Could another large planet in a nearby orbit -
Many science fiction stories and movies deal with Earth-like planets orbiting gas giants. The Star Wars trilogy alone has several of these. Most of these, of course, have not been subjected to much scientific scrutiny. Such a planet would have to be distant enough from the primary to not be bathed in its radiation belts, and there would be other limitations as well. The gas giant would also have to be inside the solar system's habitable zone, which, as I understand it, may not be possible. I wouldn't mind reading some detailed science papers on this subject.