[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).

[Rudder]

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

In a sphere?

Howsabout just calculating surface as being the most hospitable habitat for life, as it is here, on earth?

[Rudder]

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.

And no less...

[boris]

Rare Earth: Why Complex Life Is Uncommon in the Universe -- by Peter Ward, Donald Brownlee

Makes the argument (IMHO very persuasively) that "life is common but intelligent life is exceedingly uncommon").

Basically their method is to look at all of the 0.0001 probabilities that led to us and throw all those factors at Sagans "billions and billions". Some examples:

--Planet in habitable zone
--Planet with lots of liquid water
--Planet with a large, prograde moon to stabilize its axis
--A gas giant (Jupiter) far enough out not to perturb planet's axis--but close enough to sweep up all the debris which would otherwise destroy nascent life
--Events such as the "Snowball Earth", which filtered 2% of 2% (my estimates) of all life through the event(s), meaning that all life on Earth evolved from .0004 of species previously existing--in only about 600 million years(!)

And so on.

My WAG at present is that there are >1 and <10 intelligent species in our Galaxy--which makes us essentially alone.

Then there is the Fermi Paradox...

--Boris