The rule that expresses the regularity of the
distances of the planets from the sun: Neglect Mercury and assign the value of
0.3 AU (astronomical units) to Venus. Then, to find the orbital radius of any
planet, take the orbital radius of the one next closest to the sun, double it,
and add 0.4 AU. The law works well for all planets, except that there is a
planet missing between Mars and Jupiter. The search for a planet in that orbit
led to the discovery of the asteroid belt where the missing planet should be.