Orbital stability of the unseen solar companion linked to periodic extinction events

Abstract

Recent suggestions of astronomical causes for the closely periodic extinction events in the geological record1 and the possible periodicities in the cratering record2 have involved two basic ideas. First, it has been suggested3-5 that the oscillations of the Solar System through the galactic plane with semi-periods of the order of 30 Myr make possible close encounters with giant molecular clouds, or sub-units thereof, which are expected to scatter Oort cloud comets into the terrestrial zone. Second, it has been suggested6,7 that a low-mass, and as yet unseen, companion to the Sun, tentatively named Nemesis, is in an eccentric orbit (e ∼0.7), such that perihelion passages bring it through the postulated dense inner Oort cloud8. Perturbations of cometary bodies there result in scattering of some 109 comets into the terrestrial zone. Those that impact the Earth are assumed to loft dust into the atmosphere, causing climatic chaos5,9. To achieve the period that matches the best estimates for the intervals between extinction events, semi-major axes in the range 9 × 104-1 × 105 AU are required. Whether such orbits are indeed stable over the lifetime of the Solar System is not known. We present here evidence from three-dimensional numerical modelling that only orbits with a limited range in inclination with respect to the galactic plane are formally stable for the required length of time. Perturbations bypassing stars and molecular clouds may make even these orbits unstable. © 1984 Nature Publishing Group.