[snarks_when_bored]

Well, somebody is reading this stuff!

I find myself in the camp that holds that even low-level, microbial life is pretty rare, while high-level, intelligent life is incredibly rare (I think it's quite possible that Earth holds the only instances in the entire Milky Way galaxy). But I wouldn't care to dispute heatedly with anybody who thinks otherwise, because we're still a ways from having enough information to draw sound conclusions on the question.

On the star travel question, I suspect that within the next few hundred years—provided something really awful doesn't happen—we're going to launch a probe towards Proxima Centauri. It'll be a long-term project, but perhaps we'll find some things of interest. Of course, that's assuming that we don't just innundate our local neighborhood with von Neumann probes, which I suppose could happen.

[snarks_when_bored]

A correction to my post #4. I used too large a value for the radius of the Earth, and hence my estimate of the volume of a 200-mile-deep strip of the Earth's surface was too large (I hate it when that happens!).

Using the roughly correct value of 3963 miles for the radius of the Earth, we have

BS = 37,513,386,749 cubic miles (new, smaller value)

PC = 1.131 x 10⁴¹ cubic miles (no change from earlier post)

and so

BS / PC = 3.32 x 10^-31 (new, smaller value)

or

BS / PC = .00000000000000000000000000000000332

Hence life occupies no more than

3.32 billionths of a trillionth of a trillionth

of the spatial volume in a sphere of radius 5 light years centered on the Earth.

Note that this is a considerably smaller fraction even than was found in post #4 (it's about 24% of the earlier, incorrect result).