There was a thread on the old 1.0 forum that discussed Hydaelyn's night sky (and many of the posters are actually professional astronomers and scientists) but I can't remember if there was any mention about other planets in Hydaelyn's solar system (personally I presume there would be, especially as residents of Hydaelyn use the word 'star' to also mean 'planet', including their own world), so your guess is as good as mine.

Either way, this is the thread I was talking about *alerts the Eorzean Astronomical Society by linkshell*