sojourner wrote:
Scientists need proof. They may universally agree that they may exist, but until there is proof, that's as far as they'll go.

Well, you could indirectly measure the number of aliens in the universe if you multiple the number of habitable planets in the universe by the probability of an abiogenesis process happening. If only we had any reasonable estimate about the latter...
And then if you multiple it by the probability of the evolution of sentience, you get an estimate about the number of alien civilizations. If we only knew that either...