It was the third planet out from a yellowish sun in a stellar system.
I want to make a wh question about "third". Do we use "which" or another word?
Thanks
There is no straightforward answer to your question, as you will see from these threads:
How to ask a question?
What is
right or wrong
You could ask "which planet in the solar system?" or "What position is the planet in with respect to the sun?"