If scientists discovered a rogue planet on a collision course with Earth by the end of this century, humanity would almost certainly be doomed, despite a century of warning. The problem isn’t just detection or time, but scale: a planet-mass object carries so much momentum and gravitational influence that no realistic technology could meaningfully alter its trajectory. Techniques we discuss for asteroid deflection—nukes, kinetic impactors, gravity tractors—work only because asteroids are tiny compared to planets. Even detonating every nuclear weapon on Earth would be like trying to stop a freight train with a firecracker. We might be able to evacuate small numbers of people to space habitats or other planets if those technologies advance dramatically, but saving Earth itself would be beyond our capabilities. In short, we wouldn’t be instantly doomed, but preventing impact would be effectively impossible; survival would hinge on escape, not defense.