If we find life on another world, it will be an extremely important discovery and we will have to take great care not to do anything that might endanger that life. If the life we find is sentient we will have moral obligations to that life. Whether it is sentient or not, we have a duty to ourselves to preserve it as a study object, and also because it would be commonly seen as valuable in its own right. In addition to this we would also have a duty to our fellow humans and other earthly life forms not to expose them to danger by advertently or inadvertently exposing them to potentially harmful space organisms. When space exploration turns into exploitation it will therefore be important to be able to show that a world that is up for exploitation is uninhabited before the exploitation starts. Showing that a world is uninhabited is, however, a different kind of task than showing that it is inhabited. The latter task can be accomplished through one positive finding but it is not clear how to go about the former task. In this paper I suggest that it is a gradual process asymptotically approaching certainty rather than a discovery in the traditional sense of the word. It has to be handled in two steps. The first is to connect degree of certainty with research setup. The second is to decide how certain we need to be. The first step is about the number, diversity and quality of observations. The second step is a decision we have to make based on the purpose of the investigation. The purpose and therefore the degree of certainty needed to establish that a world is uninhabited will be different for a world that is up for exploitation than for a world that is not. In the latter case it is only a matter of epistemic values. In the former case also ethical values have to be considered.