From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Tue May 28 2002 - 02:54:57 MDT
Based on a stellar population of 200 billion stars and
a spectral type G abundance of ~4% [1] and an estimate of 3%
of Sun-similar stars being studied containing "Earths" [2]
the number of "Earths" in the galaxy should be 240 million!
Even if one assumes the galactic "habitable" zone is only
1% of the galaxy, that means the number of "Earths" should
be 240,000. Using the figure that 1/3 of these planets
evolve life [2] implies there are ~80,000 life bearing planets
in the galaxy. Of those planets, 75% of them (~60,000)
should be older than the Earth on average by a billion years [4].
So, either:
(a) the risks to the development and evolving of life due to
supernovas/comets/asteroids are very very high and cannot
be compensated for by evolution (questionable); or
(b) the galactic habitable zone is << 1% of the stars; or
(c) the evolution of intelligent technological civilizations
is very very difficult; or
(d) intelligent technological civilizations see little incentive
for galactic colonization; or
(e) intelligent technological civilzations universally reject
galactic colonization as a development strategy.
Robert
1. J. S. Lewis, Worlds Without End, Perseus Books (1998), pg. 153
2. R. R. Britt, "Other Worlds Not So Strange, Top Planet Hunter
Says", space.com, (14 May 2002).
http://www.space.com/scienceastronomy/astronomy/exoplanet_status_020514-1.html
3. Charles H. Lineweaver, Tamara M. Davis, "Does the rapid
appearance of life on Earth suggest that life is common
in the Universe?", astro-ph/0205014
http://xxx.lanl.gov/abs/astro-ph/0205014
4. Charles H. Lineweaver, "An Estimate of the Age Distribution of
Terrestrial Planets in the Universe: Quantifying Metallicity
as a Selection Effect", Icarus 151(2):307-313 (2001)
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