By considering the stability of a rotating mass of fluid, and building on earlier work by J. H. Poincaré and others, he developed theories of the formation and evolution of stars, double-star systems, and spiral galaxies. In 1917 he proposed a theory, subsequently expanded on by the English mathematician Harold Jeffreys (1891–1989), in which the planets condensed from filaments pulled out of the Sun by a passing star. In 1928 Jeans advanced a cosmological theory in which matter was continually being created—a cornerstone of the later steady-state theory. He then turned to popular science writing and broadcasting.