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I'm going to be off the net for a few months (spending the summer
in sunny Santa Barbara).  I thought I'd toss one more article into
the mill before my departure.
     There has been a recent proposal that our sun has a companion
star.  Naturally such a star must be inconspicuous and therefore
small and dark.  The motivation is as follows:
    Mass extinctions tend to occur with a definite periodicity of
about 26 million years.  This is a claim advanced by someone at U. Chicago
(sorry, I can't remember the name) who is a reputable paleontologist.
Besides the evidence for a large meteoritic impact close to the time
of dinosaur extinctions there is also an analysis of the ages of large
terrestrial impact craters which shows that the ages of the craters tend
to cluster around the times of the mass extinctions.  The number of
craters involved in the analysis is small.  The claim is that the 26m
year periodicity in extinctions is at least 260 million years old.
There does not seem to be any periodicity within the solar system which
could account for such a long period.  Estimates of intervals between
comet impacts give roughly similar numbers, but one would expect that
process to be randomly distributed in time, not periodic.  Therefore
if all this is true (no opinion from this onlooker) we need to invoke
an astronomical source of periodicity.
  The easiest way to do this is to suppose that every 26m years something 
disturbs the Oort cloud ( a region about 10,000 AU from the sun where comets 
come from).  Normally the stuff there just goes about its own business, never
getting very close to the inner solar system (where we live).  Every now
and then a passing star disturbs the cloud and a few comets come our way
(and some, presumably, decamp for the great beyond).  If something really
shook up the Oort cloud then a whole lot of comets would come at once, and
some of them would rain debris down on us (and on the other planets and
moons).
   There are two ways to do this periodically.  One is to suppose that this
has something to do with our orbit through the galaxy.  As it happens,
every 26m years we pass through the galactic disk (our sun bobs up and
down as it circles the galaxy).  Since this is just the right period that
would seem to be it.  However, *right now* we are passing through the
disk, and we are about midway between extinctions.  It seems that this
periodicity is about 180 degrees out of phase with the extinctions.
Since the density of perturbing material is highest in the disk it's hard
to imagine why it should be dangerous to be *away* from the disk.
    The second idea (which is the one that's gotten a lot of press recently)
is to imagine that the sun has a companion on a 26m year orbit.  At 
perihelion (closest to the sun) it lies in the Oort cloud and kicks up
some dust.  The rest of the time it is thoroughly insignificant.  The
main problem is the one that Randy Haskins pointed out.  Such an orbit
is too large to be particularly stable.  If the periodicity really goes
back 260 million years then this explanation appears unlikely.  Passing
stars should constantly perturb the orbit.  The period wouldn't be 
regular.  The star should miss the Oort cloud every now and then, or even
be expelled from our vicinity.
   This leaves us without any good explanations for the extinctions.  Maybe,
every 26m years the elves come out and sterilize the planet.

"Only perverts use cute signoffs"      Ethan Vishniac
                                       {ut-sally,ut-ngp,charm}!utastro!ethan