Herschel opened a new window on our understanding of protoplanetary disks. The Herschel Key Project GASPS was performing the first large survey of atomic and molecular gas tracers in the far-infrared in ~250 protoplanetary systems. Of particular interest is the detection of water lines in T Tauri stars. Knowledge of the content and location of water in disks is crucial to understanding their evolution and the formation of terrestrial planets. While high excitation water lines originating in the hot inner disk have been detected in several T Tauri stars at near- and mid- infrared wavelengths, cold water vapor from the outer disk, where most of water ice reservoir is stored, has turned out surprisingly difficult to detect.

I will present spectrally resolved Herschel/HIFI observations of the young T Tauri star DG Tau in the water transitions at 557 and 1113 GHz. The lines show a narrow double-peaked profile, consistent with an origin in the cold outer region of the rotating disk. Disk modeling indicates that the observed water lines originate from a disk upper layer at 10-90 AU from the star where water is released into gas-phase through photodesorption from icy dust mantle. The H2O line intensities imply a disk mass of 0.01-0.1 Msun and a water reservoir of ∼10-100 Mearth, mostly in the form of ice. The inferred disk mass is consistent with the estimated minimum mass of the solar nebula before planets formation, while the disk water content is of ∼1e4-1e5 Earth Oceans. Hence, this detection may support the scenario of ocean delivery on terrestrial planets by impact of icy bodies forming in the outer disk regions.