Anti-Ballistic Missiles (ABMs) are the only way to destroy attacking Interplanetary Missiles (IPMs). Each Anti-Ballistic missile destroys one incoming Interplanetary Missile.
Anti-Ballistic Missiles are launched automatically whenever an approaching Interplanetary Missile is detected. Otherwise they do not take part in any attacks, thus they cannot be sent on missions or destroyed by an attacking fleet.
You can store 10 Anti-Ballistic Missiles for every Missile Silo Level on your planet (twice the amount of Interplanetary Missiles).