Are there sidhe on other planets?
Call them what you will—sidhe, elves, land-wights—wherever humans go, we seem to discover Other Peoples in the Land, the Land's Older Children: not quite gods, perhaps, but of a kind with them, if perhaps a lesser kind.
Resources for answering this question are meager, since in few places does human memory extend to a time before there were humans in the Land. I can think of fewer than a handful of examples.
Still, the stories all agree. When the Norse reached Iceland, land-wights were already there to meet them.
Let me broaden the question. Are there land-wights in Antarctica?
Of course, by the time that humans first arrived in Antarctica, we had mostly ceased to “believe” in such beings, and so did not expect to encounter them. By analogy with Iceland, though, I would expect the answer to the question to be “yes.” Surely, if ever there was a land of trolls and frost-etins, it would be Antarctica.
There was, of course, life in Antarctica before humans got there. Does that make a difference?
Are there land-wights on a planet with sentient, but non-human, life?
Are there land-wights on a planet without sentient life?
Are there land-wights on a planet with no life at all?
Are there land-wights on the Moon?
We have no way to answer such questions.
Who, one might ask, are the land-wights? Are they not, if effect, Nature looking back?