Wouldn’t the water vapor just stay in the atmosphere? Where would it go?
Blasted into space from their sun’s radiation over millenia.....................
It would for awhile, but if the star it orbits is dying then it’s likely getting bigger and bigger (Red Giant). When that happens solar radiation increases and starts blowing atmospheric gases off of the planet, sending that water vapor and various other gases into space. So as the planet orbits the star it has a comet like tail that always points away from the star. Until the entire atmosphere has been blown away. Then you’re left with a really hot rock.