If the planet ran out of oil, there are alternative options for cars (electricity, biofuel, hydrogen) but what about airplanes? How would we be able to power airliners that fly across oceans?

(may be broke/outdated!)
If the planet ran out of oil, there are alternative options for cars (electricity, biofuel, hydrogen) but what about airplanes? How would we be able to power airliners that fly across oceans?