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We investigate whether it is possible that viable microbes could have been transported toEarth from the planets in extra-solar systems by means of natural vehicles such as ejecta expelled by comet or asteroid impacts on such planets. The probabilities of close encounters with other solar systems are taken into account as well as the limitations of bacterial survival times inside ejecta in space, caused by radiation and DNA decay. The conclusion is that no potentially DNA/RNA life-carrying ejecta from another solar system in the general Galactic star field landed on Earth before life already existed on Earth, not even if microbial survival time in space is as long as tens of millions of years. However, if the Sun formed initially as a part of a star cluster, as is commonly assumed, we cannot rule out the possibility of transfer of life from one of the sister systems to us. Likewise, there is a possibility that some extra-solar planets carry life that originated in our solar system. It will be of great interest to identify the members of the Sun's birth cluster of stars and study them for evidence for planets and life on the planets. The former step may be accomplished by the GAIA mission, the latter step by the SIM and DARWIN missions. Therefore it may not be too long until we have experimental knowledge on the question whether the natural transfer of life from one solar system to another has actually taken place. Subject headings: astrobiology ---stellar dynamics ---meteors, meteoroids ---(stars:) planetary systems Curt Mileikowsky 02/09/08 Natural Transfer of Viable Microbes Part 2 3

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It is conceivable that microorganisms could be transported over long distances in space inside natural «vehicles» such as molecular clouds, comets or meteors. To traverse a distance of 30 ly at speeds of 10 km/s (to another planetary system) would then take a million years. Due to the intense radiation in space by both photons and cosmic ray charged particles, microorganisms, including the most radiation-resistant spores known, need radiation protection in order to survive the millionyear dormancy. From the absolute energy distribution in the interstellar medium of the photon radiation of energies from 0.5 keV and up – known by measurement – the dose rates, in Megarads per million years (Mrad/My), have been calculated for spores of volume (1μm3which are assumed to have escaped out into interstellar space. Taking into consideration the energy dependence of photon attenuation in organic material, of absorption restriction in small bodies and of attenuation in the shielding material, these calculations have been done for unprotected spores, for spores in the central regions of seven different molecular clouds characterized by radial column densities from 0.01 to 1.3 g/cm2, and for spores inside porous meteors and in comets. Unprotected spores get almost 1000 Mrad/My from photons over 0.5 keV. Dose reduction by a factor of 1000, necessary for survival against photon radiation can be achieved by: 1) clouds with radial column densities ≥ 0.5 g/cm2; 2) meteors of radius ≥ 2-3 mm; 3) comet ice of radius > 5 mm. Cosmic ray charged particles are not treated in this Part I paper but will be the subject of a forthcomimg Part II paper.

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