All of the vegetation on the planet produced an "acid" harmful to non native lifeforms. Such an "Eden" would be a true hell for any would be colony.

Maybe finding a world in which life was just getting a foothold would be best, then we could place our own organisms on the planet before any native species had a chance of establishing themselves. The snuffing out of an embryonic planetary ecosystem might be viewed as unethical by some, but it could be the only choice we have.

As for Raoul's thought of life springing up given the slightest chance, that notion could just be wishful thinking on our part. It might be that easy but maybe, like the evolution of intelligence, if one step is missed the transition from non organic to organic won't happen no matter how favorable the environment. Only time and a great deal of exploration will answer that question.