Someone brought up the Aliens reference. Just to clear the air this was my response.
All of the worlds that were described by Malachar to Jake, Atlantis, Infernum Mons Igneus (where the word Hell came from), Coelum (Where the word Heaven came from), and various other worlds that he spoke of or implied existed. Everyone not having been born on Earth is an Alien. Atlanteans are Aliens, so are the Infernals and Angels. Malachar himself is an Alien, although he is Human. Unbeknownst to the vast majority of people living their ignorant lives on Earth, Humans spread to many other planets when there was regular magical transport between worlds.
It's not Aliens like the little grey men and area 51. The other worlds are based on magic, not science fiction. I hope that clarifies it for anyone wondering about that.
Cheers!
Chris