Until the day we find extraterrestrial life, we will always wonder whether there is life in outer space.
Until the day we find extraterrestrial life, we will always wonder if there is life in outer space.
Until the day we find extraterrestrial life, we will always wonder whether there is life in outer space.
Until the day we find extraterrestrial life, we will always wonder if there is life in outer space.
We use "if" for a condition for a specific action: "If it rains, I will wear my jacket." The word "whether" is used for an uncertain choice of possibilities and any cognitive expression of uncertainty: "ask whether," "say whether," "decide whether," "know whether," "understand whether," "predict whether," "wonder whether." In all of those cases, "if" would be incorrect.