that, in terms of those planetary systems which have been observed, the case for the solar system being a typical planetary system has yet to be established. We consider the possibility that most observed planetary systems have been formed in some quite different way. If so, it may be that none of the observed planetary systems is likely to harbour an earth-like planet. 
M.E. Beer (1), A.R. King (1), M. Livio (2) & J.E. Pringle (3) ((1)   Leicester, (2) STScI, (3) Cambridge)