Cold Accretion Theory
Cold accretion theory is a term that can be used to distinguish modern theories of planet growth from earlier theories. The mechanics of modern cold accretion are reviewed in the article Accretion.
Earlier theories visualized planets as forming out of hot blobs of solar plasma, or large, gravitationally unstable 'protoplanet' clouds which were relatively large segments of the solar nebula at any given solar distance. The planets were generally visualized as forming either from very hot matter, or (at least) forming in a molten state because of the high temperatures reached during gravitational collapse.
However, Urey (1952) and Shmidt (1958) theorized that the planets formed from innumerable small, solid (i.e. 'cold') bodies. Urey used the argument that planets are extremely deficient in inert gases. This means they could not have formed by gravitational contraction of solar or nebular clouds because those would have contained solar abundances of inert gases,