When a comet strikes the surface of a body like Earth, Moon or Mars at high speed, it creates a shock wave which throws material out to form an impact crater. For a large enough impact, some rock and dust may be thrown into space. If material ejected from the Earth contains microorganisms and spores, the question arises whether life may be carried from planet to planet within the solar system. Computations show that this can happen. The transfer of life between planets, may therefore take place within the solar system.
However, the solar system is tiny on the scale of the Galaxy, and it has generally been thought that the transfer of life between stars is impossible -- the microorganisms would be destroyed by cosmic rays, ultraviolet light and so on. Bill Napier has demonstrated that this may not be the case. Quite frequently in geological terms, the Sun passes close to molecular clouds containing star-forming regions. Life-bearing dust from the solar system may be injected quickly into these regions during such close encounters, while the microorganisms are still viable, and so be incorporated into the planets and comets forming within the star systems. The result is that if life began on Earth, it could by now have spread around the Galaxy (and life on Earth might originally have formed elsewhere). The implication that life spreads between the stars has many scientific and even philosophical consequences.