What is the etymology of the Persian word for "sun"?

This might seem a little esoteric, but does anyone have any knowledge, or even any conjectures as to the origins of the Persian (specifically Farsi) word خورشید (khorshid)?

I found this word a little puzzling, given that it doesn't seem to be an Indo-European (or Turkic or Semitic, for that matter) word. I know that Persian also uses a cognate to the Hindi word आफ़ताब (aaftaab, which I assume originally came from Persian), and that Офтоб (aftab) is more common in Tajik, but even in other Indo-Iranian languages such as Kurdish (which has a cognate to aftab), and the Indo-Aryan languages of South Asia, خورشید isn't reflected.

Where did this word come from?