Is Threnody in the same planetary system as Sel?
[No, they are not]
Folks, I hate to be a killjoy, but I just asked Brandon about this, and he says that he answered that question with "No, they are not."
Brandon says he specifically remembers this question, and he remembers saying that Threnody and Sel were not in the same system.
In any case, Threnody and Sel do not revolve around the same star. (And they're not in a binary system system either, for those trying to salvage something out of this.)