Also if may add a question I ponder is 'if' humanity makes it that far and we one day get to one of these planets and decide to colonise it do we have the right to do so? Look at it this way any planet that is suitable for colonisation would need a form of life that creates oxygen perhaps an alien type of fauna or alien micro organisms akin to algae. Now say we decide to colonise the planet we have essentially destroyed the planets chance at creating its own life giving birth to its own evolutionary ladder. Even if it's microbial or even primitive life do we have the right to take away that life's potential for evolving on its own without interference perhaps one day into not only sentiment but possibly intelligent life. Think of it as along the lines of Europeans moving into the Americas once we affected the natives way of life it was forever altered in some cases completely wiped out now imagine that on another world. Even if the 'life we find there is primitive or microbial a single human cell could be enough to contaminate the process forever altering what would ultimately evolve on that world.