There Have Probably Been Aliens

I see these mentioned a lot, and sci-fi is big on the latter. The one I rarely see come up is "entertainment."

If we were traveling amongst the stars today, with our values and morality as they currently are, and happened upon a world of what we'd consider to be primitive life, we'd probably scoop some up and put them in a zoo, or broadcast them on TV. We'd want to study them, learn what makes them tick, maybe try to see if communication was possible, but in the end, we'd get entertainment value from them.

Even if we left them be and studied from a distance, the human race would be fascinated. New life! A new place! We like new things. The news would cover these creatures ("Are they a threat to humanity? News at 11."), they'd be woven into stories and shows and movies in one form or another. If we could safely bring them back, people would want to see them in person.

We'd get a lot of entertainment value out of these new creatures. We're a curious race who manages to get bored of spectacular things quickly, which is a large reason we continue to innovate and push forward.

There's no way to know for sure what would motive another intelligent race to advance to the point where they'd reach for the stars, but if any of them shared a degree of curiosity, I doubt they'd pass us up if they found us. We'd be something new for them.

Maybe we'd end up in their stories, their drawings, their videos, or even their zoos. Maybe they'd be interested in how we smell, or the way we dress and decorate ourselves, or the things that we watch on TV. Our languages, our fights, our culture. Or maybe just the way we go about our business, missing some big cosmological truth that's so obvious to them, as if we're adorable little children.

Maybe we'd provide just enough entertainment value to be worth a quick trip. Or a long stay.

I'm not sure you can draw the conclusion that since there is one intelligent (as defined above) species, out of many on earth, that intelligence is unlikely to develop.

The main issue I see is that there were other species that were close to as intelligent us, but we out competed them and they went extinct.

I wouldn't be surprised if there was a sort of "first one wins" effect where the first species to reach a certain level of intelligence quickly (compared to the time it takes to evolve that level of intelligence) comes to dominate the planet wide eco-system on a level far beyond every other species, thus preventing the development of other intelligent species.

    I think what will really help us to get at a better estimate
    is to answer the question whether there exists life in our
    solar system that evolved independently of that on Earth.

That's definitely the big question. But you left out the elephant in the room: is there life right here on Earth that's evolved independently? Abiogenesis doesn't simply halt after it's achieved its "goal". Perhaps we take the common-origin hypothesis for granted (that the "tree of life" has one root, and is in fact a tree), but it's actually nontrivial, strong evidence about the rate of abiogenesis, right under our noses. If it's true.

There's no hard reason why another, orthogonal family of life couldn't coexist with DNA/RNA life. We have 10^7 extant species of DNA/RNA life; that's an existence proof that hostile, competing "types" of life can coexist. Suppose we found terrestrial "XNA life" with a biochemistry totally unrelated to anything else, with no possible common origin or lateral gene transfer. Then we would know life on Earth developed independently N >= 2 times, instead of N >= 1. That would change a lot; we could update P(life | Earth-like planet) from the subjective probability range [10^-Graham's number, 1) to something more like [10^-4, 1).

What's really surprising is that this is completely untested! We genuinely don't know if there's "alien" life on Earth, because we don't know how to look for it. There's no black-box tool to approach unknown biochemistry:

    Some have postulated the existence of a 'shadow biosphere'
    on Earth, teeming with life that has gone undiscovered
    because scientists simply don't know where to look. It could
    contain life that relies on a fundamentally different
    biochemistry, using different forms of amino acids or even
    entirely novel ways of storing, replicating and executing
    inherited information that do not rely on DNA or
    proteins. [...] The trick is deciding what to look for and
    how to detect it. The usual way that researchers search for
    new organisms — by sequencing DNA or RNA — will not pick up
    life that does not depend on them.

http://www.nature.com/news/life-changing-experiments-the-bio...

    ...We could even be oblivious to unfamiliar forms of life
    right under our noses...

http://www.sciencemag.org/news/2015/02/shadow-biosphere-migh...

In estimating a probability for the occurrence of life in the universe, the authors essentially claim to know the probability of their own existence.

For me, that's a red flag. I can't know the probability of a phenomenon which my existence depends on just by observing my existence alone, because I can't observe all of the cases where the phenomenon does not occur and I do not exist.

It seems that the probability of life appearing (in some form), given the ideal conditions, is higher than that of obtainiing the ideal conditions.