Lisa may enjoy this equation from Ward & Brownlee. You can also flip through the book on Google-- check it! Rare Earth: Why Complex Life is Uncommon in the Universe
Now, for the equation:
# Rare Earth Equation (revision of Drake Equation)
N = N* x fp x fpm x ne x ng x fi x fc x fl x fm x fj x fme
where N* = stars in Milky Way,
fp = fraction of stars with planets,
fpm = fraction of metal-rich planets,
ne = planets in star's habitable zone,
ng = stars in Galactic habitable zone,
fi = fraction of habitable planets where life arises,
fc = fraction of planets with life where complex life (eukaryotic-type cells) arises,
fl = percentage of a lifetime of planet that is marked by presence of complex life,
fm = fraction of planets with large Moon,
fj = fraction of planetary systems with Jupiter-sized planets,
fme = fraction of planets with critically low mass-extinction events
* as any term in equation approaches zero, so does final result
* Some of the terms may be interdependent -- a Moon may be necessary for the development of complex life, or the presence of a Jupiter may make mass extinctions less common. This form of the equation assumes no such connections exist!
* Remember that all our conclusions are based on one life-bearing planet; a Moon may turn out not to matter at all.
O yes, also the question is how rare versus how unique! Very different. I'm fascinated by this book because of the assumptions that are made that are necessary for the question to even be VALID.
Something to boggle your brains even further (on a Friday!): extremophiles
Gawd I hope I catch me a scientist sooner than later :)