Space
Being in space can reflect being "far away (from everyone else)," that one is isolated from humanity, and in this regard, the dream might mirror a lost connection with one's humanity or human context. The image of floating away and losing oneself in space is poignantly illustrated in David Bowie's Space Oddity.
Space is an illustration of our (collective) unconscious, with stars, cosmic forces, darkness, and the unknown. Venturing out into space, into the unknown and the different, becoming helpless, awestruck, and isolated—like a possible illustration of a journey into the unconscious—is depicted in Stanley Kubrick's 2001: A Space Odyssey.
The distant planet in the cosmos is something unattainable, entirely alien, and has similar associations to islands, but where the isolated, distant, and alien qualities are particularly pronounced; as exemplified in The Little Prince, where two people from different planets meet.
When we try to express that something is completely incomprehensible or entirely different, we sometimes say that it is "from another planet," meaning completely different and probably incomprehensible. (See also UFO.)