We reveal several new properties of the Kerr black hole previously unexplored in the Hawking era. These results provide new insights into the structure of black holes and future perspectives on astrophysical phenomena.

1. The Horizon Mass Theorem states that for all black holes: neutral, charged, or rotating; the mass observed at the event horizon is always twice the irreducible mass observed at infinity. The irreducible mass does not contain rotational energy, therefore there is no rotational energy inside a rotating black hole.

2. The rotational energy of the Kerr black hole exists completely outside the horizon. Accordingly, the extraction of rotational energy in the Penrose process occurs in the ergosphere region.

3. A black hole with an angular momentum and an angular velocity has a moment of inertia and a radius of gyration lying outside the horizon. When the rotation stops in an energy extraction process, there exists an irreducible moment of inertia for the static black hole. A Schwarzschild black hole does not have this property.

4. All black holes have the same surface area if their irreducible masses are equal in value, independent of their types: static, charged, or rotating. This is the Area Universality Law. It shows that the entropy of a black hole does not depend on its rotational or electrostatic energy. Black hole entropy has a purely gravitational origin.

These surprising properties show that the Kerr black hole has an extended structure. They would clarify the real nature of black holes, in particular, the supermassive black holes at the centers of galaxies.

References: Y. K. Ha, Int. J. Mod. Phys. D, Vol. 31, No. 4 (2022) 22300006 The Irreducible Mass of Christodoulou-Ruffini-Hawking Mass Formula

Y. K. Ha, Int. J. Modern Phys. D (2026) 2540019 A New Structure of Black Holes

Author is four-time winner of Gravity Research Foundation Award