As any astronomer knows, there are two zones in the Solar System packed with small loose bodies, two to three kilometers across, of rock and ice: The Kuiper belt out past Neptune and the Oort cloud, a mind-numbingly vast region marking the very distant outer reaches of our Sun’s territory at a distance of about a light-year. Every now and then some sort of gravitional perturbation (a close encounter with a larger object, or passing star) will disturb the orbit of one of these objects which then slowly begins to fall towards the inner solar system. As it nears the Sun, the ice and frozen gases melt and spray off, creating a vast cloud around the object, and we see this from Earth as a comet. The only mystery is what specifically causes any individual comet to begin it’s inward journey.
Sophisticated computer simulations run by researchers at the Southwest Research Institute (SWRI) suggest an interesting new theory. According to their work, our observations of comets so far imply a population of about four hundred billion objects in the Oort Cloud. Current models on the formation of the Solar System only predict six billion objects, however. One of the SWRI scientists, Hal Levison, suggests a possible explanation: Since stars form in clusters, the Sun would have been extremely close to a number of other stars very early in its life. Comets would constantly be tugged back and forth, moving from star to star until the growing solar wind from the new stars dispersed the nursery nebula and the stars began moving apart. The new theory suggests that the Sun had the bulk of the cluster’s comets when it moved away, stealing them from its siblings.
It has a certain romantic appeal, and the computer simulations seem to support the idea. Sadly, though, as with all other theories of comet formation, we can never fully test the idea – in the billions of years since our Sun was born, the movements of such light objects have been so thoroughly buffeted by the planets, by each other, and by passing stars that we can never untangle them and trace their paths backwards in time. But perhaps, as Levison and his colleagues work on refining their theory, they will provide a convincing enough case for this theory to become accepted as part of the standard model. I for one hope so!