re: Living Earths in interstellar space?
Ejected planets could be a small red or brown dwarfs, or other very dim and therefore undetectable neostellar objects. I'm sure the line between brown dwarf and Jovian gas giant is quite blurry. These worlds would generate heat as a result of slow gravitational collapse. A planet (satellite) of such an object would have two sources of energy throughout its history if it orbited its primary closely enough: the glow of the primary, and the tidal forces generated by its close proximity.