First one among these 3 is the best.
Remember about valid scale of objects. Background planet size should be well-adjusted, if somehow player is allowed to know the distance between planet and battlefield, for example a text "battle of <insert solar system name here> is taking place <insert a number> light years from <insert planet's name>" in briefing.
If players won't know the distance, then the size of background planet is self-explanatory.
Also, an addition to above: When user sees a HUGE sun in background, then there should be an announcement like "commander, navigate away from this star, as our ships would begin to melt!"