[meteorite-list] Contact! - OT - ish

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Wed Feb 1 14:49:18 2006
Message-ID: <A8044CCD89B24B458AE36254DCA2BD070B1766_at_0005-its-exmp01.us.saic.com>

Hi Mark,

> N = N* fp ne fl fi fc Fl (The Drake Equation)

I've always enjoyed jiggering with the numbers in the Drake
equation; unfortunately, most of the parameters are completely
unknown and so whatever value you choose is a complete guess.

Here's my w.a.g. at parameter values (vs. yours in parentheses):

N* represents the number of stars in the Milky Way Galaxy
N* = 500 billion (100 billion)
(Btw, that's American billion, not British billion). The actual
number of stars in the Milky Way is certainly at least 200 billion,
and could be over a trillion.

fp is the fraction of stars that have planets around them
fp = 50% (60%)

ne is the number of planets per star that are capable of sustaining life
ne = 0.1 (0.33)

fl is the fraction of planets in ne where life evolves
fl = 20% (10%)

fi is the fraction of fl where intelligent life evolves
fi = 1% (5%)

fc is the fraction of fi that communicate
fc = 5% (10%)

fL is fraction of the planet's life during which the communicating
civilizations live.
L = 5000 years (L = 1000 years)

You didn't indicate the average lifetime of the planet, but reverse
engineering your answers suggests that you assumed 10 billion years
(roughly the earth's expected lifetime). I guess planetary lifetime
is intimately tied to stellar lifetime, which of course varies a
great deal depending on star type. Since the majority of stars in
the Milky Way are red dwarfs, I would heavily weight stellar (and
thus planetary) lifetime toward the red dwarf lifetime -- around
100 billion years. So I'll say 50 billion years. So you and I still
end up with the same fraction (5000/50 billion vs. 1000/10 billion).
fL = 1E-7 (fL = 1E-7)

N = 0.25 (N = 1)

So we're within an order of magnitude of each other. The main factor
affecting the outcome is the lifetime of a communicating civilization.
Suppose that once a civilization becomes advanced enough to communicate,
it doesn't die until its star does? Then fL could be a million times
greater...

--Rob
Received on Wed 01 Feb 2006 02:48:46 PM PST