Interstellar transfer of planetary microbiota

Panspermia theories require the transport of micro‐organisms in a viable form from one astronomical location to another. The evidence of material ejection from planetary surfaces, of dynamical orbit evolution and of potential survival on landing is setting a firm basis for interplanetary panspermia. Pathways for interstellar panspermia are less clear. We compare the direct route, whereby life‐bearing planetary ejecta exit the Solar system and risk radiation hazards en route to nearby stellar systems, and an indirect route whereby ejecta hitch a ride within the shielded environment of comets of the Edgeworth–Kuiper Belt that are subsequently expelled from the Solar system. We identify solutions to the delivery problem. Delivery to fully fledged planetary systems of either the direct ejecta or the ejecta borne by comets depends on dynamical capture and is of very low efficiency. However, delivery into a protoplanetary disc of an early solar‐type nebula and into pre‐stellar molecular clouds is effective, because the solid grains efficiently sputter the incoming material in hypervelocity collisions. The total mass of terrestrial fertile material delivered to nearby pre‐stellar systems as the Solar system moves through the Galaxy is from kilograms up to a tonne. Subject to further study of bio‐viability under irradiation and fragmenting collisions, a few kg of original grains and sputtered fragments could be sufficient to seed the planetary system with a wide range of Solar system micro‐organisms.