  The hypothesis of a companion object (Nemesis) orbiting the Sun was motivated
by the claim of a terrestrial extinction periodicity, thought to be mediated by
comet showers. The orbit of a distant companion to the Sun is expected to be
perturbed by the Galactic tidal field and encounters with passing stars, which
will induce variation in the period. We examine the evidence for the previously
proposed periodicity, using two modern, greatly improved paleontological
datasets of fossil biodiversity. We find that there is a narrow peak at 27 My
in the cross-spectrum of extinction intensity time series between these
independent datasets. This periodicity extends over a time period nearly twice
that for which it was originally noted. An excess of extinction events are
associated with this periodicity at 99% confidence. In this sense we confirm
the originally noted feature in the time series for extinction. However, we
find that it displays extremely regular timing for about 0.5 Gy. The regularity
of the timing compared with earlier calculations of orbital perturbation would
seem to exclude the Nemesis hypothesis as a causal factor.
