Are periodic mass extinctions driven by a distant solar companion?

A model in which the 26-Myr mass extinction cycle of Raup and Sepkoski (1984) is associated with the orbital period of a solar companion star is investigated. The required semi-major axis is about 88,000 A.U., or 1.4 light year. Its highly eccentric orbit (e greater than about 0.9) periodically brings the companion into the dense inner region of the comet cloud where it perturbs the orbits of large numbers of comets, initiating an intense comet shower in the solar system which results in several terrestrial impacts of a period of 100,000 to a million years. The companion probably has a mass in the black dwarf range of 0.0002 to 0.07 solar masses, depending on its eccentricity and the density distribution of comets in the inner cloud, and is potentially observable in the infrared.