[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

NEMESIS - Part 2 of 4

WEISSMAN P.R. (1990) Are Periodic Bombardments Real? (Sky & Telescope,
March 1990, 266-270) - The Death Star, pp. 267-269:

An alternative explanation for the periodicity was suggested by Michael
Rampino and Richard Stothers (NASA-Goddard Institute for Space Studies)
and independently by Richard Schwartz and Philip James (University of
Missouri). They noticed that the Sun's so-called epicyclic motion, its
oscillation above and below the plane of the Milky Way, has a
half-period of 31 to 33 million years. Could something happen to the
solar system each time it passes through the galactic plane? Rampino and
Stothers proposed that close encounters with giant molecular clouds
might perturb the Oort cloud enough to send showers of comets toward the
planets.
However, Schwartz and James had some doubts. They noted that the Sun and
planets passed through the galactic plane only about 1 million years
ago, while the most recent major extinction identified by Raup and
Sepkoski was about 13 million years ago. Thus, they concluded, whatever
physical process caused extinctions might be strongest when the solar
system was at its maximum height above or below the galactic plane,
though Schwartz and James couldn't say just what that phenomenon was.
Despite this uncertainty, Rampino and Stothers went further. They
reanalyzed Raup and Sepkoski's extinction data and found that a period
between 31 and 33 million years gave a better fit. But their result only
increased the doubts of skeptical scientists. If two groups could find
different periods from the same data, then any periodicity was probably
spurious.
As this debate continued in early 1984, Nature published another paper
of particular interest. Walter Alvarez and Richard Muller had examined
the record of terrestrial impact craters for evidence of periodic
bombardment. They worked from a list of about 100 features compiled by
Canadian geologist Richard Grieve, but only used the 16 with the most
accurate ages. Sure enough, there was a periodicity, though it was
slightly different again - 28.4 million years.
Could this result also be a statistical fluke? Grieve thought so. He
generated random sets of crater ages and found that some 10 percent of
the cases yielded periodicities about as significant statistically as
the one found by Alvarez and Muller. Rampino and Stothers also analyzed
a similar list of craters and found a periodicity of 31 ± 1 million
years. Thus the alleged regularity in the crater ages became as suspect
as that claimed for extinction data from the fossil record.

Deepening doubts

Even before the papers announcing the new hypotheses appeared in print,
astronomers and geologists were examining the theories' broader
implications. For example, if the Sun did indeed have a faint companion
it would alter our ideas about the history of the solar system and, in
particular, of the Oort cloud.
Half of all the stars we see are binary or multiple systems, but pairs
with separations as great as that suggested by the Nemesis hypothesis
are exceedingly rare. The 26-million-year period implies an orbital
semimajor axis of 88,000 astronomical units, or 1.4 light-years!
The first problem is the orbital stability of the Sun's hypothetical
companion. Its path, like those of comets in the Oort cloud, would be
perturbed by passing stars. However, the attendant's orbit is far larger
than those of typical cloud members, so the gravitational bond to the
Sun is even weaker - the star would be expected to escape in about 600
million years. Hut's computer simulation showed that the companion might
stay bound for as long as 3.2 billion years, but the odds were against
it.
Hut's work also demonstrated that stellar perturbations would cause
Nemesis' orbital period to vary widely; each time around the Sun its
period would change by about 10 percent. This variation actually fit one
feature of Raup and Sepkoski's extinction data, which indicated that the
"period" changed by roughly that amount between successive dyings.
However, the dwarf's orbit would tend to "random walk" away from the
initial 26-million-year period, with less than a one-in-four chance that
it would still be close to that value after 10 orbits.

----------
Archives located at:
http://www.meteoritecentral.com/list_best.html

For help, FAQ's and sub. info. visit:
http://www.meteoritecentral.com/mailing_list.html
----------