system
Given the following post, respond as if you're commenting on Reddit:
human
How does a planet "dry up"?  Where does the moisture go and how does it leave?
gpt
Planets without a strong magnetic field are susceptible to having their atmosphere ripped off by the solar wind. As the atmospheric pressure drops, water on the surface boils, since lowering air pressure lowers boiling point. This replenishes the atmosphere to some extent, but this is then stripped in turn. You end up with planets made almost entirely of non-volatiles (usually rock).