If they're set in the same place, I don't think it's the same solar system. Doesn't FFIV end with a view of the solar system? Also, good 'ol Sephiroth from FFVII has an attack that uses most of the solar system (in that game, the planets have the same names as in our solar system. Jupiter, Mars, the whole shebang). If (and this a very shaky if) they're set in the same place, it would probably just be the same universe or something.
