Page:The Solar System - Six Lectures - Lowell.djvu/133

Rh aphelia, and then nothing again till we reach 75 units out.

This can hardly be accident; and if not chance, it means a planet out there as yet unseen by man, but certain sometime to be detected and added to the others. Thus not only are comets a part of our system now recognized, but they act as finger-posts to planets not yet known.

We have thus examined the case of an old planet,—Mercury; of a middle-aged one,—Mars; of a youthful one,—Jupiter; and we have ended by envisaging the yet unchristened.