Even if the environment of a planet could be altered deliberately, the feasibility of creating an unconstrained planetary environment that mimics Earth on another planet has yet to be verified. Mars is usually considered to be the most likely candidate for terraforming.
Besides, how long would it take to terraform a planet?
Chris McKay in one of his papers estimates 100,000 years for the complete process. If you look at the rate at which plants and algae can create oxygen then you can come up with a far more optimistic centuries long scale for making an oxygen rich atmosphere once you release the CO2.
Also, which planet would be the easiest to terraform?
How is Mars terraformed?
Mars is thought to have been warm in the past (due to evidence of liquid water on the surface) and terraforming would make it warm again. At these temperatures oxygen and nitrogen would escape into space much faster than they do today.
Can we breathe on Mars?
Carbon dioxide atmosphere By comparison, Mars' atmosphere is 95 percent carbon dioxide. "We need to breathe oxygen," said Lee. "There's no free oxygen in the Martian atmosphere. You cannot breathe this gas.