Question

Why are the planets placed at the particular distances from the sun that they are at?

This is a question that has interested astronomers since the time of Johannes Kepler, in the early seventeenth century, who spent years trying to match their spacing using the relative dimensions of various regular solids such as pyramids and tetrahedrons. In the eighteenth century astronomers came up with the famous Bode rule (or Titius-Bode rule) that represents the spacing in terms of a simple numerical sequence; this rule is still often quoted in astronomy textbooks, even though it does not match the spacings of the outer planets Uranus and Neptune. Much more sophisticated computer models were used in the late part of the twentieth century to try to reproduce the spacing of planetary orbits based on the formation process by which the planets condensed out of the solar nebula. However, much of this thinking was swept away when we began to discover other planetary systems, in which the spacing and size of the planets was entirely different from those in our own solar system. Thus we must now conclude that the particular spacing is partly a matter of chance in the way the planets formed, and partly the result of migration of one planet under the gravitational force of another. For instance, the orbits of Uranus and Neptune have likely shifted under the gravitational influence of Jupiter. David Morrison
NAI Senior Scientist
February 28, 2008