I would have thought the gravitational pull would keep the vapor around the planet.
The radiation from their sun breaks apart the water molecules into gases of hydrogen and oxygen.
A constant pressure form the radiation will eventually strip molecules and atoms form the planet’s atmosphere, one at a time.
http://gizmodo.com/5882517/did-you-know-that-earth-is-getting-lighter-every-day
Earth’s magnetic field provides vital protection | Astronomy.com http://bit.ly/1HVtAtO via @AstronomyMag