Thanks! I'm no expert in either technology or chemistry or physics, but I do have a gamemaster's interest in creating distinct, unusual words for the PCs to visit. :)
Ooooh, oooh! I thought of another possible use for a chlorine planet. Chlorine can be used to form
aqua regia, also known as nitrohydrochloric acid, which can be used to dissolve gold. Since gold is a valuable catalyst in nanoassembly, and since even today, substances such as aqua regia are used to liquefy gold so it can be leached from tailings in strip mines, it's cheaper to get gold there — on that planet, the chlorine won't hurt the environment!