No. 33: May-Jun 1984
As reported in SF#31, the geological record seems to show that widespread biological extinctions have occurred about every 26 million years. Coupled with this is Walter Alvarez's recent observation that terrestrial impact craters 10kilometer-diameter and up have been blasted out episodically -- every 28.4 million years on the average. This figure is close enough to 26 million years to impel some astronomers to search for a periodic source of cosmic projectiles. R.A. Muller and M. Davis, at Berkeley, think they have found one. They postulate that the solar system is really a double-star system. Our sun's companion star has only about 0.1 solar mass and is so faintly luminous that we have not found it visually. It does, however, now cruise along its orbit some 2.4 light years away. But it will be back! In fact, it returns every 26 million years to jostle the Oord Cloud of comets that hovers on the fringe of the solar system. This nudging periodically sends a large shower of comets careening around the inner solar system. The earth intercepts one or more of these projectiles each visit and -- bang -- we have new craters and another biological catastrophe.
(Anonymous; "A Star Named George," Scientific American, 250:66, April 1984.)
Comment, Ho hum! Still another cometary impact scenario. Ignatius Donnelly was pretty convincing in this matter a century ago.