Re: ET AGI is only a marginal concern

From: Dani Eder (
Date: Wed Jul 20 2005 - 15:28:37 MDT

Former astrophysics major jumping in here.

The number of galaxies in the universe is much too
small if the inflationary hypothesis is correct.
The _visible portion_ of the universe may be on
the order of 10^11 galaxies, but the total number
would be much larger.

As for number of planets per star, we now know of
about 150 extra-solar planets. The ones we have
found so far suffer from selection bias. One method
of finding them uses shifts in spectral lines of
a star due to the gravity of a planet making the
star move around. When the star moves back and forth
the Doppler effect shifts the wavelength of the
spectral lines. This technique tends to find
close, heavy planets, since they produce larger

Another method detects the drop in light from a
star when a planet passes in front of it. This
method is also biased - towards close, large planets.
Close means the orbit is smaller, and hence more
likely to generate a transit (when a planet passes
in front of the star), and large means a larger
fraction of the star's light is blocked, which is
easier to find.

A rough idea of the number of planets per star should
be predictable from the number of planets found so
far (~150), correcting for the biases in the
observational method, the number of stars observed
so far, and the size-scaling relationship for objects.

The latter is the observation that as you go from
large stars to small stars in the local neighborhood
(where we have good statistics for the stellar
population) the number of objects scales as about
the -3.5 power of the mass (many more small stars),
and in our solar system the objects from Jupiter
down to small asteroids follows a similar

Given that, you should be able to project the numbers
of small planets from the number of Jupiter and
larger planets we are finding right now.

I have not seen such a projection, but it may have
done already, and I would expect the figure to be
good to within a factor of ten given the fairly large
number of extra-solar planets found so far.


> On Tue, Jul 19, 2005 at 01:26:49PM -0700, Phillip
> Huggan wrote:
> > 10^11 and 10^11 are the figures for the # of stars
> and galaxies in
> > the universe. Ignoring earlier 1st generation
> stars which tend to
> > be very metal poor, and marginal brown dwarf star
> surfaces and
> > radioactive asteroid cores, there are ten planets
> in each stellar
> > system capable of facilitating the emergence of
> life.
> Umm, what?
> Last I checked, we had no idea whatsoever how many
> planets the
> "average" stellar system has.
> -Robin
> --
> ***
> Reason #237 To Learn Lojban: "Homonyms: Their
> Grate!"
> Proud Supporter of the Singularity Institute -

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