Depends drastically on how easy genesis of 'living' systems is, and how likely you then are to get living systems that deal with their environment and each other in a complicated enough way to call 'intelligent' or 'sentient' (though I'm not even sure what that really means, social primates are just one of such a huge space of things...).

The first point is complicated by a LOT of things. One, with only one biogenesis to look at so far (or at least only one that has survived on Earth) we might miss types of life possible in very different environments or on very different substrates. Two, we have not reliably excluded life from the rest of THIS solar system. Promising places thus far not terribly well investigated: cloudtops of Venus, Martian soil (as opposed to rock) and deep geology, interiors of icy bodies, surface/subsurface of Titan, rings of Saturn. That being said, literally the oldest rocks ever found on Earth show signs of biochemistry. It probably happened FAST, suggesting it's not that hard in the right conditions.

Another thing to note is that even if the genesis of a biosphere occurs somewhere, it doesn't necessarily mean that the self-replicating things will become a major obvious part of that area. Earth is basically the best place in the solar system to be a living thing, for a remarkably simple reason: it combines high energy gradient with high availability of many different building blocks. It is the only place where you get huge amounts of light - a kilowatt per square meter! - hitting a rocky surface where liquid water falls from a sky of nitrogen and CO2. Those gases are all wonderful simple molecule feedstocks for building up into complicated molecules in the presence of an energy gradient. And on top of even that, you have the minerals and metals of the Earth rock itself burping up to that surface in volcanoes and eroding into the seas from continents, enriching it even further with all kinds of catalysts and cofactors and phosphates that you need for fast life.

Venus has even more light and gaseous small-molecule feedstocks, but the height at which organic molecules are stable is so far above the ground that minerals and metals hardly make it there if at all, making them the limiting factor by far. Titan has organics and small molecules in spades, and probably minerals dissolved in its volcanic water, but the energy input to the surface is either in the form of ridiculously dim light (~0.1% that of here, after the clouds) and an extremely thin rain of complicated, reduced organic compounds from the sky made only by the fraction of the weak sunlight that is UV interacting with methane in the air. Mars has minerals and sunlight in spades, but more radiation, potentially icky soil chemistry, and not as much in the way of atmospheric small-molecule feedstocks (those that exist are probably underground, away from the light). We have not excluded the possibilities of biospheres in these places, but if they exist they have to be just hanging on rather than being major constituents of the geochemistry like on Earth.

Alright, time for the bullshitting ass-pull portion. My max likelihood position goes something like this:

• Low-biomass biospheres may be semi-ubiquitous but extremely difficult to find remotely without close examination. I would not be surprised at one existing elsewhere in our own solar system; I give that even odds.

• Big biospheres like ours are dependent on the confluence of factors I mentioned above, plus possibly climactic stability. I've seen simulations to the effect that our solar system is more dynamically stable than 99.5% of all possible random arrangements of planets, but that doesn't necessarily mean much - anything that sticks around a long time is obviously stable and unstable arrangements don't last long, and there may be constraints on the way systems can form. That being said I would not be surprised at large biospheres like ours being rare, like 0.1% or less of all biospheres, especially considering the recent explanet evidence for unstable solar sytems and chaos after planetary formation.

• At this point I start pulling things out of my ass and stop using hard numbers about the commonness or lack thereof of big complicated 'multicellular'-analogue life, which we really still don't quite understand the origin of as much as we like (and is actually one of my long-term research interests).

• I then begin making weird noises about how the existence of other extremely intelligent things on earth (mostly chordates and molluscs) suggests that it ain't that hard once you have complicated things that interact with their environments in a proactive way.

• I then note that ubiquitous complicated tool use seems to only have happened once (the activity of certain birds I don't think really counts, those are all quickie improvised things) and may be THE limiting step, seeing how it seems to have come from an interesting evolutionary feedback loop.

Between all these an my own suspicion that tool-using symbolic life lasts on average into the megayears at least once it is founded, when I give it a go I usually wind up calculating perhaps a hundred or two long-lived clades of symbolic tool-users in a large stable galaxy like ours, separated by thousands of light years on average. We won't be speaking to them. Even if there were 10,000 in our glaxy, the closest would on average be nearly a thousand light years away.

Upvote. The Drake Equation and SETI seem at least as relevant as, say, Pascal's Mugging. GIGO, sure, but a standard dismissal in statistics is to say there's not enough data, more research needed. Isn't this where Bayes is supposed to win over frequentism, when it comes to imperfect or incomplete information?

Babyeater FAI would be very different, but could still give us big hints on how to make human FAI. It's the standard science process, instead of reinventing the wheel, stand on the shoulders of giants and learn what other smart people who've come before have figured out.