[net.origins] Periodic extinctions?

dmr@research.UUCP (10/26/85)

Pam Pincha-Wagener wondered about the status of the proposal that mass
extinctions are periodic (P ~ 26 Myr), and might have physical
explanations.  (The two new ideas were that the Sun oscillates about
the galactic plane, and the extra mass there might disturb the Oort
cloud of comets; or that some dark body ("Nemesis") might be orbiting
the Sun, and on its closest approaches might likewise perturb comets
that then hit the Earth.)

The periodic-extinction hypothesis seems a lot less popular than it was
last year.  Hoffman had a letter in Nature (315 p. 659 20 June 1985)
that challenged the apparent periodicity itself, mainly by arguing that
the data were too thin to identify extinction events well, and the
events too poorly dated anyway.  This article seemed persuasive enough
to the editors of Nature that the major "News and Views" article in
that issue was entitled "Periodic extinctions undermined."  On the
other hand, Science (229 p. 640 16 August 1985) had a counter-editorial
"Catastrophism Not Yet Dead" pointing out some arguments against
Hoffman's approach (but really, I think, saying that Nature's
editors went too far).

I've also seen articles attacking the proposed mechanisms; one, I
recall, said that a 26 Myr-period companion would not be stable enough
against nearish approaches by random stars to last the 260 Myr time the
"periodicity" has been followed.  Thaddeus and Chanan (Nature 314 p.
73 1985) claimed that the concentration of matter in the galactic
plane is not great enough to cause the required extra perturbation
(i.e. the gradient is too small).

It's an interesting hypothesis, but the evidence for it is not overwhelming.

	Dennis Ritchie