This hypothesis was mentioned by Carl Sagan in the 1985 book "Comet" and first raised by Rampino and Stothers in a 1984 issue of Nature. It is still very controversial, but so was the Dinosaur/Impact hypothesis until the early 1990s.
See also:
American Scientist article Perturbing the Oort Cloud (new URL) by Michael Szpir, Jan/Feb 1997 issue.
Astrophysical Journal letter Tidal Imprint on Oort Comet Cloud by Matese & Whitmire (subscribers only).
The recent announcement of a possible 10th planet amongst the Oort cloud raises another possible source of periodic impacts. However, if such a planet does exist its orbital period is likely to be less than 10 million years.
Nemesis: Origin of the theory by Richard Muller
CCNet: PERIODICITY IN THE TERRESTRIAL RECORD by Bill Napier
Royal Astronomical Society of Canada Dec 98: Impact cratering through geologic time by Gene Shoemaker (3Mb PDF). Gene argued that we are in a peak for comet impacts and the Australasian Tektite event is an example of this peak.
5 Jul 05 NewSci (subs): Mass extinction theory on the rocks
The Oort Comet Cloud and Its Interaction With Our Milky Way Galaxy - John Matese
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Last update 16 Feb 2001
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