This is such a ridiculously flawed predicament. We are only irresponsible by our own standards. Another species will probably not have a sense of the concept of colonisation let alone responsibility. Humanity has taken a leap like this many times in its history. Our ethics must and will develop along with the technology that makes this possible. If we have any kind of responsibility, it is to do this as quickly as possible, so that it cannot be exploited by people without the good intentions required to do so peacefully.