  Water is crucial for the emergence and evolution of life on Earth. Recent
studies of the water content in early forming planetary systems similar to our
own show that water is an abundant and ubiquitous molecule, initially
synthesized on the surfaces of tiny interstellar dust grains by the
hydrogenation of frozen oxygen. Water then enters a cycle of
sublimation/freezing throughout the successive phases of planetary system
formation, namely, hot corinos and protoplanetary disks, eventually to be
incorporated into planets, asteroids, and comets. The amount of heavy water
measured on Earth and in early forming planetary systems suggests that a
substantial fraction of terrestrial water was inherited from the very first
phases of the Solar System formation and is 4.5 billion years old.
