The problem is is that life on another planet is pretty much unfeasible because the colonists would eventually succumb to cabin fever and the like. The only planet with gravity similar to Earth's within conceivable reach is Mars, which has an unlivable surface because it's bombarded with enough radiation to kill a human being in seconds, so our people and our crops would be forced to live underground in caves. Terraforming isn't realistic or practical because Mars is too cold and the radiation would make living on the surface impossible to live on or grow crops anyways. The air pressure is too low as well, seeing as how the psi is similar to that of extremely tall mountains on Earth, and infinitely colder. Basically, until we discover faster-than-light travel, we will never colonize another planet.