What would convince you that aliens existed? The question came up recently at a conference on astrobiology, held at Stanford University in California. Several ideas were tossed around—unusual gases in a planet’s atmosphere, strange heat gradients on its surface. But none felt persuasive. Finally, one scientist offered the solution: a photograph. There was some laughter and a murmur of approval from the audience of researchers: yes, a photo of an alien would be convincing evidence, the holy grail of proof that we’re not alone.
But why would a picture be so convincing? What is it that we’d see that would tell us we weren’t just looking at another pile of rocks? An alien on a planet orbiting a distant star would be wildly exotic, perhaps unimaginably so. What, then, would give it away as life? The answer is relevant to our search for extraterrestrials, and what we might expect to find.