2004ApJ...605L..65J

The increase with time in the luminosity of a main-sequence star can eventually lead to substantial evaporation of the oceans on an orbiting terrestrial planet. Subsequently, the gas-phase H₂O in the planet's upper atmosphere can be photodissociated by stellar ultraviolet, and the resulting atomic hydrogen then may be lost in a wind. This gaseous envelope may pass in front of the host star and produce transient, detectable ultraviolet absorption in the Lyman lines in systems older than 1 Gyr.