Water was formed early in the universe

Science : As much as 50% of the water now found on Earth may have formed more than 4.5 billion years ago—before the Sun came into existence. To try to determine where the water came from, L. Ilsedore Cleeves of the University of Michigan and colleagues focused on deuterium. Although there are just 26 deuterium atoms for every million hydrogen atoms in the universe, the proportion of deuterium in water on Earth and in the rest of the solar system is six as great. The chemical reactions that created heavy, or deuterium-rich, water were faster than those that created normal water. To work, the reactions require very cold conditions, abundant oxygen, and ionizing radiation. Those conditions are found in the cold molecular clouds from which stars form. However, according to calculations by Cleeves and her colleagues, the conditions are much less prevalent in the protoplanetary disks that form around stars after the clouds’ initial collapse. Although a protoplanetary disk is cold and contains oxygen, it is shielded from the most potent source of ionizing radiation, charged cosmic rays, by the star’s magnetic field.