The planes must be able to IMMEDIATELY land.
The planes defend in the air until the end of the battle. They then attempt to land after all of the attacker's combats are resolved and before the attacker's noncombat movements begin. If no landing space can be found at that point, they are lost.
So, if there is another aircraft carrier in the zone, or a friendly land zone adjacent to the originating zone (such as an island) then the planes can land there. They cannot use more moves than that.
They may also land on a friendly carrier in an adjacent sea zone.