@peter jeffrey cobb,
The odds that the bacteria find a planet in the first place and not drift between the stars and their respective solar systems is the first main nearly unsurmountable obstacle.
The odds that the planet if ever found is at the perfect range distance wise from the planet's respective star (not too close and not too far) is the second nearly unsurmountable obstacle.
Let's give these bacteria the benefit of the doubt and that they do stumble onto a perfect Goldie Locks ~just perfect from the planet's star
planet. An extinction level event can easily wipe out the bacteria before they can evolve past the single cell lifeforms: neighboring stars going supernova and irradiating the planet; the planet's own star sending out a killer solar storm; the planet's own tectonic and volcanic activity obliterating any atmospheric advances the bacteria has made over the lifetime of the colony of bacteria; unforeseeable events that remove the present water and/or ice from the planetary equation; etc...
The odds of a successful seeding of a planet are insanely small. And your timescale is off at least by millions of years. At that rate, the human race may have been extinct by a few million years. The bacteria need a very long time to completely create an atmosphere even remotely habitable for us humans.
And what if the bacteria evolve and inevitably change the atmosphere so the planet is somehow more inhospitable then before they arrived in the first place.
This is the most ambitious plan for the continuing survival of the an intelligent species. For a civilization to pull this off, the likelihood is that they are using this in terms of a very, very, very, very long term project: success is measured in tens of millions if not billions of years.
AS optimistic as I am regarding the technological advance of humanity in the immediate decades worth of our combined future, we as a species don't even give that much attention to the simple things that can wipe us off the planet. Just look at how small the budget is for extinction level asteroid detection and prevention.
Update on NASA FY2011 budget and NEO funding: Near Earth Object Observation (NEOO) Initiative
As you may recall when the NASA FY2011 Budget was released there seemed to be an increase in the funding for NEO specific work, from the approximately US$4M to about US$16M. Here is more detail.
That's M for million dollars not billion dollars and even if it was a typo. How much protection can an additional 16 billion dollars provide?!
Named after the late congressman, the George E. Brown, Jr., Near-Earth Object Survey section of the 2005 NASA Authorization Act called upon the space agency to detect, track, catalogue and characterize the physical characteristics of at least 90 percent of potentially hazardous NEOs larger than roughly 460 feet (140 meters) in diameter by the end of the year 2020.
Study the two most important numbers in this paragraph. There is a long time between 2011 and 2020.
For arguments sake, let us say nothing happens in this time span of 9 years.
1. Whose to say that by this time, the always short sighted dumbasses in Congress have not completely pulled the project's funding and the system already up there in space is either in need of maintenance, never worked in the first place, or completely dismantled to resell the recyclable materials, etc....
2. We know 90% of the thousands and thousands of orbiting asteroids and comets in our system. We are still missing a large 10% of ELO (extinction level objects) that are arbitrarily flying death traps in the sky. All it takes is 1 rogue asteroid....
3. I had another point but I already forgot it.
This plan is great fodder for science fiction. It's a great plan for other uberintelligent and well protected alien civilizations but for the human race? We need something done in a more timely fashion.