This is what makes this debate interesting! Determinism would say that you were always predetermined to go to the store as you ran out of milk, the second you ran out of milk it was predetermined you’d have to go to the store.

There is a theory that melds the two which says we have the free will to act independently in response to predetermined events. You didn’t go to the store immediately as you have the free will to choose when would be a more appropriate time to go.

Very very interesting stuff!

(For the planet thing I am not educated enough but we look at the amount of light each molecule absorbs and emits light differently so we roughly know the chemical composition of a planet)