Maybe Could Hitch a Ride on Asteroids and Colonize Other PlanetsThere's a scientific hypothesis called panspermia. It theorizes that life could be seeded on certain hospitable planets (potentially including Earth) from outer space, via asteroids and organisms that can survive the very harsh conditions of the void (radiations, cold, vacuum, etc).
On this topic, Wikipedia says: "Panspermia proposes that life that can survive the effects of space, such as extremophile archaea, become trapped in debris that is ejected into space after collisions between planets that harbor life and Small Solar System Bodies (SSSB). Bacteria may travel dormant for an extended amount of time before colliding randomly with other planets or intermingling with protoplanetary disks. If met with ideal conditions on a new planet's surfaces, the bacteria become active and the process of evolution begins. Panspermia is not meant to address how life began, just the method that may cause its sustenance."
Now obviously that is a very hard hypothesis to confirm, but we can certainly say that it is more or less probable by running experiments on organisms to see how well they would survive in space.
That's what the European Space Agency (ESA) has done on board the ISS:
In 2008 scientists sent the suitcase-sized Expose-E experiment package to the Space Station filled with organic compounds and living organisms to test their reaction to outer space.
When astronauts venture on a spacewalk, hours are spent preparing protective suits to survive the hostile conditions. No effort was made to protect the bacteria, seeds, lichen and algae attached to the outside of the Space Station, however.
[T]he space samples endured the full power of the Sun’s rays. The samples were insulated somewhat by the Space Station but still had to cope with temperatures changing from –12ºC to +40ºC over 200 times as they orbited Earth.
Now that these samples have been brought back to Earth and studying, we've discovered that lichen was the toughest of that batch. Back on Earth after being irradiated and in a dormant state for months, it started growing again.
"It seems possible that organisms could colonise planets by hitching rides on asteroids. ESA is probing this intriguing theory further on future Station missions with different samples."
Above is a photo of the Expose-E module placed on the International Space Station's European laboratory module Columbus. Expose allows long exposures to space conditions and solar-UV radiation on the International Space Station. Several trays filled with organisms were installed on the outside of the European Columbus laboratory.