Geoscience Reference
In-Depth Information
Stothers, had found. Although no one knows exactly what effect the
crossing of the galactic plane has, it could be that astronomical or
climatic changes are somehow induced, which, in turn, drive the
periodicity. However, the Sun is now close to the galactic plane, so
that there should have been a recent mass extinction, yet the latest
one that Raup and Sepkoski recognized occurred in the middle
Miocene, about 10 million to 11 million years ago.
The second explanation, proposed in two of the papers in
Nature,
is that the Sun has a small companion star. Because most
stars that we can observe are binary, the existence of a companion
would not be a surprise. The Sun's fellow traveler might be on a
highly eccentric orbit that takes it far out in space but periodically
brings it back nearer the outer boundaries of the solar system, where
lies the Oort cloud, a vast conglomeration of comets. Although no
one has seen this cloud, there is good reason for believing that it
exists and that it is the source of Halley's Comet and the other
"long-period" comets that approach the Sun from all over the solar
system. As the putative companion star passes near the Oort cloud,
its gravity could pull comets out of their present orbits and launch a
few on a collision course with Earth, where they would strike, pro-
ducing craters and mass extinctions. The astronomers who wrote in
Nature
agreed that the companion star must be quite small and now
be located about two light years from the Sun.
But why, since the buddy star would be closer to the earth by
half than any other, have astronomers never seen it? It turns out that
it could easily have been missed—only a small number of stars have
ever been observed and catalogued—or it might have been mistaken
for a brighter star much farther away. But would its orbit have
remained stable over the 250 million years of geologic history that
Raup and Sepkoski examined, or would it not have been degraded
by the gravity of nearby stars? One calculation showed that it could
have remained constant for as much as a billion years, more than
enough time.
The authors of one of the papers suggested that the companion
star be named Nemesis, after the Greek goddess who punished
earthly beings for attempting to usurp the privileges of the Gods.
9
In fact, they proposed other names, but the editors of
Nature
chose
Nemesis and it stuck. (Muller and his co-authors noted that if the
companion were never found, the paper claiming that it existed
might turn out to be
their
nemesis.) Muller launched a program to
search the heavens for Nemesis, using an automated telescope sys-
tem that examines about 10 stars per night. So far, over 3,000 can-
didates have been studied, but none has yet turned out to have the
