I have a question about a certain scenario involving IGs on planets. Say a player is in a sector with enemy L6 planets. All the planet have IG turned. The player attempts to leave. Which planet loses 500 fuel to keep the player in sector? All of them? the lowest number?

All of them.

Not to start an argument with PHX but wouldnt it depend on other circumstances as well to determine what planet lost fuel. For instance the way I remember is like this:

Say you have 5 L6 planets, all IG's on, all planet set for 20k sector damage.
A stock ISS with 50 k fighters tries to leave, the first IG, they fire in order of appearance in sector, grabs him and fires, the second grabs him and fires, then the third grabs him and his pod flees to the adjacent sector. Would that then mean that only the first 3 planets lost fuel for the IG's also or does it still take from all of them for the IG's.

It would be nice if someone could clarify this as it would be good to know.

I've never actually though to test that, I have no idea.

This is why you only have the last planet to shoot put its interdictor on

I have tested it in live games...yes they all loose the 500 ore, that is why (as sunshine said) only the last planet need have an IG on, as its fuel will last the longest while all the other planets are using their ore to fire cannons and will run out faster.

Knowing that, youd best have some good defenses on that last planet cause if someone knows you are working that way, they might go after that one first.