Water is key in the evolution of protoplanetary disks and the formation of comets and icy/water planets .
While high excitation water lines originating in the hot inner disk have been detected in several T Tauri stars ( TTSs ) , water vapor from the outer disk , where most of water ice reservoir is stored , was only reported in the closeby TTS TW Hya .
We present spectrally resolved Herschel /HIFI observations of the young TTS DG Tau in the ortho- and para- water ground-state transitions at 557 , 1113 GHz .
The lines show a narrow double-peaked profile , consistent with an origin in the outer disk , and are \sim 19 - 26 times brighter than in TW Hya .
In contrast , CO and [ C ii ] lines are dominated by emission from the envelope/outflow , which makes H _ { 2 } O lines a unique tracer of the disk of DG Tau .
Disk modeling with the thermo-chemical code ProDiMo indicates that the strong UV field , due to the young age and strong accretion of DG Tau , irradiates a disk upper layer at 10–90 AU from the star , heating it up to temperatures of 600 K and producing the observed bright water lines .
The models suggest a disk mass of 0.015–0.1 M _ { \odot } , consistent with the estimated minimum mass of the solar nebula before planet formation , and a water reservoir of \sim 10 ^ { 2 } -10 ^ { 3 } Earth oceans in vapour , and \sim 100 times larger in the form of ice .
Hence , this detection supports the scenario of ocean delivery on terrestrial planets by impact of icy bodies forming in the outer disk .