Could aliens use planets as spaceships?

In principle, an extremely advanced civilization could use a planet as part of a larger propulsion system, but not in the sense of a normal engine on the planet itself. The most plausible speculative versions involve moving or steering an entire star system rather than pushing a single planet alone.

For example, a Shkadov thruster is a proposed giant mirror that would reflect a star’s light asymmetrically, producing a tiny net thrust over a very long time and shifting the star along with its planets. A Caplan thruster is another speculative concept that would use stellar material and fusion-driven jets to steer a star system much more aggressively.

Directly propelling a planet by itself would be far beyond known engineering. The obstacles are enormous mass, the energy required to change its motion, and the risk of catastrophic disruption to the planet’s crust, atmosphere, and any biosphere. In a more extreme idea, using the planet’s own mass as propellant would likely strip away its atmosphere and devastate its surface.

So the best answer is: maybe in very advanced theoretical scenarios, but only as a highly speculative civilization-scale project, not as something we know how to do or that is currently feasible.