Randomness in the solar system

Most motion observed in the solar system on human timescales is highly regular—though sometimes intricate, as in the sequence of numbers of days between successive new moons shown below. In the mid-1980s, however, work by Jack Wisdom and others established that randomness associated with sensitive dependence on initial conditions could occur in certain current situations in the solar system, notably in the orbits of asteroids. Various calculations suggest that there should also be sensitive dependence on initial conditions in the orbits of planets in the solar system—with effects doubling every few million years. But there are so far no observational signs of randomness resulting from this, and indeed the planets—at least now—mostly just seem to have orbits that are within a few percent of circles. If a planet moved in too random a way then it would tend to collide or escape from the solar system. And indeed it seems quite likely that in the past there may have been significantly more planets in our solar system—with only those that maintained regular orbits now being left. (See also page 1021.)