Although it will be difficult to prove that life has been transported through our Solar System, the chances for the different steps of the process to occur can be estimated. These include:
(1) The escape process from a planet, i.e. the removal to space of biological material that has survived being lifted from a planetary surface to high altitudes where the material can escape to space.
(2) Travel conditions in space, i.e. the survival of the biological material over timescales comparable with the interplanetary passage.
(3) The entry process, i.e. the non-destructive deposition of the biological material on another planet.
Following the identification of some meteorites of lunar and probably some of Martian origin, we may conclude that the escape of material ranging from small particles up to boulder-size from a planet after the impact of a large comet or asteroid is evidently a feasible process. Recent studies showed that bacterial spores can survive [link to Horneck article ESA SP] shockwaves produced by a simulated meteorite impact.
In order to study step (2), the survival of resistant microbial forms in the upper atmosphere and free space, microbial samples have been exposed in situ aboard balloons, rockets and spacecraft - such as Gemini, Apollo, Spacelab, the Long-Duration Exposure Facility (LDEF), Foton and Eureca - and their responses investigated after recovery. The ESA Microgravity Programme has continued to support such exposure-experiments for studying life in space environment.
Figure: This image shows the EXPOSE facility. It will be used to
study the influence of space environment to microorganisms and will be
installed on an external structure on the ISS.