Well, another forum I was on just said the ROTS novel says that Emperor wasn’t disfigured by his lighting but the lighting made him drop the illusion and his real look came out. So it looks like I might have been right about Emporer being a different species after all, also there is this:
Possibly more members of his species.
Though I do prefer the idea that the Force lightning revealed his true self instead of scarring him, I’m not too sure about Palpatine being an alien. That doesn’t really fit in with his, and the Empire’s, poor treatment of non-humans. I do seem to remember the ROTJ novelization saying that Palpatine’s appearance was due to him being corrupted by the Dark Side. So it makes sense that his “old” look is his true Dark-side corrupted self and that he made himself look younger to tric the Jedi and the senate, but him being alien seems a bit weird and inconsistent to me. Also his advisers just look old to me, not melted like Palpatine does, and it’s not impossible that they too are Dark-side users to some degree.