From: Thomas Buckner (tcbevolver@yahoo.com)
Date: Mon Oct 10 2005 - 16:48:20 MDT
--- Dani Eder <danielravennest@yahoo.com> wrote:
> 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.
It is possible, although awfully improbable, that
we really are the very first-born. Someone has to
be! And even assuming civilizations like ours
don't self-destruct, there are many pitfalls
given the necessary time to evolve. It only takes
one supernova to clear out a big chunk of
inhabited space. For example, a supernova might
get the blame for the Ordovician extinction:
http://www.cnn.com/2004/TECH/science/01/08/earth.extinction.ap/
That one was 10k light years away, while one at
750 ly might have killed off the mammoths:
http://www.lbl.gov/Science-Articles/Archive/NSD-mammoth-extinction.html
Supernovae are rare historically but on an
evolutionary time scale, they can be dangerously
common (especially toward the galactic center,
where they're so common due to collisions that
life there may be just plain impossible, unless
it's visiting).
Tom Buckner
__________________________________
Yahoo! Music Unlimited
Access over 1 million songs. Try it free.
http://music.yahoo.com/unlimited/
This archive was generated by hypermail 2.1.5 : Wed Jul 17 2013 - 04:00:52 MDT