Earlier posts in this blog have postulated that life does not exist just on earth but is rather widespread across the universe albeit in planets too far from each other to make contact, it further postulated that life spreads across space through comets as bacteria and viruses, the only form of life that may survive as frozen life in the long journey from one planet to another and that comets may be formed as fragments of life bearing planets.
Once such bacteria arrive on a planet that has sunlight, temperatures in the appropriate range and gravity enough to hold an atmosphere etc. for life to exist, then life begins to unfold slowly on the planet over billions of years. True such planets would be rare but seeing that the universe is a virtual infinity of stars they are likely to be many. The earlier articles were written more than eight years ago and can be found as early posts in this blog. Just check out the archives in the left sidebar. Since then new discoveries have substantiated this theory and none has gone against it. The most recent is discoveries of the Rosetta mission. It has found oxygen, water and carbon on the comet, all indicating that this is a fragment of a planet that had life on it.