By reconstructing conditions in the disk of gas and dust in which the Solar System formed, scientists have concluded that the Earth and other planets must have inherited much of their water from the cloud of gas from which the Sun was born 4.6 billion years ago, instead of forming later. The authors say that such interstellar water would also be included in the formation of most other stellar systems, and perhaps of other Earth-like planets. The dense interstellar clouds of gas and dust where stars form contain abundant water, in the form of ice. When a star first lights up, it heats up the cloud around it and floods it with radiation, vaporizing the ice and breaking up some of the water molecules into oxygen and hydrogen. Until now, researchers were unsure how much of the ‘old’ water would be spared in this process. If most of the original water molecules were broken up, water would have had to reform in the early Solar System. But the conditions that made this possible could be specific to the Solar System, in which case many stellar systems could be left dry, says Ilsedore Cleeves, an astrochemist at the University of Michigan in Ann Arbor, who led the new study. But if some of the water could survive the star-forming process, and if the Solar System’s case is typical, it means that water “is available as a universal ingredient during planet formation”, she says.

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