The Swede Svante Arrhenius suggested a century ago that life in the form of spores could survive in space and be spread from one planetary system to another. According to Arrhenius, spores escape by random movement from the atmosphere of a planet, and are spread throughout interstellar space by the weak but persistent radiation pressure exerted by starlight. In a variation on this theme, others have suggested that the spores might be spread deliberately by intelligent beings. This theory is called "directed panspermia."The modern version of the panspermia idea starts from observations of prebiotic chemicals in interstellar clouds. It seems certain that some of these raw materials, such as amino acids, fell on to the young Earth and kick-started life.
Some researchers, notably the late Fred Hoyle and Chandra Wickramasinghe, have argued that not just complex organic substances, but even complete living organisms, albeit bacteria, might have evolved in space on the surface of dust grains, then been carried down to Earth by an impacting comet. There is also the possibility of "ballistic panspermia," when rocks from one planet are blasted into space by impacts and travel to another planet. The discovery on Earth of meteorites from Mars' surface means that we might even be descended from Martian bugs.by John Gribbin, 30-Second Theory