In the Copernican description of the planetary system there are six planets instead of the Ptolemaic seven, the moon having become a subsidiary body of a type new to astronomers – and for which Kepler was to invent the term satellite in 1611. In 1540, Rheticus felt the need to defend this new number of the planets:

As Rosen remarks in his note on his translation of this passage, Rheticus's numerological argument finds no parallel in the work of Copernicus himself. It does, however, find an answer in Kepler's defense of Copernicanism in the Mysterium cosmographicum.

Kepler's own explanation of the number of the planets is geometrical: There are exactly six orbs because there are exactly five regular solids to define the spaces between them. As Kepler points out, the fact that there are exactly five such solids is proved in a scholium to the last proposition of Elements, Book XIII. Kepler had, however, considered the possibility of a numerical explanation of the structure of the planetary system, in connection with his earliest attempts to find a pattern in the ratios of the dimensions of the planetary orbs.