From: Dani Eder (danielravennest@yahoo.com)
Date: Mon Oct 10 2005 - 12:07:25 MDT
> By the way, did anyone else who has read the book
> also disagree with Ray's
> conclusions on the Fermi Paradox? Page 347
> specifically.
Until 10 years ago, one of the factors in the
Drake Equation, the number of planets per star,
was a complete unknown. Since then we have
detected about 170 extra-solar planets. This
is a large enough sample to notice something
interesting.
Astronomers call all elements above hydrogen and
helium 'metals'. Originally the universe consisted
almost entirely of H and He. Over time stars are
creating more of the heavier elements. The amount
of heavier elements a star has is called it's
'metallicity'. It is determined by looking at the
spectrum of the star, and scaled to the Sun's
level, which is normalized as 1.0. The interesting
thing is the likelyhood of finding a planet around
a star is strongly correlated with metallicity.
It is something like 30 times higher for high
metallicity stars vs. low ones.
High metallicity stars are generally younger, so the
implication is that there are not many planets
around old stars. Thus when considering the
Drake Equation, it would be incorrect to assume
a uniform number of planets over the history of
the universe.
This tends to make the hypothesis that we are an
early civilization more likely. Younger stellar
systems are more likely to have planets, but have
not had as much time for evolution to progress.
Older systems are much less likely to have planets.
Daniel
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