Stellar perturbations of the cometary cloud

Abstract

The existence of a cloud of ∼2 × 10¹¹ comets surrounding the Solar System and extending out to interstellar distances has been suggested by Oort¹ based on the observed distribution of inverse semi-major axes of the long-period comets. Simple statistical arguments can be used to update the current understanding of how stellar perturbations act on the Oort cloud, and to infer some of the cloud properties. Using such an approach it is shown here that the cloud radius is ∼10⁵ AU, the mean ‘thermal’ velocity in the cloud is ∼110 m s⁻¹, and the resulting perihelion distribution in the planetary region is uniform with q. Stars passing through the Oort cloud over tfie history of the Solar System have ejected at least 9% of the initial population, and have randomized the orbits of the remaining comets so as to leave little record of their initial state.