Near the end of the movie where they expelled the brown dwarf star piece. The moon should have been the one that's expelled while the star piece stayed. The mass of the star is twice of the Earth's. The moon is 1/100 the mass of the Earth or 1/200 of the star. The end of the world will still happen but without the moon. Perhaps they miscalculated the star's mass, and the actual mass is less than the moon's.