The Solar System may be defined as consisting of all those objects that are governed by the Sun’s gravitational field. Other effects arising 1 the proximity of the Sun could equally 2 be used as criteria, such as radiation pressure or interaction with the solar wind. 3 any of these definitions the Solar System extends 4 to a distance of about two light-years; the closest star, Proxima Centauri, 5 lying at a distance of slightly more than four light-years. Our knowledge of this region of space certainly does not reach as far as this, 6, because the most distant Solar-System objects that we know about, the comets, seem to originate at a distance of no more than 50,000 astronomical units, or less than a third of the total distance. 7 for the other Solar-System bodies known to us, they lie at distances of less than a few hundred AU. Our study is, therefore, confined 8 what is primarily the central region of the Solar System.