Which statement accurately describes how spin orientation affects the ISCO around a rotating black hole?

• A. Spin does not affect the ISCO
• B. Prograde orientation moves the ISCO outward
• C. Retrograde orientation moves the ISCO inward
• D. Prograde orientation moves the ISCO closer to the horizon while retrograde moves it farther out

Answer: (D)

Spin of a black hole changes the space around it in a way that depends on whether the orbit goes with or against the spin. This is why the innermost stable circular orbit, or ISCO, moves: prograde orbits (same direction as the hole’s spin) benefit from frame dragging and can remain stable closer to the horizon, while retrograde orbits (opposite direction) feel the dragging as a destabilizing effect and must stay farther out. As the spin increases in the prograde case, the ISCO moves inward toward the horizon; in the retrograde case, it moves outward, even further from the horizon. In the most prograde situation the ISCO can lie very close to the horizon, while in the most retrograde case it lies farther out. This description matches the statement that prograde moves the ISCO closer to the horizon and retrograde moves it farther away.