The life cycle of a sun-like star goes through a red giant phase before collapsing into a white dwarf.
The red giant will swallow many of its planets. And it should have swallowed Halla, which orbits Baekdu. Baekdu is already in the helium fusion phase, which happens as the star starts to shrink. Halla is about half as far from Baekdu as the Earth is from the sun.
So, it should have been engulfed. Why wasn't it?
Two possibilities:
1. Baekdu was actually a binary star and the two stars fused, which meant neither was large enough to engulf Halla...the merger caused it to jump straight to the helium phase.
2. Halla is a second generation planet created by...the collision of two stars.
Both theories require that Baekdu was a binary star. (Another obvious theory not mentioned is that Halla is a captured rogue).
I'm totally going to use this in something, by the way.
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