When the Sun formed from its protostar, around 4.56 billion years ago, the solar wind blew all gas particles from the central part of the Solar Nebula. In this way most lighter volatiles (e.g. hydrogen, helium, oxygen, carbon dioxide), that had not yet condensed in the inner, warmer regions of the nebula, were lost. This fractionation process is the reason why the terrestrial planets and asteroid belt are relatively enriched in heavy elements in respect to the Sun or the gas planets.