From: Tennessee Leeuwenburg (tennessee@tennessee.id.au)
Date: Mon Oct 10 2005 - 18:32:11 MDT
Great observation, but there is one further subtlety.
Have younger stars *always* been more likely to have planets around
them? For example, a million years ago, would there have actually been
fewer planets?
Obviously, a million (/billion/whatever) years ago, all the stars would
have been correspondingly younger. Assuming the same distributions we
see today, there would have been just as many young stars, ergo just as
many planets.
Either older stars once had planets, and they have since disappeared
through cosmic misadventure, or perhaps planets *per se* are a
relatively new phenomenon.
Which is it?
Cheers,
-T
> > 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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