I suppose the question is whether "dwarf" is a modifier to "planet" or whether "dwarf planet" is a compound word that cannot be split without losing meaning.

I would lean toward the former -- if there was not room in the headline for "Dwarf planet Ceres", referring to it as "Planet Ceres" still adds information and clarifies helpfully. "Ceres" may not be familiar enough to most readers for them to instantly think, "Oh, they must be talking about the largest asteroid/eighth largest known dwarf planet", but adding "planet" makes it clear that we're talking about space stuff.