Recent results of quantum field theory on a curved spacetime suggest that extremal black holes are not thermal objects. In particular, the notion of temperature is ill-defined for them. If this is correct, one may have to go to a full semiclassical theory of gravity, including backreaction, in order to make sense of the third law of black hole thermodynamics. Alternatively it is possible that we shall have to drastically revise the status of extremal black holes.
