AP Photo

WHERE science-fiction leads, science sometimes follows. In 2009, when James Cameron made “Avatar”, he set his colonial allegory not on an alien planet but on the moon of such a planet. Perhaps he felt that exoplanets themselves were passé as settings for fiction, since in fact so many had been discovered. If so, he got in just in time, for reality may soon imitate art in the matter of moons, as well as planets.