They assumed the volatiles had been trapped in the form of hydrates and incorporated in the planetesimals flying in the protoplanets’ feeding zones.
In melted zones their heavier elements sank to the center, while lighter elements rose to the surface.
During the 20th century many theories were proposed including the planetesimal theory of Thomas Chamberlin and Forest Moulton (1901), tidal model of Jeans (1917), accretion model of Otto Schmidt (1944), protoplanet theory of William McCrea (1960) and finally capture theory of Michael Woolfson.