a1 Centre for Astrobiology, Cardiff University, Cardiff, UK e-mail: email@example.com
The Solar System passes within 5 pc of star-forming nebulae every ∼50–100 million years, a distance which can be bridged by protected micro-organisms ejected from the Earth by impacts. Such encounters disturb the Oort cloud, and induce episodes of bombardment of the Earth and the ejection of microbiota from its surface. Star-forming regions within the nebulae encountered may thus be seeded by significant numbers of microorganisms. Propagation of life throughout the Galactic habitable zone ‘goes critical’ provided that, in a typical molecular cloud, there are at least 1.1 habitable planets with impact environments similar to that of the Earth. Dissemination of microbiota proceeds most rapidly through the molecular ring of the Galaxy.
(Received January 20 2007)
(Accepted January 22 2007)
(Online publication April 16 2007)
c1 Correspondence to Professor W.M. Napier, Kilbeg South, Bandon, County Cork, Ireland.