Extrasolar life statistics

[҉]

Everyone here's probably seen this, but I was researching it recently and it's pretty much the coolest thing I know so here it is again.


The Milky Way contains between 100 and 400 billion stars (the difference in estimates is because it's hard to tell how many mostly-dead dwarf stars there are; they're hard to detect). Let's go with the low estimate, 100 billion.
Most of those stars have a system of planets of some sort: there are more extrasolar planets than there are stars, but to make things easy let's say there are the same number, because 100 billion is a really nice number.
Let's assume that life requires really specific conditions, that a planet that can support life has to be almost exactly the same as Earth and has to orbit a star almost exactly the same as Sol. There's no reason whatsoever to assume that. Life is probably way less picky, but we're gonna go with it. The Milky Way contains perhaps 11 billion planets that fit these strictest requirements.
Now, not all of these planets necessarily have life, and not all of that life is necessarily sentient, especially given that there's probably a high likelihood of sentient species annihilating themselves pretty soon after obtaining spaceflight-level technology. Let's guess that maybe one in a million of these planets actually hosts a spacefaring sentient civilization right now.
That gives us an extremely rough estimate of 11,000 sentient species with technology similar to our own, out there right now, in the Milky Way alone. There are perhaps a hundred billion other galaxies in the observable universe, which may represent as little as 1/10^23rd of the entire universe. Alien life exists. Given the absolutely absurd number of places where it might, the chance that it doesn't is functionally zero. We'll just never contact it.

[CMA]

I take statistics, it's impossible to say anything until we actually find extra-solar life. Everything is just speculation at this point, it's fun, but meaningless.

[Professor Fenway]

We may find life at some point outside our own, but it will be very difficult to find and statistically impossible.

Finding a space-faring species outside our own would be like achieving successful and reproducible cold fusion in the next 10 years. We know it happens, we just can't do it ourselves.

[CommanderKobialka]

Wasn't it just recently that NASA estimated that 1 in 5 stars with planets have at least one in the Goldilocks zone?

[Error]

Something like that. Failing everything else, they might make nice colony worlds.