How special is the Solar system?

Abstract

Most mechanisms proposed for the formation of planets are modified versions of the mechanism proposed for the formation of the Solar system. Here we argue that, with reference to those planetary systems that have been observed, it has yet to be established that the Solar system is a typical planetary system. We consider the possibility that most observed planetary systems formed in a different way. If this is so, it may be that none of the observed planetary systems is likely to harbour an Earth-like planet.