[meteorite-list] Periodicity of Falls?

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:02 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C8692C5E73_at_US-Torrance.mail.saic.com>

Hi Sam,

> Mass extinctions tend to follow a 28 million year periodicity.

I've heard various figures, with several million year error bars,
but there does seem to be some evidence of periodicity. Naturally,
one would like to identify the "trigger".

> 1. A large hitherto unseen companion star to the Sun-the so called
"Nemisis star."

Actually, the proposed Nemesis star would have to be a red
dwarf or a brown dwarf. It would not be that difficult with
current technology to see such a star at aphelion (which is
about where the star would be for the current cycle). But
simply observing the star is only part of the battle; the
hard part is observing it for a long enough baseline to
determine that it is co-orbiting with the sun.

Phil Bagnall once e-mailed me that the proposed orbit of this
star would have to be so elongated that it would not remain
stable (and that the orbit could not have survived for the
duration of the Solar System). He may be correct -- I cannot
say without seeing the model assumptions and the resulting
data. A 26-million-year period requires an orbital semi-major
axis of 87,760 a.u. Assuming the orbit is extremely elliptical,
and the star is currently at aphelion, its distance would be
twice this, or ~175,500 a.u. This is about 2.8 lightyears --
more than half the distance to the nearest known star system.

Depending on the plane of the orbit relative to Alpha-, Beta-
and Proxima Centauri (and/or possibly Barnard's star, Wolf 359,
and the other 6 stars within 10 lightyears of the solar system),
the orbit might be unstable. At best it is chaotic long-term
because of the gravitational perturbations of the Centaurus
trinary system. But suppose perihelion lies between our solar
system and Centaurus. If so, it would take quite some time
(tens of orbits, more?) before the line of apsides would precess
to bring aphelion in the direction of Centaurus. If the Nemesis
period has remained at roughly 26 million years, then there hasn't
been time for even 200 orbits.

If I can get the masses and 3-D proper motions for the nearest
half-dozen or so stars, I'll try to throw together a numerical
integrator to see what happens to the various Nemesis candidate
orbits over time. (No doubt, a reinventing-the-wheel calculation,
but a fun weekend project none-the-less). --Rob
Received on Mon 22 Jan 2001 08:06:00 PM PST