From Protoplanets to Protolife: The Emergence and Maintenance of Life
  Despite great advances in our understanding of the formation of the Solar System, the evolution of the Earth, and the chemical basis for life, we are not much closer than the ancient Greeks to an answer of whether life has arisen and persisted on any other planet. The origin of life as a planetary phenomenon will probably resist successful explanation as long as we lack an early record of its evolution and additional examples. It is widely thought that the geologic record shows that life emerged quickly after the end of prolonged bombardment of the Earth. New data and simulations contradict that view and suggest that more than half a billion years of unrecorded Earth history may have elapsed between the origin of life and LUCA. The impact-driven exchange of material between the inner planets may have allowed earliest life to be more cosmopolitan. Indeed, terrestrial life may not have originated on the Earth, or even on any planet. Smaller bodies, e.g. the parent bodies of primitive meteorites, offer alternative environments for the origin of life in our Solar System. The search for past or present life on Mars is an obvious path to greater enlightenment. The subsurface oceans of some icy satellites of the outer planets represent the best locales to search for an independent origin of life in the Solar System because of the high dynamical barriers for transfer, intense radiation at their surfaces, and thick ice crusts. The ``ultimate'' answer to the abundance of life in the Cosmos will remain the domain of speculation until we develop observatories capable of detecting habitable planets - and signs of life - around the nearest million or so stars. 
Eric Gaidos and Franck Selsis