Abstract

Avi Loeb has defended the hypothesis that the interstellar object, ‘Oumuamua, detected in 2017, is in fact an extraterrestrial artefact. His hypothesis has been widely rejected by the scientific community. On examination however it is not clear why. The puzzle is at the level of argument structure. The scientific community's responses to Loeb's hypothesis appear to point to explanations of ‘Oumuamua's properties that are mere possibilities. Yet this is something that Loeb does not contest. I appeal to broadly philosophical considerations to understand and bolster the response to Loeb. These considerations concern the structure of his argument, the role of prior confidences within it, and the presence of ‘unconceived alternative’ explanations. I then generalise. ‘Oumuamua will surely not be the last object that does not admit of straightforward natural explanation and that is claimed to be evidence of an extraterrestrial artefact. I use the preceding discussion of Loeb's argument and the scientific community's response to make some general remarks for future debate about similar cases.