In Descartes’ cosmology, each star lies at the center of a “vortex,” or gigantic pool of circulating fluid. Stars and vortices are mortal, passing into and out of existence. As a star like the Sun becomes encrusted with sunspots, it changes into a comet and wanders from vortex to vortex, visiting other solar systems. Then, with further encrustations, it develops into an Earth-like planet, complete with mountains, oceans and minerals. As a leading proponent of the mechanical philosophy, Descartes sought to explain all of nature, including gravity and magnetism, by reference only to matter and motion, rather than appealing to hidden forces.