Dear Dr. SETI:
What are the current estimations of fraction of stars with
planets and mean numbers of planets favourable for life, and
how much uncertaintity is there in current estimations?
PT (not Barnum)
The Doctor Responds:
Roughly 10% of all stars are single F, G, or K class main
sequence stars of about the right age for planets. We
consider these the best candidate stars for habitable
planets, and in fact the targeted searches (such as SETI
Institute's Project Phoenix) concentrate on such stars in
developing their target lists.
Although we are just beginning to learn how to detect
planets, about everywhere we look we find them. So I'd
say fp is pretty close to 10%.
Of about a dozen extra-solar planets detected so far, only
one has been clearly in the habitable zone of its star.
So for ne, the number of Earthlike planets per solar system,
I'd have to estimate only about 0.1. This does not consider
the possibility of habitable moons.
Both of these estimates are highly uncertain, because our
sample size is pitifully small. But we expect habitable
planets to be rather commonplace throughout the cosmos
(that's not a very specific answer, but an encouraging one).
See Solving the Drake Equation (next) for a further
perspective on this question.
Solving the Drake Equation
Dear Dr. SETI:
I read about the Drake Equation, but I find itīs pure
guesswork to predict anything specific about the number
of existing technological civilizations in our neighbourhood.
I guess it is rather a matter of belief, and one guess can
be about as good as another even if they ar wildly different.
Am I completely wrong about that?
The Doctor Responds:
Not at all, Jan! The importance of the Drake Equation is
not in the solving, but rather in the contemplation. It
was written not for purposes of quantification at all,
but rather as the agenda for the world's first SETI meeting,
in Green Bank WV in 1961. It was quite useful for its
intended application, which was to summarize all the
various factors which scientists must contemplate when
considering the question of other life.
It is interesting that when the Equation was first written,
the only factor which we had any basis to estimate with
reasonable accuracy was the first, the rate of stellar
In the four decades since, we have learned something about
extra-solar planets, so we can now estimate the second factor.
We also have direct evidence as to the existence of two
planets in their star's habitable zones, so (though the
sample size is small) we can roughly estimate the third
factor. At this rate, we should have three more factors
nailed down by the middle of the next century! A value for
the seventh will probably always elude us.
For the record, I consider the Drake Equation to be a
marvelous tool for quantifying our ignorance. But in case
you'd like to have a go at solving it, here's a downloadable spreadsheet
which will let you plug in your own estimates
for each of the seven factors, and see what results from your assumptions.
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