I believe there is an already an expert consensus currently that aliens for sure exist, in that it's highly improbable that they do not, depending upon your standard of evidence. If you use a deterministic argument, meaning, "have we directly observed Aliens?" then even the Scientific Consensus of the most high profile case, Allan Hills 84001 has been that, "morphology does not constitute evidence of biological life." However if you just take the size of the Universe, or even Galaxy, and the number of stars within a habitable zone of all known Galaxies (the band in which star density is not so great that solar systems don't get irradiated at a regular basis, wiping out amino acids), and the number of planets likely existing within the habitable band of those stars (the band at which liquid water can exist), it is literally an astrological number.

In the Milky Way galaxy alone, there is estimated to be several tens of billions of potentially habitable, Earth-size planets. There are roughly 2×10^11 galaxies in the Universe. So let's take the lower bound and say (2×10^11)*(1×10^10) = 2×10^21, which is a couple orders of magnitude under an Avagadro's Number of possible habitable planets in the Universe.

I could be off here, there might be a factor you have to add in for percent of Galaxies which are even habitable or something, but let's say, "half," then you would be still at close to an Avagadro's Number of potentially habitable planets.

So the probability that Earth is the only planet with life being 1/(~Avagadro's Number), let's say "A" is such an unfathomably low probability that 2/(~Avagadro's Number), "B" is roughly equal such that A = B.

Perhaps I'm rambling here, but, "expert consensus that aliens exist," is different from, "confirmed observations of astrobiology." No we don't have confirmed observations of astrobiology but I think there's already an expert consensus, no?

https://nap.nationalacademies.org/resource/25252/RH-astrobio.pdf

Exist or contacted?

@ICRainbow exist

@JonathanRay in more than a drake-equation way. Concrete evidence that some specific aliens exist.

@JonathanRay how about "observed"?