[PapaBear3625]

How can it be “geology” when that term is specific to planet Earth?

Because "astrology" is already taken.

[Olog-hai]

That’s OK since “aster” is Greek for “star”. “Edaphology” (study of soil) is also taken; another possibility is “chomology”, but that might be confused with “cohomology” (unless you spell the latter “co-homology”).