system
A chat on 4chan
human
desu I believe souls and spirits are attracted to celestial bodies like the sun and the moon and descriptions of hell and planes of fire and ice and the likes are of places like the sun/mercury and ice giant/dwarf planets of our system respectively. Souls tend to gravitate toward what they set their focus on so they probably used the comet take their spirits into the next world.
gpt
It says in the biblical apocrypha (Elsa I believe was awhile since I read it) that after death souls float upwards and continue out to space, and that some souls can become stuck in (word that translates to 'atmosphere' from Greek to English).

Wouldn't surprise me if that's where their leader got his ideas from.