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In 1984, the planetologists David Raup and Jack Sepkoski announced that they had
identified a periodicity of about 26 million years associated with the 12 mass
extinction events that have occurred during the last 250 million years. Practically
simultaneously, a group of astronomers published a possible explanation for this
periodicity: the Sun is not an isolated star, but has a dark companion about 1 light
year away, dubbed "Nemesis", and when it passes closer to the Sun it
gravitationally perturbs the cometary nuclei in the Oort cloud, causing many to fall
towards the inner solar system. Some of these collide with planets on relatively
short timescales, and in the case of the Earth, cause mass extinction events.
Evidence for all of this came from the ages of terrestrial and lunar craters, and
their apparent temporal concentration near the epochs of the mass extinction events.
An intriguing yet disturbing hypothesis that is now questioned by a new an
statistically more rigorous work, published by Coryn Bailer-Jones, of the Max Planck
Institute for Astronomy, in the Monthly Notices of the Royal Astronomical Society.
Using a more sophisticated statistical analysis, Bailer-Jones re-examines the ages of
craters, finding a completely different result: there are no peaks in the cratering
history separated by 26 million years, but rather a slight but steady trend for the
cratering rate to increase over the last 250 million years, something which is yet to
be explained.
According to Bailer-Jones, the conclusions reached in the '80s were influenced by the
same starting hypothesis, essentially giving more weight to the aspects of the data
that would validate the hypothesis.
In reality there were also other hypotheses proposed, such as the idea that the
motion of the Solar System through the Galactic plane brought it periodically closer
to other stars, but because such events would not necessarily be periodic and now
that there is now evidence of associated peaks in the cratering history, the mass
extinctions can just as well be explained by random impacts by asteroids or comets.
This doesn't necessarily mean that the Sun doesn't have a companion. The analysis of
data collected by the WISE mission may give a definitive answer to this question. |
