I am working on a world that is an Earth-sized planet that was captured by a gas giant planet moving inward in the solar system, then becoming a moon. Through magic and hand-waving mechanisms, the planet remains habitable following mass extinctions during its capture.
The question, then, is what would be the necessary size of a gas giant to have an Earth-sized moon in a stable orbit? Would a Jupiter-sized planet be sufficient, or would it need to be a super-Jupiter?
In case it matters, I'm intending the orbit of the moon to be far enough away to avoid catastrophic radiation and tidal forces. However, unless that makes a difference for how big the planet would be I'm saving questions of orbital distance for the next post.