Re: Extrasolar Planets vs Fermi Paradox

From: Dani Eder (
Date: Thu Sep 30 2004 - 09:06:15 MDT

Re: Fermi Paradox
> >Possibility: we're really the first.
> Given the extremely unfriendly planetary systems we
> have seen so far, this
> looks increasingly likely.

The extrasolar planets we have found so far are
a biased sample due to the way we have been looking
for them. We don't have much data relevant to
addressing the 'intelligent life is rare' answer
to the Fermi Paradox yet, but it is finally coming

The current methods used to find extrasolar planets
involve looking for a wobble in the star or a
dimming in the star's light. Astrometry measures
the side-to-side wobble of a star under the gravity
of a planet or brown dwarf. Spectroscopy measures
the doppler shift of the star's spectral lines
towards and away from us from the same cause.

Both methods are biased towards heavy objects.
Astrometry is also biased to objects in wide orbits,
since it produces larger wobbles in the star.
Spectroscopy is biased to small orbits, since it
produces larger doppler shifts in the star. Neither
method is capable of locating earth-mass objects
at present.

The third method involves looking for the dimming
of a star's light as a planet transits in front of
it. Transits are rare, since the planet's orbit
has to be aligned edge on from our point of view,
and even then they occur on the order of 1/1000 of
the orbit period. Therefore you have to monitor
a lot of stars for extended periods to find one.

Automated telescopes with big CCDs have only recently
(a few years) been able to do this kind of search,
and results are just starting to come in.

The fourth and most obvious method, direct observation
via imaging, is very difficult due to the glare of
the star. There is one disputed sighting of a
planet visually. There are telescopes and missions
planned in the next ten years or so that should
be able to succeed with visual searches.

There is a well known size distribution law in
astronomical circles both for stars and for smaller
objects in our solar system. It is observed that
the number of objects goes as some power (around
-2.5 if I remember right) of the diameter or mass.
That means there are lots more small objects than
large ones. The hundred or so extrasolar planets
found so far, when you correct for observation
bias and allow for the still large uncertainties
in their sizes, seem to be following a similar law.

This is encouraging, since it implies for every
planet we are finding in the 0.3-80 Jupiter mass
range, there should be multiple planets in the
0.0003-0.03 Jupiter (0.1 to 10 Earth) mass range.

In the next 5-10 years we should be able to assign
realistic values to the term f(p) [fraction of
stars with planets] in the Drake Equation. The
next term, [fraction of planets that can support
life], will take a little longer to move from
the stage of 'guess with no data' to 'estimate with
some data'.


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