@chris presumably this refers to humanity finding strong evidence of non-terrestrial life? because most people already expect non-terrestrial life to exist in the universe, it being so big and all that.

@Bayesian That's correct, we would both have to have credible evidence, and that evidence is widely believed in the scientific community that this confirms alien life.

Note: I'll be using the date the evidence was revealed to the world, not the date of discovery for the purposes of this market. If someone in a black ops somewhere knows there is aliens, but we don't, I don't count that, even retroactively.