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 indepe

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-biological-higgs-1.10310 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-might-be-hiding-strange-life-right-under-our-noses http://www.sciencemag.org/news/2015/02/shadow-biosphere-migh...