The Sun's motion perpendicular to the galactic plane

Abstract

The period and amplitude of the Sun's motion perpendicular to the galactic plane are important parameters in some explanations of the terrestrial mass extinctions and cratering records^1–5. Here we have calculated the range of periods and vertical excursions that are consistent with the distributions of tracer stars in the Galaxy and have also evaluated the probable phase jitter in the solar period. We find acceptable half-periods for the vertical oscillation that range from 26 to 37 Myr (including the range of periods that have been inferred from the terrestrial records on mass extinctions and on cratering), maximum heights above the plane from 49 to 93 pc, and an average phase jitter per half-period of the order of 6–9%. The largest uncertainty in all these calculations is caused by the unknown distribution of the unseen mass that must be postulated to explain the distribution of observed stars^6–7. For all the models we consider, the most recent passage of the Sun through the galactic plane occurred in the past 3 Myr provided only that the present position of the Sun is between 0 and 20 pc above the plane. We extend the argument of Thaddeus and Chanan⁸ to show that the apparent periodicity in the mass extinction and cratering records cannot be caused by an population of objects (observed or unobserved) that contributes a major fraction of the total mass density at the solar vicinity.