12% chance. The life doesn't need to be intelligent to count. Can be microbes on Mars or some exoplanet. Strong evidence means that a majority of scientists would agree that it's aliens and not something else (note: not consensus, just majority). The reason I'm setting this standard is that I don't think it will be realistic to find conclusive, incontroversial evidence. What counts as strong evidence: Biosignatures on an exoplanet, once the studies showing them have been replicated, confirmed, and accepted by the scientific community Similar level of evidence inside the solar system. A confident announcement by NASA can also count, as long as it's not "maybe we found aliens" but rather "we are confident that we found aliens" UFO evidence that has been shown to have intelligent extraterristrial origin (just being unknown is not enough), well-studied and accepted by scientists. What doesn't count: Some study showing what might be a biosignature on another planet NASA saying that they found something that might be associated with life on Mars or another planet. UFOs that are confirmed by the US government, but there's no significant positive evidence that it is, in fact, aliens Mexican aliens, unless there's some very significant new evidence released.